CA1219377A - Graphic display systems - Google Patents
Graphic display systemsInfo
- Publication number
- CA1219377A CA1219377A CA000477995A CA477995A CA1219377A CA 1219377 A CA1219377 A CA 1219377A CA 000477995 A CA000477995 A CA 000477995A CA 477995 A CA477995 A CA 477995A CA 1219377 A CA1219377 A CA 1219377A
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- Prior art keywords
- area
- list
- clip
- subject
- parts list
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Links
- 238000000034 method Methods 0.000 claims abstract description 58
- 239000003086 colorant Substances 0.000 abstract description 3
- 208000033361 autosomal recessive with axonal neuropathy 2 spinocerebellar ataxia Diseases 0.000 description 2
- 208000035405 autosomal recessive with axonal neuropathy spinocerebellar ataxia Diseases 0.000 description 2
- 230000007246 mechanism Effects 0.000 description 2
- 239000007787 solid Substances 0.000 description 2
- 102100023413 GRB2-related adapter protein Human genes 0.000 description 1
- 101000829735 Homo sapiens GRB2-related adapter protein Proteins 0.000 description 1
- 238000005516 engineering process Methods 0.000 description 1
- 239000011888 foil Substances 0.000 description 1
- 230000002452 interceptive effect Effects 0.000 description 1
- 239000000203 mixture Substances 0.000 description 1
- 230000003287 optical effect Effects 0.000 description 1
Classifications
-
- G—PHYSICS
- G06—COMPUTING; CALCULATING OR COUNTING
- G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
- G06T15/00—3D [Three Dimensional] image rendering
- G06T15/10—Geometric effects
- G06T15/30—Clipping
-
- G—PHYSICS
- G06—COMPUTING; CALCULATING OR COUNTING
- G06K—GRAPHICAL DATA READING; PRESENTATION OF DATA; RECORD CARRIERS; HANDLING RECORD CARRIERS
- G06K15/00—Arrangements for producing a permanent visual presentation of the output data, e.g. computer output printers
- G06K15/22—Arrangements for producing a permanent visual presentation of the output data, e.g. computer output printers using plotters
-
- G—PHYSICS
- G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
- G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
- G09G5/00—Control arrangements or circuits for visual indicators common to cathode-ray tube indicators and other visual indicators
- G09G5/42—Control arrangements or circuits for visual indicators common to cathode-ray tube indicators and other visual indicators characterised by the display of patterns using a display memory without fixed position correspondence between the display memory contents and the display position on the screen
Abstract
UK9-84-008 ABSTRACT GRAPHIC DISPLAY SYSTEMS A method of determining the coincidence of a subject area and a clip area in a graphic display system, comprising creating three parts list, one for parts of the subject area that are outside the clip area, a second for parts of the clip area that are inside the subject area and a third for overlapping lines. The parts of the subject area which lie outside the clip area are defined by examining the parts lists. The output can then be used to control a pen plotter, in which different colours are used so that underlying areas do not have to be inked over with overlay colours.
Description
~1~; '3--8 4 - O l~ 7 GRAP~IC DISPLAY SYSTE:~!S
This invention relates to graphic display systems and in particular to such systems that include a pen plotter as an output device for pictures developed in the system.
An interactive graphic display system such as the chaxt utility of the IBM Graphics Display Data Manager (GDDM) which is described in EU patent publication 26266 gives a user the ability to create graphic pictures on a display screen such as an IBM 3279. (IBM is a Registered Trade Mark). Once created it is often desirable to have a hardcopy of the picture and pen plotters connected to the system are commonly used for this purpose.
Pen plotters provide a cheap, high quality graphics hard copy capability.
However the technology differs from that of a cathode ray tube display in one significant way, once something is drawn, it cannot be erased or changed. ~his difference presents a problem where, say, a red square is drawn over a larger blue square. On a display, the underlying blue pels are simply changed to red pels, but on a plotter, the red will be drawn ~ over the blue and a muddy brown colour will result. To provide quality output on plotters, it is therefor necessary to alter any underlying areas by clipping out any overlying areas. When the clipped underlying area is drawn and shaded, it will leave blank paper where the overlying areas will subsequently be drawn, resulting in a much more acceptable plot.
Various methods for area clipping have been published.
"Reentrant Polyson Clipping", Ivan E. Sutherland & Gary w Hodgman Comm. of ACM Jan 197~ V17 no. 1 p.32-42 describe a ~ 8~-00~ - 2 - ~ ; 7 simple method for clipping a concave polygon to a convex area.
This requires a concave clip area to be divided into convex polygons thus increasing the complexity of the calculations.
"~idden surface removal using polygon area sorting" ~evin Weiler 6 Peter Atherton Siggraph 77 proceedings Computer Graphics 11(2) Summer 77 p.214 describe another method for clipping concave polygons with holes, but this requires the polygons to be defined in a clockwise direction and holes in an anti-clockwise direction.
The method of the present invention places no restrictions on the area definitions. Both the subject and clip areas may consist of multiple, overlapping, self-intersecting polgyons. The inside of an area is defined as all points that would cross the area boundary an odd number of times if moved along a straight line to infinity.
It is an object of the invention to provide a method for a complex area definition to be clipped to another complex area definition.
The method also finds use with regard to quality plotting in:
o Implementing 'background' lines by treating each background line as a thin area of width twice that of the pen. These can then be clipped out of any parts of the picture that they overlay.
o When plotting on foils, a common problem is ink run, for example where two areas abut and are coloured differently, because the ink takes a long time to dry, the two colours blend into each other along the boundary. This can be solved by shrinking the areas bv one pen width. The easiest way to do this for an U~ 8~-008 - 3 - l~L~'7'7 arbitrary area definition is to define a thin area of twice the pen width that goes around the boundary of the area, and clip this out of the original area.
According to the present invention there is provided a method of determining the coincidence of a subject area and a clip area, in a graphic display system in which area definitions are stored in a display list store comprising the steps of:
- a) creating and storing a subject area parts list comprising separate entries for each part of one or more contiguous lines of the subject area that do not pass through the clip area b) creating and storing a clip area parts list comprising separate entries for each part of one or more contiguous lines of the clip area that pass through the subject area c) creating and storing an overlap parts list comprising separate entries for each part of one line of the subject and clip areas that overlap d) for each entry in the subject area parts list scanning unprocessed entries first in the subject area parts list secondly in the clip area parts list thirdly in the overlap parts list for an ent-y having an end point definition the same as one end point definition of the part being processed, creating a new part by joining any part found to the part being processed storing the created part in a results part list and reiterating the process for the entry in the results part list until no further parts are found whereby the results part list U~?-fi~ 7 ~
includes the definition of the po~tion of the subject area which does not coincide with tne clip area.
According to a second aspect of the invention there is provided a graphic display system, in which complex area definitions are stored in a display list buffer, including control means adapted to control the operation of the system . 10 to perform the method of determining the coincidence of a subject axea and a clip area comprising the steps of:
a) creating and storing a subject area parts list comprising separate entries for each part of one or more contiguous lines of the subject area that do not pas~ through the clip area b) creating and storing a clip area parts list comprising separate entries for each part of one or more contiguous lines of the clip area that pass through the subject area c) creating and storing an overlap parts list - comprising separate entries for each part of one line of the subject and clip areas that overlap d) for each entry in the suhject area parts list scanning unprocessed entries first in the subject area parts list secondly in the clip area parts list thirdly in the overlap parts list for an entry 3S having an end point definition the same as on end point definition of the part being processed, creating a new part by joining any part found to the part being processed storing the created part in a results part list and reiterating the process for the entry in the results part list until no further ~};q-~-008 c 1~ 7 7 parts ale found whereby the results part list includes the definition of the portion of the subject area which does not coincide with the clip area.
In order that the invention may be fully understood it will now be described with reference to the accompanying drawings in which:
Figs. 1-4 illustrate the operation of the method by way of example.
Fig. S illustrates the component procedures of the method.
Fig. 6 is a block schematic of a system in which the method can be implemented.
The method embodying the invention will clip a 'subject area' to a 'clip area' producing a result that is an area defining all parts of the subject area that are outside the clip area. If necessary, the method is easily changed to clip the other way around, i.e. to produce a definition of the parts of the subject area that are inside the clip area.
