Numéro de publication | US20090196519 A1 |

Type de publication | Demande |

Numéro de demande | US 12/345,905 |

Date de publication | 6 août 2009 |

Date de dépôt | 30 déc. 2008 |

Date de priorité | 18 janv. 2005 |

Autre référence de publication | US7486834, US20060158692 |

Numéro de publication | 12345905, 345905, US 2009/0196519 A1, US 2009/196519 A1, US 20090196519 A1, US 20090196519A1, US 2009196519 A1, US 2009196519A1, US-A1-20090196519, US-A1-2009196519, US2009/0196519A1, US2009/196519A1, US20090196519 A1, US20090196519A1, US2009196519 A1, US2009196519A1 |

Inventeurs | James R. Bailey, David A. Crutchfield, Shaun T. Love |

Cessionnaire d'origine | Lexmark International, Inc. |

Exporter la citation | BiBTeX, EndNote, RefMan |

Citations de brevets (3), Référencé par (2), Classifications (4) | |

Liens externes: USPTO, Cession USPTO, Espacenet | |

US 20090196519 A1

Résumé

Error diffusion is performed using a Floyd-Steinberg-like approach. A integer-representation of a running error is compressed by storing only its most significant bits and returning any remainder to the error diffusion processor. The running error is shifted to the right until only the desired number of significant bits remain, and this compressed error is stored. Any portion of the original running error that is lost due to the shifting is treated as a remainder and is returned to the error diffusion processor for use in calculating an adjusted current pixel value. The amount of the shift is retained in compressed form to keep track of the number of shifts needed to form a truncated running error from the compressed running error.

Revendications(24)

