Method and apparatus of correcting hybrid flash artifacts in digital images

0001The present invention relates to digital image correction, and particularly to correction of eye artifacts due to flash exposure.

0002<patcit id="pcit0001" dnum="US6873743B"><text>US patent 6,873,743</text></patcit> to Steinberg, discloses an automatic, red-eye detection and correction system for digital images including a red-eye detector module that determines without user intervention if a red-eye defect exists. If a defect is located in an image the portion of the image surrounding the defect is passed to a correction module that de-saturates the red components of the defect while preserving the other color characteristics of the defect region.

0003<patcit id="pcit0002" dnum="WO03071484A"><text>WO03/071484</text></patcit>, Pixology, discloses a method of detecting red-eye features in a digital image comprising identifying highlight i.e. glint regions of the image having pixels with a substantially red hue and higher saturation and lightness values than pixels in the regions therearound. In addition, pupil regions comprising two saturation peaks either side of a saturation trough may be identified. It is then determined whether each highlight or pupil region corresponds to part of a red-eye feature on the basis of further selection criteria, which may include determining whether there is an isolated, substantially circular area of correctable pixels around a reference pixel. Correction of red-eye features involves reducing the lightness and/or saturation of some or all of the pixels in the red-eye feature.

0004In many cases, the eye-artifact that is caused by the use of flash is more complex than a mere combination of red color and a highlight glint. Such artifacts can take the form of a complex pattern of hybrid portions that are red and other portions that are yellow, golden, white or a combination thereof. One example includes the case when the subject does not look directly at the camera when a flash photograph is taken. Light from the flash hits the eye-ball at an angle which may provoke reflections different than retro-reflection, that are white or golden color. Other cases include subjects that may be wearing contact lenses or subjects wearing eye glasses that diffract some portions of the light differently than others. In addition, the location of the flash relative to the lens, e.g. under the lens, may exacerbate a split discoloration of the eyes.

0005A technique is provided for digital image artifact correction as follows. A digital image is acquired. A candidate red-eye defect region is identified in the image. A region of high intensity pixels is identified which has at least a threshold intensity value in a vicinity of said candidate red-eye region. An eye-related characteristic of a combined hybrid region is analyzed. The combined hybrid region includes the candidate red-eye region and the region of high intensity pixels. The combined hybrid region is identified as a flash artifact region based on the analyzing of the eye-related characteristic. Flash artifact correction is applied to the flash artifact region.

0006The flash artifact correction may include red-eye correction of the candidate red-eye region. The flash artifact correction may also include correction of the region of high intensity pixels.

0007A bounding box may be defined around the candidate red-eye defect region. The identifying of the region of high intensity pixels may comprise identifying a seed high intensity pixel by locating said seed high intensity pixel within said bounding box. The seed pixel may have a yellowness above a pre-determined threshold and a redness below a pre-determined threshold. The region of high intensity pixels may be defined around the seed pixel.

0008The analyzing may include calculating a difference in roundness between the candidate red-eye region and the combined region. The red-eye correction may be applied when the roundness of the combined hybrid region is greater than a threshold value.

0009The method may include determining to apply red-eye correction when a roundness of the combined hybrid region is greater than a roundness of the candidate red-eye region by a threshold amount.

0010The method may include determining to not apply correction when the region of high intensity pixels includes greater than a threshold area. The area may be determined as a relative function to the size of said bounding box.

0011The method may include determining a yellowness and a non-pinkness of the region of high intensity pixels. The acquired image may be in LAB color space, and the method may include measuring an average b value of the region of high intensity pixels and determining a difference between an average a value and the average b value of the region of high intensity pixels.

0012The analyzing may include analyzing the combined hybrid region for the presence of a glint, and responsive to detecting a glint, determining to not correct the region of high intensity pixels responsive to the presence of glint.

0013The method may include correcting the region of high intensity pixels by selecting one or more pixel values from a corrected red-eye region and employing the pixel values to correct the region of high intensity pixels. The selected pixel values may be taken from pixels having L and b values falling within a median for the corrected red-eye region.

