1 DETECTING REDEYE DEFECTS IN DIGITAL IMAGES
This application is a Continuation of U.S. patent application Ser. No. 11/937,377, filed on Nov. 8, 2007, and this application is related to PCT application no. PCT/EP2008/ 008437, filed Oct. 7, 2008.
Embodiments of the invention relate generally to the field of digital image processing and more specifically to methods and apparatuses for detecting redeye defects in digital images, such “redeye” defects including any flash-induced artifact in an image of a human or animal eye, whether actually red or not.
Redeye is a phenomenon in flash photography where a flash is reflected within a subj ect’s eye and appears in a photograph as alight, usually red, dot where the black pupil of the subject’s eye wouldnorrnally appear. The unnatural glowing red of an eye is due to internal reflections from the vascular membrane behind the retina, which is rich in blood vessels. This objectionable phenomenon is well understood to be caused in part by a small angle between the flash of the camera and the lens of the camera. This angle has decreased with the miniaturization of cameras with integral flash capabilities. Additional contributors include the relative closeness of the subject to the camera and ambient light levels.
The redeye phenomenon can be reduced by causing the iris to reduce the opening of the pupil. This is typically done with a “pre-flash”, a flash or illumination of light shortly before a flash photograph is taken. This causes the iris to close. Unfortunately, the pre-flash is an objectionable 0.2 to 0.6 seconds prior to the flash photograph. This delay is readily discernible and easily within the reaction time of a human subj ect. Consequently the subject may believe the pre-flash is the actual photograph and be in a less than desirable position at the time of the actual photograph. Alternately, the subject must be informed of the pre-flash, typically losing any spontaneity of the subject captured in the photograph.
Digital photography eliminates the need for film as the image is digitally captured and stored in a memory array for display on a display screen on the camera itself. This allows photographs to be viewed and enjoyed virtually instantaneously as opposed to waiting for film processing. Digital photography equipment includes microprocessors for image processing and compression and camera systems control. It is possible to exploit the computation capabilities of such microprocessors for performing operations to improve redeye detection and elimination.
Existing techniques for redeye detection and correction in digital images are described in U.S. Pat. No. 6,407,777 and US Patent Application Publication No. 2005/ 0232490. However, these prior methods are not eflicient in practice.
In accordance with one embodiment of the invention, a method for detecting a redeye defect in a digital image containing an eye is disclosed; the digital image is converted into an intensity image, at least a portion of the intensity image is segmented into segments each having a local intensity maxi
mum, a corresponding portion of the digital image is thresholded to identify regions of relatively high intensity, a region from at least some of the regions of step (c) having substantially the highest average intensity is selected, and segments from step (b) intersecting the region selected at step (d) according to a pre-determined criterion are selected.
Other features and advantages of embodiments of the invention will be apparent from the accompanying drawings, and from the detailed description, that follows below.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention may be best understood by referring to the following description and accompanying drawings that are used to illustrate embodiments of the invention. In the drawings:
FIG. 1 is a block diagram of a digital camera operating in accordance with an embodiment of the present invention;
FIG. 2 is a flow diagram of the steps performed by software in the camera of FIG. 1 of an embodiment of redeye detection and correction according to the invention; and
FIGS. 3 to 7 show image data processed according to the method of FIG. 2 (for clarity, FIGS. 6(a) to 6(d), 6(f) and 7 are the negative or inverted form of the relevant images described in the text).
An image acquired with a flash may include red-eye defects. In general, these red-eye defects are detected by applying a conventional eye defect detector to the image. However images acquired with a high ISO rating, for example, greater than ISO 800, may include numerous small clusters of red-pixels indicative of noise and in such cases, the eye defect detector can identify the noise speckles as relatively small red eye defects.
Embodiments of the invention provide methods and apparatuses for detecting red eyes in high ISO flash images. For one embodiment of the invention, a digital image is acquired. One or more relatively large candidate red eye defect regions are detected in at least a portion of the image. Face detection is applied to at least a portion of the image to eliminate non-face regions and one or more relatively small candidate red eye defect regions are identified in at least a portion of the image not including the eliminated non-face regions.
In the following description, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known circuits, structures and techniques have not been shown in detail in order not to obscure the understanding of this description.
Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
Moreover, inventive aspects lie in less than all features of a single disclosed embodiment. Thus, the claims following the Detailed Description are hereby expressly incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment of this invention.
Embodiments of the invention are applicable to wide range of systems in which image processing is effected.
FIG. 1 is a block diagram of a digital image acquisition device 20 which in the present embodiment is a portable digital camera, and includes a processor 120. It can be appreciated that many of the processes implemented in the digital camera may be implemented in or controlled by software operating in a microprocessor, central processing unit, controller, digital signal processor and/or an application specific integrated circuit, collectively depicted as processor 120. Generically, all user interface and control of peripheral components such as buttons and display is controlled by a microcontroller 122. The processor 120, in response to a user input at 122, such as half pressing a shutter button (pre-capture mode 32), initiates and controls the digital photographic process. Ambient light exposure is monitored using light sensor 40 in order to automatically determine if a flash is to be used. A distance to the subject is determined using a focus component 50 which also focuses the image on image capture component 60. In the present specification, the term “image” refers to image data and does not necessarily imply that an actual viewable image is present at any stage of the processing.