Description

- [0001]None.
- [0002]None.
- [0003]None.
- [0004]1. Field of the Invention
- [0005]The present invention is directed to systems and methods for implementing error diffusion when processing an image, such as for printing.
- [0006]2. Description of the Related Art
- [0007]When printing an image using an output device which places discreet units of colorants (ink drops, toner, etc.) on media, it is necessary to reduce the range of the image pixels to match the reproduction capabilities of the printing device. This typically means a reduction in the bit resolution of the image.
- [0008]Most often, the reduction in bit resolution is accomplished by halftone transformation. Halftone transformation results, on a pixel-by-pixel basis for all image pixels, in the replacement of an original non-binary, or “gray-level” value of, e.g., 8 bits, with a binary value after comparison with some threshold. The threshold itself may vary dynamically depending on the non-binary pixel value, and other factors. The original 8-bit value at each pixel is thus substituted by either a “0” (representing an 8-bit value of 0) or a “1” (representing an 8-bit value of 255). The consequence of such a transformation at a pixel is that the overall “brightness” of the image is changed. To mitigate this, the change, or “error”, may be diffused to nearby, as yet untransformed pixels through a technique known as error diffusion. Error diffusion works by spreading the inaccuracy, or error, of the halftone decision at one pixel in the output image among nearby pixels, creating a visually superior transformation. Each original pixel value is adjusted based on the error contributed by adjacent and nearby pixels, and these contributions are taken into account in calculating the correct transformed value for the pixel.
- [0009]There are a number of error diffusion techniques, each of which uses a different combination of thresholding approaches, collection of nearby pixels to which the error is spread, error weightings to each of these nearby pixels, and other factors. The Floyd-Steinberg algorithm, developed in 1975 and known to those skilled in the art, is one of the more well-known implementations of error diffusion. This algorithm generates a series of error values for each image element as an image line is transformed. These error values are calculated by taking a fraction of nearby pixel error values and adding them together to represent a pixel location.
- [0010]In the Floyd-Steinberg algorithm, the error at a transformed pixel
**420**is spread to a collection of four specific nearby pixels in the fashion shown inFIG. 4A . The error from a just-transformed pixel**420**is spread to pixels**422**,**424**,**426**and**428**using error spread weights 7/16, 1/16, 5/16 and 3/16, respectively, the error spread weights representing the proportion of error at transformed pixel**420**that is spread to each adjacent untransformed, error-receiving pixel. Thus, from the perspective of a just-transformed pixel**420**, its total error is spread to “Next Back” pixel**428**(with “send backward coefficient” 3/16), “Next Below” pixel**426**(with “send below coefficient” 5/16), “Next Forward” pixel**424**(with “send forward coefficient” 1/16), and “Current Right” pixel**422**(with “send right coefficient” 7/16). In the foregoing nomenclature, the prefix “Next” refers to the next line to which the corresponding errors are spread. - [0011]
FIG. 4B shows receipt of partial errors from the perspective of a pixel**450**that is about to be transformed using Floyd-Steinberg error diffusion. Soon-to-be transformed pixel**450**receives a portion of the error from each of four nearby, previously transformed pixels**452**,**454**,**456**and**458**, using error spread weights of 7/16, 1/16, 5/16 and 3/16, respectively. Of these, pixels**454**,**456**and**458**are on the previous line (“above”), while recently-transformed pixel**452**is immediately to the left of untransformed pixel**450**, on the current line. From the perspective of untransformed pixel**450**, error is received from “Previous Back” pixel**454**(with “receive backward coefficient” 1/16), “Previous Above” pixel**456**(with “receive above coefficient” 5/16), “Previous Forward” pixel**458**(with “receive forward coefficient” 3/16), and “Current Left” pixel**452**(with “receive left coefficient” 7/16). In the foregoing nomenclature, the prefix “Previous” refers to the previous line from which the corresponding errors are received. - [0012]From the foregoing description, it can be seen that in the Floyd-Steinberg algorithm the error created from transforming a pixel is spread to four adjacent pixels. Furthermore, prior to transformation, each pixel receives a portion of the error from each of the four adjacent pixels that have previously been transformed.
- [0013]The Floyd-Steinberg algorithm typically operates in row order (sometimes called “line order”). That is, an entire row, or line, of an image is transformed before the next row or line is transformed. Transformation of a row results in the storage of a large number of error values. For instance, if an image has a resolution of 600 pixels per inch (PPI), and each row of the image is 9 inches wide, then 5400 pixels worth of error data, each error datum comprising anywhere from 1 color (for a black & white printer) to 3 or more colors (for a color printer), may need to be stored.
- [0014]Originally, the Floyd-Steinberg algorithm was implemented in software with data being read from, and written to a main memory having ample space. More recently, however, high-speed ASIC-based hardware implementations using integer arithmetic have been realized. For cost reasons, it is best to minimize the amount of memory used in such implementations.