0014The method may include determining to not apply correction when an average b value of the region of high intensity pixels exceeds a relatively low threshold or if a difference between average a and b values is lower than a pre-determined threshold.

0015The method may include converting the acquired image to one of RGB, YCC or Lab color space formats, or combinations thereof.

0016The analyzing of the acquired image may be performed in Luminance chrominance color space and the region of high intensity pixels may have a luminance value greater than a luminance threshold, and blue-yellow chrominance values greater than a chrominance threshold and a red-green value less than a red-green threshold.

0017The method may include filtering the red-eye candidate regions to confirm or reject said regions as red-eye defect regions, and selecting a subset of the rejected red-eye candidate regions.

0018The method may be implemented within a digital image acquisition device. The method may be implemented as part of an image acquisition process. The method may be implemented as part of a playback option in the digital image acquisition device.

0019The method may be implemented to run as a background process in a digital image acquisition device. The method may be implemented within a general purpose computing device and wherein the acquiring may include receiving the digital image from a digital image acquisition device.

0020The candidate red-eye region and/or the region of high intensity pixels may be corrected. The region of high intensity pixels may be corrected after the red-eye candidate region. The correcting of the region of high intensity pixels may utilize corrected pixel values based on the candidate red-eye region. Results of correcting the candidate red-eye region and the region of high intensity pixels may be combined in such a manner as to obfuscate a seam between the regions. The method may include smoothing a seam region between the candidate red-eye region and the region of high intensity pixels.

0021The eye-related characteristic may include shape, roundness, and/or relative pupil size.

0022A further method is provided for digital image artifact correction. A digital image is acquired. A candidate red-eye defect region is identified in the image. A seed pixel is identified which has a high intensity value in the vicinity of the candidate red-eye region. An eye-related characteristic of a combined hybrid region is analyzed. The combined hybrid region includes the candidate red-eye region and the seed pixel. The combined hybrid region is identified as a flash artifact region based on the analyzing of the eye-related characteristic. Flash artifact correction is applied to the flash artifact region.

0023The flash artifact correction may include red-eye correction of the candidate red-eye region. The flash artifact correction may also include correction of a second region that includes the seed pixel.

0024The seed pixel may have a yellowness above a pre-determined threshold and a redness below a pre-determined threshold.

0025The method may include filtering the red-eye candidate regions to confirm or reject the regions as red-eye defect regions, and selecting a subset of the rejected red-eye candidate regions.

0026The method may be implemented within a digital image acquisition device. The method may be implemented as part of an image acquisition process. The method may be implemented as part of a playback option in the digital image acquisition device.

0027The method may be implemented to run as a background process in a digital image acquisition device. The method may be implemented within a general purpose computing device, and the acquiring may include receiving the digital image from a digital image acquisition device. The analyzing may include checking whether an average b value exceeds a relatively low threshold. The analyzing may include checking whether a difference between an average a value and the average b value is lower than a given threshold.

0028Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which: <ul id="ul0001" list-style="none"><li><figref idref="f0001">Figure 1</figref> illustrates an image in which several defect candidate regions have been identified and surrounded by bounding boxes;</li><li><figref idref="f0001">Figure 2</figref> shows in more detail a candidate region exhibiting a half-red half-white/golden defect; and</li><li><figref idref="f0002">Figure 3</figref> illustrates a flow diagram of an embodiment of image correction software according to the present invention.</li></ul>

0029The preferred embodiments provide improved methods for detecting defects in subjects' eyes as well as methods for correcting such defects.

0030A preferred embodiment may operate by examining a candidate red eye region, looking in its neighborhood or vicinity for a possible yellow, white and/or golden patch belonging to the same eye, and, if any, under certain conditions correcting one or both of the red-eye or golden patch.

0031Using a technique in accordance with a preferred embodiment, the quality and acceptability of automatic eye correction can be increased for half red - half white/golden defects.