Each area definition is represented by a list of poly~on definitions, each polygon definition is represented by a ring of vertices. The resultant area definition is in the same format so that it can be further clipped to other areas if required.
The method involves building three lists. ~ne (the subject parts list) contains the parts of the subject area boundary that are outside the clip area. The second (the clip parts list) contains the parts of the clip area boundary that U~9-84-0~8 ~ ~ c3~r~-7 are inside the subject area, and the third all clip area bounda~y lines that cverlap s~bject area boundary lines (referl-ed to as: 'overlapping' clip par.s).
Each list is built up by considering each line in .he area and clipping it to the inside/outside of the other area.
This requires a line clipping routine that can clip an arbitrary line to an arbitrary area definition. In particular it must cope with lines that (i) ~ may intersect the area at a vertex, (ii) may overlap one (or more) of the area boundary lines; and areas that may consist of multiple overlapping, self-intersecting polygons. Such a routine is described below The resultant area definition is produced by joining the parts in the three lists together to form polygons.
The joining process proceeds by taking a subject part and attempting to join it, l~t : to a non-overlapping clip part, 2nd : to another subject part, and 3rd : to an overlapping clip part.
: - Thus the subject part is extended, and the total number of parts is reduced. ~hen a polygon is formed, it is removed from the subject parts list and added to the resultant area definition. This process continues.for each subject part until there are non left.
Finally, any remaining clip parts are joined (where possible) in an attempt to form more polygons. These polygons are added to the resultant area definition unless they were formed only from overlapping clip parts. In this case, it is determined whether this polygon (which is common to both subject and clip areas) has the same sence (hole or solid) in both the subject and clip areas. The polygon is only added to ~J~;9-84-OOd - ~ 9377 the result if its sense is different in the subject and clip areas.
In the rare situation of clipping to a self intersecting area with many bounda_y lines that are common or overlap the subject boundary, it is possible that after joining various parts, a dead end is reached, i.e. no part can be found that will join. This situation may occur when there are two or more overlapping clip parts that can be joined to the subject part, and the wrong one happens to be chosen. This occurs very infrequently and the simplest solution is to shift the subject area by a tiny amount in the x and y direction, then restart the clipping process.
Figures 1 to 4 illustrate the method by way of example.
A complex polygon such as shown in 1 (a) is displayed on a display screen. This may be part of a graph for example. A
second area shown at 1 (b) is to be overlaid on the area of 1 (a) to give the result shown in Fig. 1 (c). It is then necessary to clip the shaded area of 1 (b) from the area of 1 (a) - Figures 2a and 2b illustrate the contents of a subject parts list.
Figures 3a and 3b illustrate the contents of a clip parts list.
Figure 4a and 4b illustrate the contents of an overlap parts li~t.
Of course all lines and parts are actually stored in the form of end point coordinates and the drawings are used for illustration only.
U~ 4-008 - ~ ?~r7 ~
In general te~s the first step is for all lines of the subiect area to be examined to determine whether or not they pass through the clip area. Starting at the top left corner line 10 has a portion 11 which overlaps the ciip area. This is added to the overlap parts list (Fig. 4a). The second portion 12 is added to the subject parts list Fig. 2.
The lines 13, 14, 15 and 16 are then examined and are added to the subject parts list. As each has a common coordinate with the previous line a complex part is developed.
The line 18 has a portion 19 which passes through the clip area and portions 17 and 20 which do not. The portion 17 is added to the complex part 2 (a) in the subject parts list, the portion 19 is ignored and the portion 20 starts a new part in the subject parts list. The line 21 is added to the part 20 in the subject parts list and the line 22 is added to the overlap parts list (4 (b).
The clip area i~ then examined for lines that pass through or are contained in the subject area. The line 11 is a keady in the overlap parts list. Line 23 has two parts portion 24 is added to the clip parts list and portion 25 is ignored as is portion 27 of line 26. However portion 28 and portion 29 of line 30 are added to the clip parts list (3 (b~). Portion 22 of line 29 is already in the overlay parts li t.
In order to build the subject area minus the clip area, the first entry in the subject parts list is examined and then in the following order remaining entries in the clip parts list and then the overlap parts list are examined to determine the first that has a common end point coordinate with the first entry. This part found is then added to the subject part and the result placed in a result parts list. The process is reiterated using the entry in the results parts U~9-8~-008 - ? - 1~13~ ~ ~
li~t until no parts having a common end point with the resultant part is found. In all cases the result sha ~ be a closed polygon. The next entry in the subject parts list is then examined and the processes repeated until there are no further parts in the subject parts list. The result parts list will then contain a definition of the subject area minus the clip ar~a.
r- 10 There now follows a detailed description of the method.
The method has been divided into small procedures and Fig. 5 shows how they are related.
Area Clip 50 (Fig. 5) This procedure controls the whole of the area clipping process. The 'subject area' is clipped to the 'clip area'.
The result is a new subject area that defines all those parts of the original subject area that are outside of the clip area.
Call 'BUILD PARTS LIST' to bulld the sub~ect parts 11st whlch ; - contains all parts of the 'subject area' that are outside the 'clip area .
Call 'BUILD PARTS LIST' to build the clip parts list which contains all parts of the 'clip area' that are inside the 'clip area'.
This li s~ will also contain 'overlapping clip parts'.
Scan both parts lists and move any parts that are polygons to the result area.
SCAN:
~9~ 008 - ]0 D0 for each subj~ct part ti~ILE subject parts list is not emDty IF there is a non-overlapping clip part that will join THE~
CALL 'JOIN TWO PARTS' ITERATE the SC~N loop ENDIF
IF there is a subject part that will join THEN
CALL 'JOIN TWO PARTS' ITERATE the SCAN loop ENDIF
IF there is an overlapping clip part that will join THEN
CALL ' JOIN TWO PARTS' ITEP~ATE the SC~ loop ENDIF
If this point is reached, we took a wrong turning further 'oack which has lead us to a dead end.
Scan the subject area definition and shift it by one unit RESTART the clipping procedure.
ENDDO
The subject 7ist is now empty, try to construct re polygons from any remaining clip parts.
SCAN2:
DO WHILE clip parts list is not empty Take the 1st clip part DO for each subsequent clip part IF t will join to the first clip part THEN
CALL 'JOIN TWO PARTS' ITERATE the SCAN2 loop ENDIP
1'~19377 -008 - ll -ENDDC) Delete the 1st clip part, it does not form a polygon E~DDO
Clipping is now ccmplete Delete the original subject area and replace with the result area.
Build Parts List (51) This procedure is passed two area definitions, the subject area is to be divided into parts that are either inside or outside of the clip area. Note that the terms 's~bject area' ~ 'clip area' are with respect to the processing done by this procedure, and are independent of the same terms used in the AREA CIIP procedure.
DO for each polygon in the subject area definition DO for each vertex in the polygon Consider the line from the current vertex to the next vertex CALL 'WINDoW' to test if we can trivially reject the line IF line can be rejected THEN
IF parts are to be outside the clip area THEN
Result = current line F r.c~.
Result = null ELSE
CALL 'CLIP LINE TO AREA' to clip the line ENDIF
DO for each line in the result If this line is not flagqed as overlapping T~EN
~1~9-84-008 -- 1 - 1A~ 77 CALL 'ADD LINE TO PARTS LIST' ELSE
only process overlapping lines when building a list of parts that are inside the clip area IF parts are to be inside the clip area THEN
each overlapping line is further clipped to its containing area to split it up if it is intersected by its own boundary.
CALL 'CLIP LINE TO AREA' to produce the split result DO for each line in the split result CALL 'ADD LINE TO PARTS LIST' Delete the line from the split result ENDDO
ENDIF
ENDIF
ENDDO
. Delete the line from the result .
ENDDO
ENDDO
IF the first vertex in the first part is the same as the last vertex in the last part AND neither of the parts is flagged as overlapping THEN
CALL 'JOIN TWO PARTS' to join the first part to the last part ENDIF
Add Line To Parts List 52 -S4-008 - 13 - ~ 7 7 Add a single line to a parts list. Lines flagged as overlapping are held individually in the parts list. Other lines will be appended to the end of the current part if that is possible.