- [0015]The target platform for a system in accordance with the present invention is a device, such as a printer, that is configured to perform error diffusion on image data pixels, each image data pixel comprising a non-binary pixel value.
- [0016]In one aspect, the present invention is directed to an error diffusion system configured to perform halftoning of image pixel data. The system comprises an error diffusion processor configured to receive a current pixel from a current image line and output an error diffused current pixel in response thereto; a first decompressor connected to the error diffusion processor and configured to decompress a compressed previous running error of a pixel belonging to a previous line of the image for use in calculating an adjusted pixel value of said current pixel; and a first compressor connected to the error diffusion processor and configured to compress a current running error of said current pixel to thereby form a compressed current running error for the current pixel.
- [0017]In another aspect, the present invention is also directed to a method for handling running error values during a halftoning process of an image. The method entails decompressing a compressed previous running error of a pixel belonging to a previous line of said image to form a truncated previous running error for use in calculating an adjusted pixel value of a current pixel in a current line of an image; and compressing a current running error of that same current pixel to thereby form a compressed running error for that pixel.
- [0018]In still another aspect, the present invention is directed to an error diffusion system for halftoning image data pixels one image line at a time, the system configured to calculate an adjusted pixel value for a current pixel in a current image line, the adjusted pixel value including partial errors from pixels on a previous image line, wherein the partial errors from the pixels on the previous image line are calculated only after halftoning of entire previous line has been completed.
- [0019]The invention is now described with reference to the attached drawings in which:
- [0020]
FIG. 1 shows an error diffusion system in accordance with the present application; - [0021]
FIG. 2A shows a first embodiment of an error spread coefficient subsystem in accordance with the present application; - [0022]
FIG. 2B shows a second embodiment of an error spread coefficient subsystem in accordance with the present application; - [0023]
FIG. 2C shows a third embodiment of an error spread coefficient subsystem in accordance with the present application; - [0024]
FIG. 2D shows a first embodiment of an error spread coefficient subsystem in accordance with the present application; - [0025]
FIG. 3 shows a block diagram of an error diffusion processor implementation in accordance with an embodiment of the present application; - [0026]
FIG. 4A illustrates the prior art principle of error spread weights applied to the error of a transformed pixel; and - [0027]
FIG. 4B illustrates the prior art principle of error spread weights received by a pixel to be transformed. - [0028]
FIG. 1 shows a block diagram of an error diffusion system**100**in accordance with an embodiment of the present invention. The system**100**can belong to a printer that receives an image with multi-bit data pixels and outputs halftone images while using an error diffusion algorithm. The system can be part of a stand-alone printer of the sort capable of printing photographs directly from a digital camera without first having to download the photographs to a personal computer. - [0029]The system
**100**includes a general purpose microprocessor**110**that is connected to a main memory**104**. Main memory**104**typically stores the input pixel data**106**of an image whose pixels are to be transformed from a non-binary format to a binary format, using error diffusion. The microprocessor**110**is part of an Application Specific Integrated Circuit (ASIC)**102**(represented by the dashed) configured to implement error diffusion. The dotted arrows represent connections between the microprocessor**110**and the other components of the ASIC, through data buses, control buses and other structures known to those skilled in the art of integrated circuit design. While in this embodiment, the error diffusion is performed using hardware on the ASIC, it can instead be performed entirely in software by the microprocessor**110**. It is further understood that in some embodiments, the main memory**104**can also be part of the ASIC**102**, or the input pixel data can be stored in a local memory on-board the ASIC. - [0030]In addition to the microprocessor
**110**, the ASIC**102**includes an error diffusion processor**120**, and error spread coefficient subsystem**130**, threshold generation circuitry**140**, and a running error compression/decompression subsystem, shown generally at**190**. - [0031]The error diffusion processor
**120**receives pixel data**106**from the main memory**104**, error spread coefficients**132**from the error spread coefficient system**130**, and threshold information**142**from threshold generation circuitry**140**. The error diffusion processor**120**uses this information, along with the truncated previous line running error information**124**provided by the error compression/decompression subsystem**190**, to transform the pixel data**106**into error diffused pixel data**126**which is stored in the main memory or in another memory location. Control signals**121**are sent from the error diffusion processor**120**to the error spread coefficient system**130**for requesting coefficients and performing other functions. - [0032]The error spread