0032Implementations of the preferred embodiments can take advantage of the red part of the eye defect being detected by one automatic red-eye detection processing method, perhaps utilizing a conventional technique or a new technique, so the detection of the non-red regions can be applied as a pre-correction stage, and so that this method may take full advantage of existing or new detection methods. The correction parts of such red-eye processing may be altered to implement a technique in accordance with a preferred embodiment, while non correction parts preferably are not altered.

0033A technique in accordance with a preferred embodiment may provide a qualitative improvement in image correction with relatively little processing overhead making it readily implemented in cameras that may have limited processing capability and/or without unduly effecting the camera click-to-click interval.

0034It will be seen that pixels belonging to a red-eye defect may be corrected by reducing the red value of the pixel. As an example, image information may be available in Luminance-Chrominance space such as L*a*b* color space. This may involve reducing the L* and a* value of a pixel to a suitable level. In many cases, reduction of the a* value may automatically restore the chrominance of the eye thus restoring a true value of the iris.

0035However, for white/golden pixels of a half red - half white/golden eye defect, the L and possibly b characteristics of the pixel may also be either saturated and/or distorted. This means that unlike red eye defects, in these cases the original image information may be partially or even totally lost. The correction may be performed by reducing the overall L* value as well as reduction of the a* and b*. However, because L* may be very high, the chrominance may be very low, thus there may not be significant color information remaining. In an additional preferred embodiment, correction of the white/golden portion of the defect involves reconstructing the eye, as opposed to the restoration described above from information from the corrected red eye portion of the defect.

0036Referring now to <figref idref="f0002">Figure 3</figref>, a digital image 10 may be acquired 30 in an otherwise conventional manner and/or utilizing some innovative technique. Where the embodiment is implemented in a device separate from a device such as a camera or scanner on which the image was originally acquired, the image may be acquired through file transfer by another suitable means including wired or wireless peer-to-peer or network transfer. Otherwise the image correction process described below, if suitably speed optimized, can either be implemented within the image acquisition chain of the image acquisition device for displaying a corrected image to a user before the user chooses to save and/or acquire a subsequent image; or alternatively, the image correction process can be analysis optimized to operate in the background on the image acquisition device on images which have been stored previously.

0037Next, during red-eye detection 32, red-pixels 20 are identified and subsequently grouped into regions 22 comprising a plurality of contiguous (or generally contiguous) pixels (see, e.g., <figref idref="f0001">Figure 2</figref>). These regions can be associated 34 with larger bounding box regions 12,14,16,18 (see, e.g., <figref idref="f0001">Figure 1</figref>). The candidate regions contained within these bounding boxes are then passed through a set of filters 36 to determine whether the regions are in fact red-eye defects or not. Examples of such falsing filters are disclosed in <patcit id="pcit0003" dnum="US6873743B"><text>US 6,873,743</text></patcit>.

0038One possible reason a filtering process might reject a candidate region, such as a region of red-pixels 20 as illustrated at <figref idref="f0001">Figure 2</figref>, is that it lacks the roundness expected of a typical red-eye defect. Such regions as well as regions failed for other suitable reasons may be preferably passed as rejected regions 38 for further processing to determine if they include a half red - half white/golden eye defect - and if so for the defect to be corrected accordingly. Much of the operation of this processing can be performed in parallel with other red-eye processing (in for example a multi-processing environment) or indeed processing for each rejected region could be carried out to some extent in parallel.