IF parts list is empty OR current part is flagged as overlapping OR line is f~agged as overlapping OR last vertex of current part is different from first vertex on line THEN
Start a new part Copy the first and last vertex of the line into the new part ELSE
Add the last vertex of the line to the current part ENDIF
Window 53 Thls procedure takes a llne and an area, and sets a flag to indicate whether the line is completely outside the incidence rectangle of the area. This enables the line to be trivially rejected from clipping.
A suitable technique can be found in (3) expand reference (3) Join Two Parts 54 This procedure takes two parts (partl and part2) and joins them together. It is assumed that the caller has determined that the last vertex in partl is the same as one of the end vertices of part2, i.e. that part2 can be joined to the end of partl.
~c3~ 0~ 9~ 7 ~
Part2 is removed from its containing list and appended to partl, if partl now defines a polygon, it is removed from its list and added to the Result area definition.
IF last vertex of partl = last vertex of part2 THEN
reverse the order of the vertices in part2 ENDIF
Delete the last ertex in partl , ~~ Move part2 onto the end pf partl IF the last vertex is the same as the first vertex in partl THEN
A polygon has been formed Remove partl from its list IF all lines in the polygon are flagged as overlapping THEN
This polygon is common to both areas, we must determine its sense Define a horizontal line that bisects the incidence retangle of the polygon, i.~. xmin,(ymax-ymin)~2 to xmin,(ymax-ymin)/2 '~''` "
CALL 'CLIP LINE TO AREA' to clip line inside of subject area CALL 'CLIP LINE TO AREA' to clip line inside of clip area compare these two results IF 1st point in each result is different OR only one of the results is null THEN
Polygons have different sense (i.e. one is a hole and the other is a solid) Move polygon to the result area definition ELSE
IK 4 ~ -OO'1 _ I c Delete pol~gon ELSE
~love polvgon to the result area definition ENDIF
ENDIF
Clip Line To Area 55 This procedure clips a single 'subject line' to the inside or the outside of an area. The result is a number of lines, represented by a ist containing an even number of ordered points. Each odd-even pair of points define the two end points of a line.
Determine the major axis of the line IF end point 1 > end point 2 along the major axis THEN
SW2p the lines two end points over remember that the line has been reversed ENDIF
CALL 'GET INTERSECTION LIST' to build a list of all intersections that the line (extended to infinity in both directions) makes with the area.
CALL 'PROCESS INTERSECTION LIST' to build the list of vertices representins the clipped line IF the original line was reversed THEN
swap the lines two end points over u~9~ -no8 - 16 Get Intersection List 56 Find all intersections of a line extended to infinity with an area boundary~
The intersection of the subject line with each boundary line is calculated, if the intersection is within the extent of the boundary line, it is added to the intersection list.
Boundary lines that are colinear to the subject line are added to a list of colinear boundary lines, this list is used by 'IDENTIFY OVERLAPPING LINES' to flag any parts of the clipped line that overlap a boundary line.
Once sorted, the intersection list defines each point where the extended subject line enters and leaves the area,
This invention relates to graphic display systems and in particular to such systems that include a pen plotter as an output device for pictures developed in the system.
An interactive graphic display system such as the chaxt utility of the IBM Graphics Display Data Manager (GDDM) which is described in EU patent publication 26266 gives a user the ability to create graphic pictures on a display screen such as an IBM 3279. (IBM is a Registered Trade Mark). Once created it is often desirable to have a hardcopy of the picture and pen plotters connected to the system are commonly used for this purpose.
Pen plotters provide a cheap, high quality graphics hard copy capability.
However the technology differs from that of a cathode ray tube display in one significant way, once something is drawn, it cannot be erased or changed. ~his difference presents a problem where, say, a red square is drawn over a larger blue square. On a display, the underlying blue pels are simply changed to red pels, but on a plotter, the red will be drawn ~ over the blue and a muddy brown colour will result. To provide quality output on plotters, it is therefor necessary to alter any underlying areas by clipping out any overlying areas. When the clipped underlying area is drawn and shaded, it will leave blank paper where the overlying areas will subsequently be drawn, resulting in a much more acceptable plot.
Various methods for area clipping have been published.
"Reentrant Polyson Clipping", Ivan E. Sutherland & Gary w Hodgman Comm. of ACM Jan 197~ V17 no. 1 p.32-42 describe a ~ 8~-00~ - 2 - ~ ; 7 simple method for clipping a concave polygon to a convex area.
This requires a concave clip area to be divided into convex polygons thus increasing the complexity of the calculations.
"~idden surface removal using polygon area sorting" ~evin Weiler 6 Peter Atherton Siggraph 77 proceedings Computer Graphics 11(2) Summer 77 p.214 describe another method for clipping concave polygons with holes, but this requires the polygons to be defined in a clockwise direction and holes in an anti-clockwise direction.
The method of the present invention places no restrictions on the area definitions. Both the subject and clip areas may consist of multiple, overlapping, self-intersecting polgyons. The inside of an area is defined as all points that would cross the area boundary an odd number of times if moved along a straight line to infinity.
It is an object of the invention to provide a method for a complex area definition to be clipped to another complex area definition.
The method also finds use with regard to quality plotting in:
o Implementing 'background' lines by treating each background line as a thin area of width twice that of the pen. These can then be clipped out of any parts of the picture that they overlay.
o When plotting on foils, a common problem is ink run, for example where two areas abut and are coloured differently, because the ink takes a long time to dry, the two colours blend into each other along the boundary. This can be solved by shrinking the areas bv one pen width. The easiest way to do this for an U~ 8~-008 - 3 - l~L~'7'7 arbitrary area definition is to define a thin area of twice the pen width that goes around the boundary of the area, and clip this out of the original area.
According to the present invention there is provided a method of determining the coincidence of a subject area and a clip area, in a graphic display system in which area definitions are stored in a display list store comprising the steps of:
- a) creating and storing a subject area parts list comprising separate entries for each part of one or more contiguous lines of the subject area that do not pass through the clip area b) creating and storing a clip area parts list comprising separate entries for each part of one or more contiguous lines of the clip area that pass through the subject area c) creating and storing an overlap parts list comprising separate entries for each part of one line of the subject and clip areas that overlap d) for each entry in the subject area parts list scanning unprocessed entries first in the subject area parts list secondly in the clip area parts list thirdly in the overlap parts list for an ent-y having an end point definition the same as one end point definition of the part being processed, creating a new part by joining any part found to the part being processed storing the created part in a results part list and reiterating the process for the entry in the results part list until no further parts are found whereby the results part list U~?-fi~ 7 ~
includes the definition of the po~tion of the subject area which does not coincide with tne clip area.
According to a second aspect of the invention there is provided a graphic display system, in which complex area definitions are stored in a display list buffer, including control means adapted to control the operation of the system . 10 to perform the method of determining the coincidence of a subject axea and a clip area comprising the steps of:
a) creating and storing a subject area parts list comprising separate entries for each part of one or more contiguous lines of the subject area that do not pas~ through the clip area b) creating and storing a clip area parts list comprising separate entries for each part of one or more contiguous lines of the clip area that pass through the subject area c) creating and storing an overlap parts list - comprising separate entries for each part of one line of the subject and clip areas that overlap d) for each entry in the suhject area parts list scanning unprocessed entries first in the subject area parts list secondly in the clip area parts list thirdly in the overlap parts list for an entry 3S having an end point definition the same as on end point definition of the part being processed, creating a new part by joining any part found to the part being processed storing the created part in a results part list and reiterating the process for the entry in the results part list until no further ~};q-~-008 c 1~ 7 7 parts ale found whereby the results part list includes the definition of the portion of the subject area which does not coincide with the clip area.
In order that the invention may be fully understood it will now be described with reference to the accompanying drawings in which:
Figs. 1-4 illustrate the operation of the method by way of example.
Fig. S illustrates the component procedures of the method.
Fig. 6 is a block schematic of a system in which the method can be implemented.
The method embodying the invention will clip a 'subject area' to a 'clip area' producing a result that is an area defining all parts of the subject area that are outside the clip area. If necessary, the method is easily changed to clip the other way around, i.e. to produce a definition of the parts of the subject area that are inside the clip area.
Each area definition is represented by a list of poly~on definitions, each polygon definition is represented by a ring of vertices. The resultant area definition is in the same format so that it can be further clipped to other areas if required.