coefficient system
**130**receives input**112**from the microprocessor**110**and pixel data**106**in the case of data-driven determinations of the error spread coefficients. The error spread coefficient system**130**provides the error diffusion processor**120**with the error spread coefficients**132**to be used in allocating the error from a transformed pixel. As discussed further below, the error spread coefficient system**130**may be implemented in a number of different ways. - [0033]The threshold generation circuitry
**140**creates a threshold**142**that is used to compare with each adjusted non-binary (e.g., 8 bit) gray level pixel datum to determine whether the corresponding pixel is to be set to “0” or “1”. The threshold generation circuitry**140**is under the control of the microprocessor**110**and can include pseudo-random circuitry or the like to form a dynamic threshold, in a known manner. - [0034]As is known to those skilled in the art, the error diffusion processor
**120**typically processes image pixel data in line order—each pixel belonging to one line of an image is error diffused, before pixels of the next line are processed. The running error compression/decompression subsystem**190**, described in further detail below, is used to efficiently store the total error at each transformed pixel in an immediately preceding row of image data for use in adjusting a current pixel value of a pixel in a current row of image data. - [0035]
FIG. 3 illustrates the operation of the error diffusion processor**120**ofFIG. 1 , which performs most operations using 16-bit integer math for increased precision. - [0036]The first step
**301**is to receive the 8-bit value of a current pixel, from main memory**104**. The next step**302**is to shift this pixel value left by four bits, which is equivalent to multiplying by 16. At this point, the original 8-bit pixel has been transformed into having 12 significant bits. The transformed pixel is then input into a summer**304**along with five other inputs designated**306**A,**306**B,**306**C,**306**D and**340**, to form a 16-bit adjusted pixel value**308**. - [0037]Input error
**306**A is the partial error received by the current pixel from the transformed pixel in the preceding row and to the left (“previous back”). Input error**306**A is the product of an 8 to 10 bit representation of the truncated running error from transformed “previous back” pixel and a 4-bit representation of the “receive backward coefficient”**350**A, both of which are input to a first integer multiplier**352**A. - [0038]Input error
**306**B is the partial error received by the current pixel from the transformed pixel in the preceding row and directly above (“previous above”). Input error**306**B is the product of an 8 to 10 bit representation of the truncated running error from transformed “previous above” pixel and a 4-bit representation of the “receive above coefficient”**350**B, both of which are input to a second integer multiplier**352**B. - [0039]Input error
**306**C is the partial error received by the current pixel from the transformed pixel in the preceding row and to the right (“previous forward”). Input error**306**C is the product of an 8 to 10 bit representation of the truncated running error from transformed “previous forward” pixel and a 4-bit representation of the “receive forward coefficient”**350**C, both of which are input to a third integer multiplier**352**C. - [0040]Input error
**306**D is the partial error received by the current pixel from the just-transformed pixel immediately to the left in the current row (“current left”). Input error**306**D is delayed by delay**334**and is the delayed product of an 8 to 10 bit representation of the truncated running error from the immediately preceding (i.e., just-transformed pixel) and a 4-bit representation of the “receive left coefficient”**350**D, both of which are input to a fourth integer multiplier**352**D. - [0041]In the foregoing discussion of the input errors
**306**A,**306**B,**306**C,**306**D, the term ‘truncated running error’ refers to the fact that the least significant bits of the various running errors have been set to zero by retaining only the most significant bits and/or shifting to the right, as discussed further below. - [0042]Finally, input error
**340**is delayed by delay**336**and is the delayed version of a 4 to 6 bit current remainder**352**(**152**B inFIG. 1 ) comprising the least significant bits that have been stripped off the current running error by the error compression/decompression subsystem**190**during the compression process. - [0043]Once the 16-bit adjusted current pixel value
**308**has been formed by the summer**304**, a decision**312**is made to determine whether it exceeds a threshold. If the 16-bit adjusted current pixel value**308**exceeds the threshold**142**, then, in block**314**, the current pixel is set to “1” and the current running error**322**is calculated. If, on the other hand, the 16-bit adjusted current pixel value**308**does not exceed the threshold**142**, then, in block**318**, the current pixel is set to “0” and the 16-bit adjusted pixel value is used as the current running error**322**. Thus, the current running error**322**is the outcome of the halftoning decision represented by decision block**312**and blocks**314**,**318**. - [0044]In either case, the current running error
**322**is tapped, as shown by line**315**, and input to a shifter**326**where it undergoes a right shift (i.e., a divide by 16). The output of the shifter**326**is then input to the aforementioned fourth multiplier**352**D to help form the ‘current left’ error**306**D which is delayed by delay**334**and which is to be provided to the pixel immediately to the right on the same line for use in the next iteration. - [0045]One consequence of the design shown in
FIG. 3 is that the partial errors**306**A,**306**B,**306**C, contributed by the three pixels on a previous image line are calculated only after halftoning of the entire previous line has been completed. For a current pixel on a current image line, the partial errors from pixels on the previous line are calculated only after halftoning of the immediately preceding pixel (the pixel to the left) on the current image line has been completed. Moreover, these partial errors from the pixels on the previous image line are calculated only after decompressing compressed running errors corresponding to those pixels. - [0046]The current running error