Ifa flash is to be used, processor 120 causes the flash 70 to generate a photographic flash in substantial coincidence with the recording of the image by image capture component 60 upon full depression of the shutter button. The image capture component 60 digitally records the image in colour. The image capture component preferably includes a CCD (charge coupled device) or CMOS to facilitate digital recording. The flash may be selectively generated either in response to the light sensor 40 or a manual input 72 from the user of the camera. The high resolution image recorded by image capture component 60 is stored in an image store 80 which may comprise computer memory such a dynamic random access memory or a non-volatile memory. The camera is equipped with a display 100, such as an LCD, for preview and postview of images.
In the case of preview images which are generated in the pre-capture mode 32 with the shutter button half-pressed, the display 100 can assist the user in composing the image, as well as being used to determine focusing and exposure. Temporary storage 82 is used to store one or more of the preview images and can be part of the image store 80 or a separate component. The preview image is preferably generated by the image capture component 60. For speed and memory efliciency reasons, preview images preferably have a lower pixel resolution than the main image taken when the shutter button is fully depressed, and are generated by sub-sampling a raw captured image using software 124 which can be part of the general processor 120 or dedicated hardware or combination thereof. Depending on the settings of this hardware subsystem, the pre-acquisition image processing may satisfy some predetermined test criteria prior to storing a preview image. Such test criteria may be chronological, such as to constantly replace the previous saved preview image with a new captured preview image every 0.5 seconds during the pre-capture mode 32, until the high resolution main image is captured by full depression of the shutter button. More sophisticated criteria may involve analysis of the preview image content, for example, testing the image for changes, before deciding whether the new preview image should replace a previously saved image. Other criteria may be based on image analysis such as the sharpness, or metadata analysis such as the exposure condition, whether a flash is going to happen, and/or the distance to the subj ect.
If test criteria are not met, the camera continues by capturing the next preview image without saving the current one. The process continues until the final high resolution main image is acquired and saved by fully depressing the shutter button.
Where multiple preview images can be saved, a new preview image will be placed on a chronological First In First Out (FIFO) stack, until the user takes the final picture. The reason for storing multiple preview images is that the last preview image, or any single preview image, may not be the best reference image for comparison with the final high resolution image in, for example, a red-eye correction process or, in the present embodiment, mid-shot mode processing. By storing multiple images, a better reference image can be achieved, and a closer alignment between the preview and the final captured image can be achieved in an alignment stage discussed later.
The camera is also able to capture and store in the temporary storage 82 one or more low resolution post-view images. Post-view images are low resolution images essentially the same as preview images, except that they occur after the main high resolution image is captured.
A redeye detection and correction filter 90 can be integral to the camera 20 or part of an external processing device 10 such as a desktop computer, a colour printer or a photo kiosk. In this embodiment, the filter 90 receives the captured high resolution digital image from the store 80 and analyzes it, 92, to detect redeyes. The analysis 92 is performed according to the principles of the invention as described in the embodiments to follow. If redeyes are found, the filter modifies the image, 94, to remove the redeye from the image using wellknown techniques. The modified image may be either displayed on image display 100, saved on a persistent storage 112 which can be internal or a removable storage such as CF card, SD card or the like, or downloaded to another device via image output means 110 which can be tethered or wireless. The redeye filter 90 can be brought into operation either automatically each time the flash is used, or upon user demand via input 30. Although illustrated as a separate item, where the filter 90 is part of the camera it may be implemented by suitable software on the processor 120.
FIG. 2 is a flow diagram of the steps performed by software in the camera and/or external processing device of FIG. 1 of an embodiment of redeye detection and correction according to the invention.
1. Eye Detection
The first step 200 of the embodiment is to run an eye detection algorithm to identify regions of the image suspected to contain an eye. Examples of such algorithms are described in PCT Application No. PCT/EP2006/008342 and U.S. Application Nos. 60/865,375 filed Nov. 10, 2006; U.S. 60/865,622 filed Nov. 13, 2006; and U.S. 60/915,669 filed May 2, 2007 whose output is a detection rectangle which ideally should contain an eye but might be a false positive (a non-eye region). Moreover, even in the case where the detection rectangle contains an eye, in most cases it is a non-defect eye, which should not be modified by a sub sequent correction algorithm. Therefore, the detection rectangle should be filtered to discard both non-eye and normal-eye regions before applying correction.
The main difliculty of the detection task is the wide variety of redeyes, both in terms of color (covering all possible hues from bright red to orange, yellow, white, and also combinations of them) and size. Moreover, in many cases, the defect is far from being uniform in terms of color and/or intensity. Thus, the object is to find an invariant to all of the defect eyes, i.e., a property that holds true in substantially all cases. This
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