The method involves building three lists. ~ne (the subject parts list) contains the parts of the subject area boundary that are outside the clip area. The second (the clip parts list) contains the parts of the clip area boundary that U~9-84-0~8 ~ ~ c3~r~-7 are inside the subject area, and the third all clip area bounda~y lines that cverlap s~bject area boundary lines (referl-ed to as: 'overlapping' clip par.s).
Each list is built up by considering each line in .he area and clipping it to the inside/outside of the other area.
This requires a line clipping routine that can clip an arbitrary line to an arbitrary area definition. In particular it must cope with lines that (i) ~ may intersect the area at a vertex, (ii) may overlap one (or more) of the area boundary lines; and areas that may consist of multiple overlapping, self-intersecting polygons. Such a routine is described below The resultant area definition is produced by joining the parts in the three lists together to form polygons.
The joining process proceeds by taking a subject part and attempting to join it, l~t : to a non-overlapping clip part, 2nd : to another subject part, and 3rd : to an overlapping clip part.
: - Thus the subject part is extended, and the total number of parts is reduced. ~hen a polygon is formed, it is removed from the subject parts list and added to the resultant area definition. This process continues.for each subject part until there are non left.
Finally, any remaining clip parts are joined (where possible) in an attempt to form more polygons. These polygons are added to the resultant area definition unless they were formed only from overlapping clip parts. In this case, it is determined whether this polygon (which is common to both subject and clip areas) has the same sence (hole or solid) in both the subject and clip areas. The polygon is only added to ~J~;9-84-OOd - ~ 9377 the result if its sense is different in the subject and clip areas.
In the rare situation of clipping to a self intersecting area with many bounda_y lines that are common or overlap the subject boundary, it is possible that after joining various parts, a dead end is reached, i.e. no part can be found that will join. This situation may occur when there are two or more overlapping clip parts that can be joined to the subject part, and the wrong one happens to be chosen. This occurs very infrequently and the simplest solution is to shift the subject area by a tiny amount in the x and y direction, then restart the clipping process.
Figures 1 to 4 illustrate the method by way of example.
A complex polygon such as shown in 1 (a) is displayed on a display screen. This may be part of a graph for example. A
second area shown at 1 (b) is to be overlaid on the area of 1 (a) to give the result shown in Fig. 1 (c). It is then necessary to clip the shaded area of 1 (b) from the area of 1 (a) - Figures 2a and 2b illustrate the contents of a subject parts list.
Figures 3a and 3b illustrate the contents of a clip parts list.
Figure 4a and 4b illustrate the contents of an overlap parts li~t.
Of course all lines and parts are actually stored in the form of end point coordinates and the drawings are used for illustration only.
U~ 4-008 - ~ ?~r7 ~
In general te~s the first step is for all lines of the subiect area to be examined to determine whether or not they pass through the clip area. Starting at the top left corner line 10 has a portion 11 which overlaps the ciip area. This is added to the overlap parts list (Fig. 4a). The second portion 12 is added to the subject parts list Fig. 2.
The lines 13, 14, 15 and 16 are then examined and are added to the subject parts list. As each has a common coordinate with the previous line a complex part is developed.
The line 18 has a portion 19 which passes through the clip area and portions 17 and 20 which do not. The portion 17 is added to the complex part 2 (a) in the subject parts list, the portion 19 is ignored and the portion 20 starts a new part in the subject parts list. The line 21 is added to the part 20 in the subject parts list and the line 22 is added to the overlap parts list (4 (b).
The clip area i~ then examined for lines that pass through or are contained in the subject area. The line 11 is a keady in the overlap parts list. Line 23 has two parts portion 24 is added to the clip parts list and portion 25 is ignored as is portion 27 of line 26. However portion 28 and portion 29 of line 30 are added to the clip parts list (3 (b~). Portion 22 of line 29 is already in the overlay parts li t.
In order to build the subject area minus the clip area, the first entry in the subject parts list is examined and then in the following order remaining entries in the clip parts list and then the overlap parts list are examined to determine the first that has a common end point coordinate with the first entry. This part found is then added to the subject part and the result placed in a result parts list. The process is reiterated using the entry in the results parts U~9-8~-008 - ? - 1~13~ ~ ~
li~t until no parts having a common end point with the resultant part is found. In all cases the result sha ~ be a closed polygon. The next entry in the subject parts list is then examined and the processes repeated until there are no further parts in the subject parts list. The result parts list will then contain a definition of the subject area minus the clip ar~a.
r- 10 There now follows a detailed description of the method.
The method has been divided into small procedures and Fig. 5 shows how they are related.
Area Clip 50 (Fig. 5) This procedure controls the whole of the area clipping process. The 'subject area' is clipped to the 'clip area'.
The result is a new subject area that defines all those parts of the original subject area that are outside of the clip area.
Call 'BUILD PARTS LIST' to bulld the sub~ect parts 11st whlch ; - contains all parts of the 'subject area' that are outside the 'clip area .
Call 'BUILD PARTS LIST' to build the clip parts list which contains all parts of the 'clip area' that are inside the 'clip area'.
This li s~ will also contain 'overlapping clip parts'.
Scan both parts lists and move any parts that are polygons to the result area.
SCAN:
~9~ 008 - ]0 D0 for each subj~ct part ti~ILE subject parts list is not emDty IF there is a non-overlapping clip part that will join THE~
CALL 'JOIN TWO PARTS' ITERATE the SC~N loop ENDIF
IF there is a subject part that will join THEN
CALL 'JOIN TWO PARTS' ITERATE the SCAN loop ENDIF
IF there is an overlapping clip part that will join THEN
CALL ' JOIN TWO PARTS' ITEP~ATE the SC~ loop ENDIF
If this point is reached, we took a wrong turning further 'oack which has lead us to a dead end.
Scan the subject area definition and shift it by one unit RESTART the clipping procedure.
ENDDO
The subject 7ist is now empty, try to construct re polygons from any remaining clip parts.
SCAN2:
DO WHILE clip parts list is not empty Take the 1st clip part DO for each subsequent clip part IF t will join to the first clip part THEN
CALL 'JOIN TWO PARTS' ITERATE the SCAN2 loop ENDIP
1'~19377 -008 - ll -ENDDC) Delete the 1st clip part, it does not form a polygon E~DDO
Clipping is now ccmplete Delete the original subject area and replace with the result area.
Build Parts List (51) This procedure is passed two area definitions, the subject area is to be divided into parts that are either inside or outside of the clip area. Note that the terms 's~bject area' ~ 'clip area' are with respect to the processing done by this procedure, and are independent of the same terms used in the AREA CIIP procedure.
DO for each polygon in the subject area definition DO for each vertex in the polygon Consider the line from the current vertex to the next vertex CALL 'WINDoW' to test if we can trivially reject the line IF line can be rejected THEN
IF parts are to be outside the clip area THEN
Result = current line F r.c~.
Result = null ELSE
CALL 'CLIP LINE TO AREA' to clip the line ENDIF
DO for each line in the result If this line is not flagqed as overlapping T~EN
~1~9-84-008 -- 1 - 1A~ 77 CALL 'ADD LINE TO PARTS LIST' ELSE
only process overlapping lines when building a list of parts that are inside the clip area IF parts are to be inside the clip area THEN
each overlapping line is further clipped to its containing area to split it up if it is intersected by its own boundary.
CALL 'CLIP LINE TO AREA' to produce the split result DO for each line in the split result CALL 'ADD LINE TO PARTS LIST' Delete the line from the split result ENDDO
ENDIF
ENDIF
ENDDO
. Delete the line from the result .
ENDDO
ENDDO
IF the first vertex in the first part is the same as the last vertex in the last part AND neither of the parts is flagged as overlapping THEN
CALL 'JOIN TWO PARTS' to join the first part to the last part ENDIF
Add Line To Parts List 52 -S4-008 - 13 - ~ 7 7 Add a single line to a parts list. Lines flagged as overlapping are held individually in the parts list. Other lines will be appended to the end of the current part if that is possible.
IF parts list is empty OR current part is flagged as overlapping OR line is f~agged as overlapping OR last vertex of current part is different from first vertex on line THEN
Start a new part Copy the first and last vertex of the line into the new part ELSE
Add the last vertex of the line to the current part ENDIF
Window 53 Thls procedure takes a llne and an area, and sets a flag to indicate whether the line is completely outside the incidence rectangle of the area. This enables the line to be trivially rejected from clipping.