**322**is also input to the error compression/decompression block**320**, representing the error compression/decompression subsystem**190**ofFIG. 1 . The output of the error compression/decompression block**320**comprises the aforementioned current running error remainder**352**, (**152**B inFIG. 1 ) and the truncated previous line running error**324**, (**124**inFIG. 1 ). The previous line truncated running error**324**comprise the total error at each transformed pixel on the previous line, and three of these are needed at any given time, the three corresponding to the “previous back”, previous above” and “previous forward” total errors. It is understood that pipelined systems, buffers and other hardware in the ASIC accommodates this. - [0047]Returning to
FIG. 1 , the error compression/decompression subsystem**190**includes a first error compressor**150**for compressing each current running error value**122**. As it comes in, each current running error**122**is an N=16 bit value, and so the current running error has bit positions in the range [15:0], with 0 denoting the least significant bit. While it is possible to store all N=16 bits, this would mean that one would have to store on the order of 5400 (assuming 600 ppi and 9 inches) pixel's worth of data, or roughly 86,400 bits per line. The first error compressor**150**helps reduce this total. In particular, the first error compressor**150**stores a maximum of m most significant bits (MSB) of each 16-bit current running error value 122. M is selected to be equal to 8 bits (m=8), although it can be some other number. - [0048]During operation, the first error compressor
**150**checks to see the position of the most significant bit in the current running error**122**. - [0049]If it is determined that the position of the most significant bit in the current running error
**122**is between bit positions J=0 and J=11, then the current running error**122**is shifted to the right by k=4. The thus-shifted version of the current running error**122**is considered to be the compressed current running error**152**A (since the most significant bit, after shifting, is now between bit positions 0-7), and the lowest four bits of current running error**122**(originally in bit positions 0-3) are simply returned**152**B (**352**inFIG. 3 ) to the error diffusion processor**120**as a remainder. - [0050]If, however, it is determined that the position of the most significant bit in the current running error
**122**is between bit positions J=12 and J=14 then the current running error**122**is shifted to the right by an amount necessary to cause the m=8 most significant bits to occupy bit positions 0-7 to thereby create the compressed current running error**152**A. - [0051]For example, given that the most significant bit is in bit position J=12, the first error compressor
**150**shifts the entire current running error value to the right an appropriate amount (in this example, k=5 shifts to the right) until the most significant bit falls into the bit position 7. This way, only m=8 bits need to be stored as the compressed current running error**152**A, along with the shift data value**154**. - [0052]Continuing with this example, the k=5 least significant bits (LSBs) are packed into an 8-bit word and returned
**152**B (**352**inFIG. 3 ) to the error diffusion processor**120**as a remainder for use in summing errors for the next pixel, as discussed above. Therefore, in the system ofFIG. 1 , the error compressor**150**outputs an error shift value**154**in addition to the m-bit compressed running error**152**A. - [0053]In a sense, one can consider the m=8 MSBs of the compressed running error to be a mantissa and the error shift value k=5 to be an exponent. The m=8 bit mantissa can then be shifted to the left by the k=5 error shift value to form a truncated previous line running error
**124**which has a magnitude on the order of the its original running error**122**, and differs from its original running error**122**by just the k=5 least significant bits which, in any event, have been recycled as remainder**152**B (**352**inFIG. 3 ). - [0054]As stated above, N is a 16 bit value, and so the current running error has bit positions in the range [15:0], with 0 denoting the least significant bit. It is understood in the foregoing example that if the most significant bit were in bit position J=11, instead of bit position J=12 in the N=16 bit current running error
**122**(**322**inFIG. 