0039Processing in accordance with an exemplary embodiment which may be involved in checking for half red - half white/golden eye defects is outlined in more detail as follows: <ol id="ol0001"><li>1. The bounding box 12-18 of an already detected red part of the eye artifact is searched 40 for a point, say 26 (see <figref idref="f0001">Figure 2</figref>) having: <ol id="ol0002"><li>a. High intensity (<i>I</i> > threshold)</li><li>b. High yellowness (<i>b</i> > threshold)</li><li>c. Low redness (<i>a</i> < threshold)</li></ol> In this example, it is assumed that the image information for a region is available in Lab color space, although another embodiment could equally be implemented for image information in other formats such as RGB, YCC or indeed bitmap format. If such a point does not exist, then STOP (i.e., the decision is taken that no white/golden patch exists in the vicinity of the red area) and confirm that the region is to be rejected 42. </li><li>2. Starting from a point detected in Step 40, grow 44 a region 24 (see <figref idref="f0001">Figure 2</figref>) based on luminance information, for example, if luminance is greater than a threshold, a point is added to the white/golden region 24. If the region 24 exceeds a predefined maximum allowable size, step 46, then STOP and confirm that the region is to be rejected 42. The maximum allowable size can be determined from a ratio of the bounding box area vis-à-vis the overall area of the red 22 and white/golden region 24.</li><li>3. Yellowness and non-pinkness of the white region are then assessed 48 by checking that average <i>b</i> value exceeds a relatively low threshold, and the difference between average "<i>a</i>" and average "<i>b</i>" is lower than a given threshold. If at least one test fails, then STOP and confirm that the region is to be rejected 42.</li><li>4. In this embodiment, the increase of roundness of the combination of initial red 22 and detected white/golden regions 24 from the original red region 22 is checked 50. Thus, the roundness of the union of the red and white/golden regions is computed and compared with that of the red region 22. If roundness is less than a threshold value or decreased or not increased sufficiently by "adding" the white/golden region 24 to the red one 22, then STOP and reject the region 42. Roundness of a region is preferably computed using the formula <maths id="math0001"><math display="block"><mi>Roundness</mi><mo>=</mo><mfrac><msup><mi>Perimeter</mi><mn>2</mn></msup><mrow><mn>4</mn><mo>⁢</mo><mi>π</mi><mo>⋅</mo><mi>Area</mi></mrow></mfrac></math><img file="EP1812901B1_D0001.tif" /></maths> Prior to assessing roundness, a hole filling procedure is preferably applied to each region 22,24 to include for example pixel 28 within the union. </li><li>5. If the region passes one or more and preferably all of the above tests, it is added to the list of confirmed red-eye regions. At this point, the red part of the eye defect can be corrected 52 in any of various manners, for example, by reducing the a value of pixels in Lab color space, while the pixels that were corrected are marked to be used in further processing.</li><li>6. For white/golden regions that were added to the list of red-eye defect regions, further correction of the white/golden portion of the defect can be applied, after some further checks. One such check is to detect glint 54. In RGB space, glint candidates are selected as high luminance pixels (for example, min(R, G) >= 220 and max(R, G) == 255). If a very round (e.g, in one or both of aspect ratio and elongation), luminous, and desaturated region is found within the interior of the current "red ∪ white" region 22,24, its pixels may be removed from the "pixels-to-correct" list. The glint may be the entire high luminance region but in most cases only a small part of the high luminance region will satisfy the criteria for glint pixels.</li><li>7. Where a glint is not detected or is small relative to the size of the white/golden region, the non-red eye artifact pixels 24 can be corrected 56 preferably taking color information from red pixels 22 which where already corrected at step 52, if such information after the correction exists. Alternatively, the correction can be done by reduction of the Luminance value. In the preferred embodiment, color information is derived from a selection of ex-red pixels with L and b values which lie in the median for that region (between the 30% and 70% points on a cumulative histogram for L and b). These color samples (from the already corrected red part of the eye) are used to create the same texture on both the red and non-red defect parts of the eye. It should be noted that the L and b histograms may be generally available from preprocessing steps, for example, those for determining various thresholds, and won't necessarily have changed during correction as the red correction may just involve reducing the a value of a pixel. It is possible that the correction of the red-eye region and the one for the high intensity region may show an unpleasant seam between the regions. In an alternative embodiment, the corrected region will be smoothed in such a manner that the seams between the two regions if exist, will be eliminated.</li></ol>

0040In methods that may be performed according to preferred embodiments herein and that may have been described above and/or claimed below, the operations have been described in selected typographical sequences. However, the sequences have been selected and so ordered for typographical convenience and are not intended to imply any particular order for performing the operations.