A suitable technique can be found in (3) expand reference (3) Join Two Parts 54 This procedure takes two parts (partl and part2) and joins them together. It is assumed that the caller has determined that the last vertex in partl is the same as one of the end vertices of part2, i.e. that part2 can be joined to the end of partl.
~c3~ 0~ 9~ 7 ~
Part2 is removed from its containing list and appended to partl, if partl now defines a polygon, it is removed from its list and added to the Result area definition.
IF last vertex of partl = last vertex of part2 THEN
reverse the order of the vertices in part2 ENDIF
Delete the last ertex in partl , ~~ Move part2 onto the end pf partl IF the last vertex is the same as the first vertex in partl THEN
A polygon has been formed Remove partl from its list IF all lines in the polygon are flagged as overlapping THEN
This polygon is common to both areas, we must determine its sense Define a horizontal line that bisects the incidence retangle of the polygon, i.~. xmin,(ymax-ymin)~2 to xmin,(ymax-ymin)/2 '~''` "
CALL 'CLIP LINE TO AREA' to clip line inside of subject area CALL 'CLIP LINE TO AREA' to clip line inside of clip area compare these two results IF 1st point in each result is different OR only one of the results is null THEN
Polygons have different sense (i.e. one is a hole and the other is a solid) Move polygon to the result area definition ELSE
IK 4 ~ -OO'1 _ I c Delete pol~gon ELSE
~love polvgon to the result area definition ENDIF
ENDIF
Clip Line To Area 55 This procedure clips a single 'subject line' to the inside or the outside of an area. The result is a number of lines, represented by a ist containing an even number of ordered points. Each odd-even pair of points define the two end points of a line.
Determine the major axis of the line IF end point 1 > end point 2 along the major axis THEN
SW2p the lines two end points over remember that the line has been reversed ENDIF
CALL 'GET INTERSECTION LIST' to build a list of all intersections that the line (extended to infinity in both directions) makes with the area.
CALL 'PROCESS INTERSECTION LIST' to build the list of vertices representins the clipped line IF the original line was reversed THEN
swap the lines two end points over u~9~ -no8 - 16 Get Intersection List 56 Find all intersections of a line extended to infinity with an area boundary~
The intersection of the subject line with each boundary line is calculated, if the intersection is within the extent of the boundary line, it is added to the intersection list.
Boundary lines that are colinear to the subject line are added to a list of colinear boundary lines, this list is used by 'IDENTIFY OVERLAPPING LINES' to flag any parts of the clipped line that overlap a boundary line.
Once sorted, the intersection list defines each point where the extended subject line enters and leaves the area,
2~ thus when scanning through the list, lines from odd to even nurnbered intersections are within the area, and lines from even to odd numbered intersection are outside the area.
It is important to ensure that the correct nurrlber of --. intersections is added to the list, when the subject line intersects a vertex or is colinear to a boundary line, see figure 4. This is handled by the routines 'PROCESS PARALLEL
LINE' and 'ADD AN INTERSECTION'.
DO for each polygon in the area DO for each vertex in the polygon
It is important to ensure that the correct nurrlber of --. intersections is added to the list, when the subject line intersects a vertex or is colinear to a boundary line, see figure 4. This is handled by the routines 'PROCESS PARALLEL
LINE' and 'ADD AN INTERSECTION'.
DO for each polygon in the area DO for each vertex in the polygon
3 Consider the boundary line pl-p2 'rom the current vertex to the next vertex If boundary line is parallel to the subject line THE~
CALL 'PROCESS PARALLEL LINE' ELSE
IF intersection is within extent of the boundary UK9~ 00~ - 17 1~19~'7~
line THEN
CALL 'ADD AN INTERSECTION' ENDIF
ENDIF
ENDDO
ENDDO
Process Intersectic,n List 57 The intersection list is sorted and scanned to deter~ine which intersections are required to ~ake up the resultant clipped line definition. Intersections outside the extent of the subject line are removed.
Sort the intersection list along the major axis.
Delete and count the intersections that are before or at the first end point of the subject line.
If the count is odd, the 1st end point of the subject - line is inside the area, otherwise it is out~ide.
( Add the 1st end point of the subject line to the start of the intersection list according to the following table:
Lines 1st end point Type of clipping Add 1st point to list?
inside area inside yes inside area outside no outside area inside no out~ide area outside yes Scan and count the intersection list to find the las.
intersection that is before the subject lines 2nd end U~9-8~-00~ - 1i3 1~1<3;~';"7 point.
IF the count is odd THEN
Add the subject lines 2nd end point to the end of the intersec~ion list ENDIF
Delete any remaining intersections after the 2nd end point.
IF the subject line was reversed by 'GET INTERSECTION
LIST' THEN
reverse the order of the intersections.
ENDIF
The intersection list now defines the clipped subject line CALL 'IDENTIFY OVERLAPPING LINES' to flag any portions of the result that over ap an area boundary line -Identify Overlapping Lines 58 Lines and gaps in the resultant clip line definition are compared with the boundary lines in the colinear boundary lines list. Any lines that overlap with a boundary line are f agged as over apping. Also any gaps that overlap with a boundary line are added to the clipped line de finition as over apping lines-DO for each odd numbered point in the clipped line definition IF line from current point to next point overlaps one of the boundary lines in the colinear boundary lines J~9~ 0~ 3~; ;' list THEN
flag this line as overlapping ENDIF
IF line from next point to the next point after overlaps THEN
Add these two points after the current point to make the gap into a line flag this new line as overlapping END
ENDDO
IF the ~st end point of the subject line is not in the clipped result THEN
IF line from 1st end point of the subject line to 1st point in the clipped line is overlapping THEN
Add these two points after the current point to ma~e the gap into a line flag this new line as overlapping ENDIF
-. ENDIF
( IF the 2nd end point of the subject line is not in the clipped result THEN
IF line from the last point in the clipped result to the 2nd end point of the subject line is overlapping THEN
Add these two points after the current point to make the gap into a line flag this new line as overlapping ENDIF
ENDIF
Add An Intersection 59 ~l~?~ ( n8 - ~o - 1~1~3~77 This procedure determines whether to add none, one or two intersections to the intersection list. If the intersection occurs at Pl, it is not necessary to add it, because that particular intersection will have been processed as P2 for the previous line. If the line intersects at P2, we must determine if the subject line is tangential to the area, if it is, two intersections at P2 are added, otherwise one intersection at P2 is added. If the intersection is not at Pl or P2, the actua~ intersection is calculated and added to the intersection list.
IF intersection is at Pl THEN
do nothing ELSE
IF intersection is at P2 THEN
Consider line from P1 to P3 (the next vertex after IF subject line intersects line P1 - P3 within the extent of Pl - P3 THEN
The subject line is not tangential to the area Add point P2 to the intersection li~^t twice ( ENDIF
ELSE
Calculate the coordinates of the intersection Add the intersection point to the intersection list ENDIF
ENDIF
Process Parallel Line 60 This procedure processes a subject line that is parallel ~1~9-8~-~0~ - 2l ~ L~ 77 to the current boundary line Pl - P2. If the two lines are not colinear, no intersections are added. If they are colinear, si~ilar processins to that in 'ADD AN INTERSECTION' is done to deter~ine if the subject line is tangential to tr.e area or not.
Determine the minor axis of boundary line Pl - P2 Rotate the line by adding l to Pl and subtracting l from P2 along the minor axis.
IF the su~ject line intersects the rotated boundary line at the centre THEN
The ines are colinear Consider the line from PO (previous vertext to Pl) and (next vertex after P2) 2~
IF subject line intersects line PO - P3 w $hin the of PO - P3 THEN
The subject line is not tansential to the area Add point P2 to t~e intersection list ELSE
The subject line is tangential to the area O Add point P2 to the intersection li t twice ENDIF
ENDIF
Implementation Notes Line intersection calculation The following technicue is used to calculate the intersection of two lines. Fixecl point arithmetic can be used 8~-00~ 9 ~ ~ ~
throughout, thus avoiding rounding errors. Also the position of the intersection, relative to either O.L the lines can found by co~paring the top and bottom of the fraction forming At or Bt without having to calculate the actua intersection.