3 ), the mantissa would still be m=8 bits, but there would only be a shift of k=4. Because the amount of required shift can vary among the running errors, we refer to this process of keeping a constant number of most significant bits while varying the amount of the shift as “dynamic shifting”. Furthermore, while in the above example m=8, it is understood that m may take on another value, such as m=5 in which case the coarseness of the truncated running error would increase as well due to the lower resolution in the compressed error. - [0055]From the foregoing, it can be seen that shifting effectively compresses the original 16-bit current running error value
**122**from the original N=16 bits down to m=8 bits. The resulting shifted value is output as an m=8 bit “compressed current running error”**152**A and sent to the compressed running error buffer**180**where it is stored. - [0056]The error shift value
**154**(which in this example is k=5) is passed on to shift data compressor**160**. The error shift values**154**(the “exponents”) corresponding to the compressed current running errors**152**A for a number of successive 16-bit current running error values**122**are often the same, or vary by 1, at most. Therefore, run length encoding (RLE) of the error shift values can be performed by the shift data compressor**160**. The shift data compressor**160**outputs RLE compressed shift data**162**, in a form such as a packets, for storage in the compressed shift data buffer**182**. These RLE shift data packets**162**can be of variable length, or alternatively, of fixed length, depending on the RLE implementation chosen. - [0057]The compressed running error buffer
**180**is a FIFO buffer. Current error compressor**150**compresses a current line running error**122**for a pixel in a current line of an image to thereby form the compressed current running error**152**A which is stored in buffer**180**. - [0058]The calculation of the current line running error
**122**itself depends, in part, on the partial errors**306**A,**306**B,**306**C contributed by pixels in the previous line, as discussed above with reference toFIG. 3 . Therefore, before the current error compressor**150**compresses the current running error**122**, a compressed previous running error**156**is retrieved from the buffer**180**and is decompressed by the previous error decompressor**158**. To reconstitute the previous running error (or more precisely, a truncated version of it), the compressed previous running error**156**and its associated shift value are needed. The stored compressed previous running error**156**must first be retrieved from the compressed running error buffer**180**and then decompressed by previous error decompressor**158**. It should be noted that the decompression of the compressed previous running error**156**can be implicit. Since the decompressed previous running error is to be multiplied by a coefficient**350**, it is possible to apply the shift to the coefficient rather than apply it to the compressed previous running error. Likewise, the coefficient could be stored in a preshifted form eliminating the need for a shift at all. - [0059]To perform this decompression, the shift data decompressor
**168**first retrieves the appropriate compressed shift data**166**from the compressed shift data buffer**182**, then decompresses this to reconstitute the error shift value for each needed pixel's compressed running error in the previous line, and lastly supplies the corresponding reconstituted error shift value**170**to the previous error decompressor**158**. The previous error decompressor**158**then uses this reconstituted error shift value**170**to shift the compressed previous running error**156**by the appropriate amount to form the truncated previous line running error**124**that is supplied to error diffusion processor**120**. - [0060]In the present context, “truncation” refers to the fact that while the order of magnitude of the truncated running error is comparable to that of the original current running error, its least significant bits are not contained in that value.
- [0061]In summary, then, it can be seen that the compression/decompression subsystem
**190**includes a first compressor**150**connected to the error diffusion processor**120**and configured to retain, at most, only the m most significant bits of each of a plurality of current running errors to thereby form a corresponding plurality of compressed current running errors**152**A. The compression/decompression subsystem**190**also includes a second compressor**160**configured to compress information sufficient to create a truncated previous line running error**124**corresponding to its original current running error**122**, from the compressed previous running error**156**. - [0062]As mentioned above, error diffusion is performed in line order, and so all the pixels belonging to a single line are processed one after the other. Then, in the general case, one may consider the i
^{th }pixel in a line of image data to have an N-bit current running error with the most significant bit in bit position J_{i}, N>J_{i}. If J_{i}<12, the current running error is shifted to the right by k_{i}=4 bits, the m=8 bits in bit positions [7:0] are stored in the compressed error buffer, and the shift value k_{i }itself is sent to the second compressor**160**. If J_{i}≧12, the current running error is shifted to the right by a number of bits k_{i }such that its most significant bit ends up in bit position**7**, the m=8 bits in bit positions [7:0] are again stored in the compressed error buffer, and the shift value k_{i }itself is again sent to the second compressor**160**. Finally, the various corresponding k_{i }shift values are compressed. - [0063]Since the compressed running error buffer