Assume two lines A and B defined by their end points Axl,Ayl Ax2,Ay2 and Bxl,Byl Bx2,By2 Aa = Ayl-Ay2 Ba = Byl-By2 Ab = Ax2-Axl Bb = Bx2-Bxl bot = (Ab*Ba) - (Aa*Bb) if bot=O, the two lines are parallel Atop = Bb*(Byl-Ayl) + Ab (Byl-Ayl) At = Atop ~ bot If 0<=At<=l then intersection ls within the extent of line A
If At<0 int. is on the side of point Axl,Ayl If At>l int. is on the side of point Ax2,Ay2 !
Btop = Aa*(Bxl-Axl) Bt = Btop / bot If O<=Bt<=l then intersection is within the extent of line B
If Bt<0 int. is on the side of point Bxl,Byl If Bt>l int. is on the side of point 8x2,By2 The intersection point Ix,Iy can be calcu ated using Bt or At Ix = Ab*At + Axl = Bb*Bt ~ Bxl Iy = -Aa*At + Ayl = -Ba*Bt + Byl ['~9-84-008 - ~3 1~'3~ 7 Data st uctures Area definitions, are represented by a simple data structure consisting of a list of polygons, each polygon is represented as a ring of points (vertices).
¢ .
'3 -- 2 ~1 --Area Definition . _ . ~
¦ pap ¦area a.tributes , pap - polygon anchor pointer 5 ~ / List of polygons ¦ nxtpl prvp ¦ i ¦ A nxtp - ne~t pointer , 10 V ¦ prvp - previous pointer ....
¦ A
V ¦ POLYGON DEFINITION
¦ vap ¦ poly. attribute ~
vap - vertex anchor pointer .. ~
V ~ ~ ring of points ¦ nxtp ¦ prvp¦
¦ A
v 30 1 ....
I
POINT DEFINITION
l _ point attributes I
~ 008 - ~ 7 ~
Area attributes will include all information pertaining to the area, e.g.
The areas incidence rectangle (min & max ~ & Y~values) Colour Shading pattern Polygon attributes may include:
The polygons incidence rectangle Vertex count.
Point attributes include the co-ordinate of the point, and attri~utes pertaining to the line from this point to the next point. e.g.:
X co-ordinate Y co-ordinate colour linetype overlapping flag.
A line definition is simply two or more point definitions chained together, thus a single boundary line is passed to the clipping routine simply by passing the current polygon vertex.
30 The result of the clipping is a list of point definitions, lines exist between each odd numbered point and its next point, gaps exist between each even numbered point and its next point.
The method described above may be implemented on a system such as is illustrated schematically in Fig. 6. A host central processing unit 61 has a connection to a display control unit 62. The display control unit 62 controls the display and display screen 63 and receives input signals from 40 a Keyboard 64 and an cursor control device 65 which may be a ~ R~_OO~ 7 ~
light pen, a tablet or an optical mouse. Hardcopy of the display pictures is provided by a plotter 66 also controlled by the display control unit. A graphic picture may be developed by a user interactively on the display screen and stored for later use in the host central processing unit.
Alternatively a hardcopy may be produced by the plotter 66, it is then that the process described above is used.
The control mechanism for controlling the operation of the system to perform the process may be a control routine stored in the central process unit or may be a control routine store on a diskette and loaded into the random access store of the display control unit. A further embodiment includes a programmable read only memory which is included in the display control unit and storing the routines to perform the process.
All such possible mechanisms are contemplated to fall within the scope of the following claims.
_,~
CALL 'PROCESS PARALLEL LINE' ELSE
IF intersection is within extent of the boundary UK9~ 00~ - 17 1~19~'7~
line THEN
CALL 'ADD AN INTERSECTION' ENDIF
ENDIF
ENDDO
ENDDO
Process Intersectic,n List 57 The intersection list is sorted and scanned to deter~ine which intersections are required to ~ake up the resultant clipped line definition. Intersections outside the extent of the subject line are removed.
Sort the intersection list along the major axis.
Delete and count the intersections that are before or at the first end point of the subject line.
If the count is odd, the 1st end point of the subject - line is inside the area, otherwise it is out~ide.
( Add the 1st end point of the subject line to the start of the intersection list according to the following table:
Lines 1st end point Type of clipping Add 1st point to list?
inside area inside yes inside area outside no outside area inside no out~ide area outside yes Scan and count the intersection list to find the las.
intersection that is before the subject lines 2nd end U~9-8~-00~ - 1i3 1~1<3;~';"7 point.
IF the count is odd THEN
Add the subject lines 2nd end point to the end of the intersec~ion list ENDIF
Delete any remaining intersections after the 2nd end point.
IF the subject line was reversed by 'GET INTERSECTION
LIST' THEN
reverse the order of the intersections.
ENDIF
The intersection list now defines the clipped subject line CALL 'IDENTIFY OVERLAPPING LINES' to flag any portions of the result that over ap an area boundary line -Identify Overlapping Lines 58 Lines and gaps in the resultant clip line definition are compared with the boundary lines in the colinear boundary lines list. Any lines that overlap with a boundary line are f agged as over apping. Also any gaps that overlap with a boundary line are added to the clipped line de finition as over apping lines-DO for each odd numbered point in the clipped line definition IF line from current point to next point overlaps one of the boundary lines in the colinear boundary lines J~9~ 0~ 3~; ;' list THEN
flag this line as overlapping ENDIF
IF line from next point to the next point after overlaps THEN
Add these two points after the current point to make the gap into a line flag this new line as overlapping END
ENDDO
IF the ~st end point of the subject line is not in the clipped result THEN
IF line from 1st end point of the subject line to 1st point in the clipped line is overlapping THEN
Add these two points after the current point to ma~e the gap into a line flag this new line as overlapping ENDIF
-. ENDIF
( IF the 2nd end point of the subject line is not in the clipped result THEN
IF line from the last point in the clipped result to the 2nd end point of the subject line is overlapping THEN
Add these two points after the current point to make the gap into a line flag this new line as overlapping ENDIF
ENDIF
Add An Intersection 59 ~l~?~ ( n8 - ~o - 1~1~3~77 This procedure determines whether to add none, one or two intersections to the intersection list. If the intersection occurs at Pl, it is not necessary to add it, because that particular intersection will have been processed as P2 for the previous line. If the line intersects at P2, we must determine if the subject line is tangential to the area, if it is, two intersections at P2 are added, otherwise one intersection at P2 is added. If the intersection is not at Pl or P2, the actua~ intersection is calculated and added to the intersection list.
IF intersection is at Pl THEN
do nothing ELSE
IF intersection is at P2 THEN
Consider line from P1 to P3 (the next vertex after IF subject line intersects line P1 - P3 within the extent of Pl - P3 THEN
The subject line is not tangential to the area Add point P2 to the intersection li~^t twice ( ENDIF
ELSE
Calculate the coordinates of the intersection Add the intersection point to the intersection list ENDIF
ENDIF
Process Parallel Line 60 This procedure processes a subject line that is parallel ~1~9-8~-~0~ - 2l ~ L~ 77 to the current boundary line Pl - P2. If the two lines are not colinear, no intersections are added. If they are colinear, si~ilar processins to that in 'ADD AN INTERSECTION' is done to deter~ine if the subject line is tangential to tr.e area or not.
Determine the minor axis of boundary line Pl - P2 Rotate the line by adding l to Pl and subtracting l from P2 along the minor axis.
IF the su~ject line intersects the rotated boundary line at the centre THEN
The ines are colinear Consider the line from PO (previous vertext to Pl) and (next vertex after P2) 2~
IF subject line intersects line PO - P3 w $hin the of PO - P3 THEN
The subject line is not tansential to the area Add point P2 to t~e intersection list ELSE
The subject line is tangential to the area O Add point P2 to the intersection li t twice ENDIF
ENDIF
Implementation Notes Line intersection calculation The following technicue is used to calculate the intersection of two lines. Fixecl point arithmetic can be used 8~-00~ 9 ~ ~ ~
throughout, thus avoiding rounding errors. Also the position of the intersection, relative to either O.L the lines can found by co~paring the top and bottom of the fraction forming At or Bt without having to calculate the actua intersection.