**180**is a FIFO buffer, as the first compressor**150**accepts a new running error**122**, the previous error decompressor**158**outputs a truncated previous line running error**124**, the appropriate location in the compressed running error buffer**180**being overwritten in the process. It is further understood fromFIG. 3 that since three such truncated error values (“receive back total”, “receive above total”, and “receive forward total”) are needed at one time by the summer**304**, the diffusion processor**120**must include appropriate registers, circuitry, buffers and the like, all well within the knowledge of one skilled in the art, to accommodate this requirement. - [0064]Considerable savings in buffer memory from using the two compressors
**150**,**160**with the dynamic shifting can be realized. Assuming that m=8 MSBs are stored in the compressed running error buffer**180**, and further assuming that RLE compression of the error shift values**154**requires 1 bit for every 10 pixels, a line of 5400 pixels requires that roughly 5400×8+540=43,740 bits of data be stored by compressed running error buffer**180**and compressed shift data buffer**182**. This is a savings of about 42,660 bits, or roughly 49% fewer bits than would be required if all 5400×16=86,400 bits of the current running error**122**were stored. If, instead, only m=4 MSBs were stored (in which case the remainder**122**(**352**inFIG. 3 ) returned to the error diffusion processor would be 8 or 9 bits), then only 5400×8+540=22140 bits of data would have to be stored, for a savings of about 74% over storing all 86,400 bits. - [0065]
FIGS. 2A-2D present four different embodiments for the error spread coefficient subsystem circuitry**130**seen inFIG. 1 . - [0066]In
FIG. 2A , the error spread coefficient subsystem circuitry**130**A uses a static 16-bit error spread vector**210**. The error spread vector**210**comprises error spread information that is used by the error diffusion processor**120**to determine how to weight the total error from each of four previously transformed adjacent pixels in preparation for summer**304**. In one embodiment, this error spread information comprises four 4-bit error spread coefficients**212**. For this, the 16-bit error spread vector**210**can be considered as four 4-bit numbers ranging from 0-15, each number corresponding to the relative weight one of the four error spread weights discussed above. Thus, as seen inFIG. 2A , the first four bits [15:12] of the 16-bit error spread vector**210**comprises “0001”, which corresponds to a weight of 1 (for the “back” coefficient”**350**A), the next four bits [11:8] comprises “0101”, which corresponds to a weight of 5 (for the “above” coefficient**350**B), next four bits [7:4] comprises “0011”, which corresponds to a weight of 3 (for the “forward” coefficient**350**C), and the last four bits [3:0] comprises “0111”, which corresponds to a weight of 7 (for the “left” coefficient**350**D). - [0067]The error diffusion processor
**120**uses each of these 4-bit values as a relative weighting, and so multiplies each of the truncated running errors by an appropriate corresponding 4 bit value to create four partial errors**306**A,**306**B,**306**C and**306**D used in the summer**304**within the error diffusion processor**120**. When the 16-bit error spread vector**210**is provided to the error diffusion processor**120**, the latter understands the meaning of the four groupings of bits and uses them accordingly. - [0068]In
FIG. 2B , the error spread coefficient subsystem**130**B uses a pixel data-driven paradigm to determine the error spread coefficients. The 8-bit gray-level value of the pixel data**106**being transformed is used to select from among a predetermined set of four error spread coefficients, each set comprising a 16-bit error spread vector. In particular, the 8-bit pixel data is input to a 256×16 bit lookup table (LUT)**224**. Each entry in the lookup table**224**comprises a set of four 4-bit error spread coefficients, one for each of the 256 possible 8-bit gray level values (0-255), which are used to index the appropriate entry in the lookup table**224**. - [0069]In the case of error spread coefficient subsystem
**130**B, each of the**256**error spread vectors comprises four 4-bit weights dictating how to spread the running error from the current pixel to adjacent untransformed pixels. This contrasts with embodiment**130**A where the fixed error spread vector**210**dictates how to weight the total error from each of the previously transformed pixels in preparation for summer**304**. - [0070]In response to a particular 8-bit pixel value input thereto, the lookup table
**224**supplies the appropriate 16-bit error spread vector and splits it in two parts. The first part comprises a 4-bit weight that is sent via output**222**A to the error diffusion processor**120**for use as the “receive left” coefficient**350**D for the next pixel (and also happens to be the “send right” coefficient for the current pixel). The second part is a 12 bit value**228**which comprises the “send back”, “send below” and “send forward”, coefficients for the current pixel. These are stored in a 5400×12-bit spread buffer**226**(12 bits for each pixel in a row) again assuming 600 ppi by 9 inch line length. The error diffusion processor**120**retrieves the appropriate set of previous line “receive” coefficients via output**222**B of the error spread coefficient subsystem**130**B. - [0071]
FIG. 2C shows a third embodiment of an error spread coefficient subsystem**130**C. The error spread coefficient subsystem**130**B ofFIG. 2B required a 5400×12-bit spread buffer**226**, totaling 64,800 bits of data. The error spread coefficient subsystem**130**C ofFIG. 2C uses less memory by compressing a plurality of consecutive sets of the 12-bit error spread coefficient data that ultimately will be used in the error diffusion processor**120**as the “receive back”, “receive above” and “receive forward” coefficients**350**A,**350**B,**350**C, respectively. - [0072]The error spread coefficient subsystem
**130**C ofFIG. 2C includes a 256×16 data-driven lookup table**232**which behaves much the same as lookup table**224**in error spread coefficient subsystem**130**B ofFIG. 2B . Thus, an indexed**16**-bit error spread vector is split into two parts, the first part being the same 4-bit weight provided to the error diffusion processor via output**242**A. The second part, which is the 12 bit value**232**A comprising the current set of “send backward”, “send below”, and “send forward” 4-bit error spread coefficients is sent to block compressor**234**. - [0073]Block compressor accepts a plurality of consecutive sets of 12-bit error spread coefficients, and outputs compressed error spread data
**234**A corresponding to these consecutive 12-bit error spread coefficients. These compressed error spread data can be in the form of multi-bit, such as 64-bit, compressed data blocks**234**A. These blocks**234**A of compressed 12-bit error spread coefficients are then stored in compressed spread buffer**236**. Thus, in this third embodiment**130**C, the compressor**234**is configured to compress a plurality of sets of at least three of said four error spread coefficients to thereby form compressed error spread data**234**A. - [0074]When the error diffusion processor
**120**needs to retrieve a set of previous line “receive” coefficients, the decompressor**238**selectively retrieves the appropriate compressed error spread coefficient block**236**A from the compressed spread buffer**236**, decompresses it, and provides the required information to the error diffusion processor**120**via output**242**B. - [0075]Compression of consecutive 12-bit sets is possible because of their redundancy. This redundancy is due to a combination of two factors: (1) although the lookup table
**232**stores one error spread vector for each gray level value, these vectors are not unique—as few as only 16 or so different vectors may need to be stored—thus, two gray level values that are close to each other typically will index entries comprising identical error spread vectors; and (2) in a line of an image, due to the relatively low spatial frequencies, it is not uncommon for runs of adjacent pixels' gray level values to be identical or very close to one another, and so these map onto the same error spread vector. The degree of compression depends on such factors as the spatial frequencies present in image, the number of different error spread vectors in the lookup table**232**, the correlation between neighboring gray level values and the error spread vectors onto which they map, and the like. - [0076]
FIG. 2D shows a fourth embodiment of an error spread coefficient subsystem**130**D. The error spread coefficient subsystem**130**D includes a 16×4 bit left pixel coefficient array**264**, and a 16×12 error spread coefficient array which stores 16 predetermined sets of three 4-bit coefficients which ultimately will be used in the error diffusion processor**120**as the “receive back”, “receive above” and “receive forward” coefficients**350**A,**350**B,**350**C, respectively. - [0077]In this embodiment, incoming 8-bit pixel data
**106**first indexes 256×4 bit pointer lookup table**252**. In response to a pixel value, the pointer lookup table**252**outputs a 4-bit error spread pointer**252**A to left pixel coefficient array**264**and to pointer compressor**254**. - [0078]The 4-bit pointer
**252**A selects one from among 16 possible (2^{4}) entries in the 16×4 bit left pixel coefficient array**264**. In response to the 4-bit error spread pointer**252**A, left pixel coefficient array**264**provides the error diffusion processor**120**, via output**262**A, the “receive left” coefficient**350**D for use by the next pixel that is processed. - [0079]The 4-bit error spread pointer
**252**A is also supplied to a pointer compressor**254**which compresses a plurality of consecutive pointers to thereby from compressed error spread pointer information**254**A. In one embodiment this compressed error spread pointer information**254**A is formed as 64-bit compressed pointer blocks**254**A which are then stored in a compressed pointer buffer**256**. - [0080]When the error diffusion processor
**130**needs the “receive back”, “receive above” and “receive forward” coefficients**350**A,**350**B,**350**C, respectively, it sends appropriate control signals**121**D to the pointer decompressor**258**within the error spread coefficient subsystem**130**D. - [0081]Decompressor
**258**then obtains the correct compressed pointer block(s)**256**A from the compressed pointer buffer**256**, decompresses the compressed error spread pointer information and thereby forms at least one decompressed 4-bit error spread pointer. The appropriate decompressed 4-bit error spread pointer(s) are then used to retrieve the needed coefficients. In this regard, it should be noted that in embodiments where the coefficient array**266**comprises the triplet of “send backward”, “send below” and “send forward” 4-bit error spread coefficients, more than one such 12-bit triplet may need to be retrieved, since the “receive backward”, “received above” and “receive forward” coefficients**350**A,**350**B,**350**C, respectively, may belong to as many as 3 different entries within the 16 entry coefficient array**266**. - [0082]The present invention has been described with respect to specific embodiments. However, it will be appreciated that modifications and variations of the present invention are covered by the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention.

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Référencé par

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Classifications

Classification aux États-Unis | 382/252 |

Classification internationale | G06K9/00 |

Classification coopérative | H04N1/4052 |

Classification européenne | H04N1/405B2 |

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