Assume two lines A and B defined by their end points Axl,Ayl Ax2,Ay2 and Bxl,Byl Bx2,By2 Aa = Ayl-Ay2 Ba = Byl-By2 Ab = Ax2-Axl Bb = Bx2-Bxl bot = (Ab*Ba) - (Aa*Bb) if bot=O, the two lines are parallel Atop = Bb*(Byl-Ayl) + Ab (Byl-Ayl) At = Atop ~ bot If 0<=At<=l then intersection ls within the extent of line A
If At<0 int. is on the side of point Axl,Ayl If At>l int. is on the side of point Ax2,Ay2 !
Btop = Aa*(Bxl-Axl) Bt = Btop / bot If O<=Bt<=l then intersection is within the extent of line B
If Bt<0 int. is on the side of point Bxl,Byl If Bt>l int. is on the side of point 8x2,By2 The intersection point Ix,Iy can be calcu ated using Bt or At Ix = Ab*At + Axl = Bb*Bt ~ Bxl Iy = -Aa*At + Ayl = -Ba*Bt + Byl ['~9-84-008 - ~3 1~'3~ 7 Data st uctures Area definitions, are represented by a simple data structure consisting of a list of polygons, each polygon is represented as a ring of points (vertices).
¢ .
'3 -- 2 ~1 --Area Definition . _ . ~
¦ pap ¦area a.tributes , pap - polygon anchor pointer 5 ~ / List of polygons ¦ nxtpl prvp ¦ i ¦ A nxtp - ne~t pointer , 10 V ¦ prvp - previous pointer ....
¦ A
V ¦ POLYGON DEFINITION
¦ vap ¦ poly. attribute ~
vap - vertex anchor pointer .. ~
V ~ ~ ring of points ¦ nxtp ¦ prvp¦
¦ A
v 30 1 ....
I
POINT DEFINITION
l _ point attributes I
~ 008 - ~ 7 ~
Area attributes will include all information pertaining to the area, e.g.
The areas incidence rectangle (min & max ~ & Y~values) Colour Shading pattern Polygon attributes may include:
The polygons incidence rectangle Vertex count.
Point attributes include the co-ordinate of the point, and attri~utes pertaining to the line from this point to the next point. e.g.:
X co-ordinate Y co-ordinate colour linetype overlapping flag.
A line definition is simply two or more point definitions chained together, thus a single boundary line is passed to the clipping routine simply by passing the current polygon vertex.
30 The result of the clipping is a list of point definitions, lines exist between each odd numbered point and its next point, gaps exist between each even numbered point and its next point.
The method described above may be implemented on a system such as is illustrated schematically in Fig. 6. A host central processing unit 61 has a connection to a display control unit 62. The display control unit 62 controls the display and display screen 63 and receives input signals from 40 a Keyboard 64 and an cursor control device 65 which may be a ~ R~_OO~ 7 ~
light pen, a tablet or an optical mouse. Hardcopy of the display pictures is provided by a plotter 66 also controlled by the display control unit. A graphic picture may be developed by a user interactively on the display screen and stored for later use in the host central processing unit.
Alternatively a hardcopy may be produced by the plotter 66, it is then that the process described above is used.
The control mechanism for controlling the operation of the system to perform the process may be a control routine stored in the central process unit or may be a control routine store on a diskette and loaded into the random access store of the display control unit. A further embodiment includes a programmable read only memory which is included in the display control unit and storing the routines to perform the process.
All such possible mechanisms are contemplated to fall within the scope of the following claims.
_,~
Claims (6)
1. A method of determining the coincidence of a subject area and a clip area, in a graphic display system in which area definitions are stored in a display list store comprising the steps of:
a) creating and storing a subject area parts list comprising separate entries for each part of one or more contiguous lines of the subject area that do not pass through the clip area b) creating and storing a clip area parts list comprising separate entries for each part of one or more contiguous lines of the clip area that pass through the subject area c) creating and storing an overlap parts list comprising separate entries for each part of one line of the subject and clip areas that overlap d) for each entry in the subject area parts list scanning unprocessed entries first in the subject area parts list secondly in the clip area parts list thirdly in the overlap parts list for an entry having an end point definition the same as on end point definition of the part being processed, creating a new part by joining any part found to the part being processed storing the created part in a results part list and reiterating the process for the entry in the results part list until no further parts are found whereby the results part list includes the definition of the portion of the subject area which does not coincide with the clip area.
a) creating and storing a subject area parts list comprising separate entries for each part of one or more contiguous lines of the subject area that do not pass through the clip area b) creating and storing a clip area parts list comprising separate entries for each part of one or more contiguous lines of the clip area that pass through the subject area c) creating and storing an overlap parts list comprising separate entries for each part of one line of the subject and clip areas that overlap d) for each entry in the subject area parts list scanning unprocessed entries first in the subject area parts list secondly in the clip area parts list thirdly in the overlap parts list for an entry having an end point definition the same as on end point definition of the part being processed, creating a new part by joining any part found to the part being processed storing the created part in a results part list and reiterating the process for the entry in the results part list until no further parts are found whereby the results part list includes the definition of the portion of the subject area which does not coincide with the clip area.
2. A method as claimed in claim 1 in which the graphic display system includes an output pen plotter and the result parts list is used to control the plotter when the subject area is drawn.
3. A graphic display system, in which complex area definitions are stored in a display list buffer, including control means adapted to control the operation of the system to perform the method of determining the coincidence of a subject area and a clip area comprising the steps of:
a) creating and storing a subject area parts list comprising separate entries for each part of one or more contiguous lines of the subject area that do not pass through the clip area b) creating and storing a clip area parts list comprising separate entries for each part of one or more contiguous lines of the clip area that pass through the subject area c) creating and storing an overlap parts list comprising separate entries for each part of one line of the subject and clip areas that overlap d) for each entry in the subject area parts list scanning unprocessed entries first in the subject area parts list secondly in the clip area parts list thirdly in the overlap parts list for an entry having an end point definition the same as on end point definition of the part being processed, creating a new part by joining any part found to the part being processed storing the created part in a results part list and reiterating the process for the entry in the results part list until no further parts are found whereby the results part list includes the definition of the portion of the subject area which does not coincide with the clip area.
a) creating and storing a subject area parts list comprising separate entries for each part of one or more contiguous lines of the subject area that do not pass through the clip area b) creating and storing a clip area parts list comprising separate entries for each part of one or more contiguous lines of the clip area that pass through the subject area c) creating and storing an overlap parts list comprising separate entries for each part of one line of the subject and clip areas that overlap d) for each entry in the subject area parts list scanning unprocessed entries first in the subject area parts list secondly in the clip area parts list thirdly in the overlap parts list for an entry having an end point definition the same as on end point definition of the part being processed, creating a new part by joining any part found to the part being processed storing the created part in a results part list and reiterating the process for the entry in the results part list until no further parts are found whereby the results part list includes the definition of the portion of the subject area which does not coincide with the clip area.
4. A graphic display system as claimed in claim 3 including an output pen plotter and in which the results part list is used to control the plotter when the subject area is drawn.
5. In a graphic display system a process for determining the coincidence of complex areas comprising for each subject area and clip area the steps of:
a) creating and storing a subject area parts list comprising separate entries for each part of one or more contiguous lines of the subject area that do not pass through the clip area b) creating and storing a clip area parts list comprising separate entries for each part of one or more contiguous lines of the clip area that pass through the subject area c) creating and storing an overlap parts list comprising separate entries for each part of one line of the subject and clip areas that overlap d) for each entry in the subject area parts list scanning unprocessed entries first in the subject area parts list secondly in the clip area parts list thirdly in the overlap parts list for an entry having an end point definition the same as on end point definition of the part being processed, creating a new part by joining any part found to the part being processed storing the created part in a results part list and reiterating the process for the entry in the results part list until no further parts are found whereby the results part list includes the definition of the portion of the subject area which does not coincide with the clip area.
a) creating and storing a subject area parts list comprising separate entries for each part of one or more contiguous lines of the subject area that do not pass through the clip area b) creating and storing a clip area parts list comprising separate entries for each part of one or more contiguous lines of the clip area that pass through the subject area c) creating and storing an overlap parts list comprising separate entries for each part of one line of the subject and clip areas that overlap d) for each entry in the subject area parts list scanning unprocessed entries first in the subject area parts list secondly in the clip area parts list thirdly in the overlap parts list for an entry having an end point definition the same as on end point definition of the part being processed, creating a new part by joining any part found to the part being processed storing the created part in a results part list and reiterating the process for the entry in the results part list until no further parts are found whereby the results part list includes the definition of the portion of the subject area which does not coincide with the clip area.
6. A system as claimed in claim 5 including an output pen plotter and in which the results parts list is used to control the plotter when the subject area is drawn.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US8411579 | 1984-05-05 | ||
| GB848411579A GB8411579D0 (en) | 1984-05-05 | 1984-05-05 | Graphic display systems |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1219377A true CA1219377A (en) | 1987-03-17 |
Family
ID=10560553
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000477995A Expired CA1219377A (en) | 1984-05-05 | 1985-03-29 | Graphic display systems |
Country Status (6)
| Country | Link |
|---|---|
| US (1) | US4698779A (en) |
| EP (1) | EP0163852B1 (en) |
| JP (1) | JPS60239825A (en) |
| CA (1) | CA1219377A (en) |
| DE (1) | DE3587129T2 (en) |
| GB (1) | GB8411579D0 (en) |
Families Citing this family (32)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4853971A (en) * | 1985-03-18 | 1989-08-01 | Dainippon Screen Mfg. Co., Ltd. | Method and apparatus for processing image data |
| FR2586838B1 (en) * | 1985-08-30 | 1989-07-28 | Labo Electronique Physique | HIDDEN FACES PROCESSOR FOR SYNTHESIS OF THREE-DIMENSIONAL IMAGES |
| US5003498A (en) * | 1986-01-13 | 1991-03-26 | Hitachi, Ltd. | Graphic display method |
| US4829446A (en) * | 1986-12-12 | 1989-05-09 | Caeco, Inc. | Method and apparatus for recording and rearranging representations of objects in a model of a group of objects located using a co-ordinate system |
| JP2777628B2 (en) * | 1987-06-15 | 1998-07-23 | 株式会社日立製作所 | Graphic processing method and apparatus |
| US4958305A (en) * | 1987-11-04 | 1990-09-18 | General Electric Company | Polygon edge clipping |
| US4888712A (en) * | 1987-11-04 | 1989-12-19 | Schlumberger Systems, Inc. | Guardband clipping method and apparatus for 3-D graphics display system |
| JPH0187463U (en) * | 1987-12-02 | 1989-06-09 | ||
| US5040130A (en) * | 1988-09-20 | 1991-08-13 | International Business Machines Corporation | Computer graphics boundary--defined area clippping and extraneous edge deletion method |
| US4980843A (en) * | 1988-12-02 | 1990-12-25 | Spectra-Physics | Ink saving method for plotters |
| EP0372810A3 (en) * | 1988-12-02 | 1991-07-10 | Spectra-Physics, Inc. | Inkjet printers |
| US5247609A (en) * | 1988-12-08 | 1993-09-21 | Thermo Separation Products (California) Inc. | Line density control for plotters |
| DE69033514D1 (en) * | 1989-02-08 | 2000-05-31 | Canon Kk | Figure processing device |
| US5060170A (en) * | 1989-08-09 | 1991-10-22 | International Business Machines Corp. | Space allocation and positioning method for screen display regions in a variable windowing system |
| US5146554A (en) * | 1989-09-29 | 1992-09-08 | Eastman Kodak Company | Page memory control in a raster image processor employed for digital halftoning |
| US5131049A (en) * | 1989-12-08 | 1992-07-14 | Xerox Corporation | Identification, characterization, and segmentation of halftone or stippled regions of binary images by growing a seed to a clipping mask |
| US5231695A (en) * | 1990-08-29 | 1993-07-27 | Xerox Corporation | Generalized clipping in an extended frame buffer |
| EP0521210B1 (en) * | 1991-07-05 | 2000-03-22 | International Business Machines Corporation | Graphics clipping |
| DE69132007T2 (en) * | 1991-07-05 | 2000-09-14 | Ibm | Graphic processing device, method and computer program |
| US6005574A (en) * | 1991-07-05 | 1999-12-21 | International Business Machines Corporation | Data processing system including method and apparatus for performing graphics clipping operations |
| KR940003044B1 (en) * | 1991-09-13 | 1994-04-11 | 삼성전자 주식회사 | Video signal mixing method & apparatus |
| US5563990A (en) * | 1992-10-30 | 1996-10-08 | International Business Machines Corporation | Method and apparatus for processing a pick event |
| JP2607416B2 (en) * | 1993-01-14 | 1997-05-07 | 株式会社日立製作所 | Shape selection display method |
| US5668931A (en) * | 1993-03-31 | 1997-09-16 | Dermer; Richard A. | Method for automatic trap selection for correcting for plate misregistration in color printing |
| US5613046A (en) * | 1993-03-31 | 1997-03-18 | Miles Inc. | Method and apparatus for correcting for plate misregistration in color printing |
| US5313570A (en) * | 1993-03-31 | 1994-05-17 | Miles, Inc. | Method for determining color boundaries for correcting for plate misregistration in color printing |
| US5613052A (en) * | 1993-09-02 | 1997-03-18 | International Business Machines Corporation | Method and apparatus for clipping and determining color factors for polygons |
| US5546524A (en) * | 1993-12-15 | 1996-08-13 | International Business Machines Corp. | Method and apparatus for interlocking graphical objects |
| US5757963A (en) * | 1994-09-30 | 1998-05-26 | Xerox Corporation | Method and apparatus for complex column segmentation by major white region pattern matching |
| US5699453A (en) * | 1994-09-30 | 1997-12-16 | Xerox Corporation | Method and apparatus for logically tagging of document elements in the column by major white region pattern matching |
| US5555556A (en) * | 1994-09-30 | 1996-09-10 | Xerox Corporation | Method and apparatus for document segmentation by background analysis |
| US5574802A (en) * | 1994-09-30 | 1996-11-12 | Xerox Corporation | Method and apparatus for document element classification by analysis of major white region geometry |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3889107A (en) * | 1972-10-16 | 1975-06-10 | Evans & Sutherland Computer Co | System of polygon sorting by dissection |
| US3816726A (en) * | 1972-10-16 | 1974-06-11 | Evans & Sutherland Computer Co | Computer graphics clipping system for polygons |
| US4319339A (en) * | 1976-06-11 | 1982-03-09 | James Utzerath | Line segment video display apparatus |
| JPS5326534A (en) * | 1976-08-25 | 1978-03-11 | Hitachi Ltd | Vi deo display device |
| DE3008443A1 (en) * | 1979-03-05 | 1980-09-18 | Texas Instruments Inc | DIGITAL DATA PROCESSING ARRANGEMENT |
| US4492956A (en) * | 1980-02-29 | 1985-01-08 | Calma Company | Graphics display system and method including preclipping circuit |
| US4328550A (en) * | 1980-08-08 | 1982-05-04 | Weber John M | Programmer unit with composite calculation capability |
| US4412296A (en) * | 1981-06-10 | 1983-10-25 | Smiths Industries, Inc. | Graphics clipping circuit |
| US4622545A (en) * | 1982-09-30 | 1986-11-11 | Apple Computer, Inc. | Method and apparatus for image compression and manipulation |
| US4531120A (en) * | 1983-01-20 | 1985-07-23 | International Business Machines Corporation | Superposing graphic patterns |
-
1984
- 1984-05-05 GB GB848411579A patent/GB8411579D0/en active Pending
-
1985
- 1985-01-14 JP JP60003408A patent/JPS60239825A/en active Pending
- 1985-03-29 CA CA000477995A patent/CA1219377A/en not_active Expired
- 1985-04-11 DE DE8585104226T patent/DE3587129T2/en not_active Expired - Fee Related
- 1985-04-11 EP EP85104226A patent/EP0163852B1/en not_active Expired - Lifetime
- 1985-04-30 US US06/728,718 patent/US4698779A/en not_active Expired - Fee Related
Also Published As
| Publication number | Publication date |
|---|---|
| JPS60239825A (en) | 1985-11-28 |
| EP0163852A3 (en) | 1990-03-21 |
| EP0163852B1 (en) | 1993-03-03 |
| EP0163852A2 (en) | 1985-12-11 |
| GB8411579D0 (en) | 1984-06-13 |
| DE3587129D1 (en) | 1993-04-08 |
| DE3587129T2 (en) | 1993-09-16 |
| US4698779A (en) | 1987-10-06 |
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| MKEX | Expiry | ||
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Effective date: 20050329 |