Patent Publication Number: US-8977056-B2

Title: Face detection using division-generated Haar-like features for illumination invariance

Description:
PRIORITY INFORMATION 
     This application is a continuation of U.S. patent application Ser. No. 12/843,805, entitled “Face Detection Using Division-Generated Haar-Like Features For Illumination Invariance”, filed on Jul. 26, 2010, which was related to U.S. patent application Ser. No. 12/825,280, entitled “Methods And Apparatuses For Half-Face Detection,” filed on Jun. 28, 2010. The entire contents of both of these applications are hereby incorporated by reference for all purposes. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to digital image processing and more particularly to techniques for face detection and recognition. 
     BACKGROUND OF THE INVENTION 
     The approaches described in this section are approaches that could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section. 
     Many existing digital image processing algorithms utilize facial recognition and detection techniques in order to identify human faces in a digital image. Identifying human faces is often a necessary or desired step in various image enhancement and image alteration applications. For example, identifying human faces can be used as a step in algorithms that enhance the separation of a subject in the foreground from the background in order to enhance depth of field or to separate the face or a body from the background altogether. Identifying human faces can also be used as a step in image correction algorithms that are used to identify and correct defects in a digital image. For example, by knowing whether an object is a face or not a face, a location of eyes can be estimated and used to increase the number of true positives identified by a redeye removal algorithm and reduce the number of false positives identified by the redeye removal algorithm. 
     A well-known fast face-detection algorithm is disclosed in U.S. Patent Application Publication No. 2002/0102024, which is hereby incorporated by reference in its entirety for all purposes. That patent application proposes a classifier chain consisting of a series of sequential feature detectors. According to one implementation, a set of training data includes known face-containing images (which are tightly cropped around the faces therein, such that the faces dominate the images&#39; areas), which have been labeled as such, and known face-omitting images, which also have been labeled as such. For each image in the training data, the values of various features (discussed in further detail below) within that image are observed, such that the same features for each such image are observed; in different images, different values may be observed for the same feature. A machine-learning mechanism processes the training data to learn, automatically, numerical ranges into which the values of features of known face-containing images tend to fall and outside of which the values of the corresponding features of known face-omitting images tend to fall; each different feature may be associated with a different numerical range. The machine-learning mechanism generates the classifier chain based on this processing. Each classifier in the classifier chain corresponds to a separate feature and associated numerical range. Classifiers that are more likely to filter out face-omitting images may be placed earlier in the classifier chain than classifiers that are less likely to do so. 
     Unlabeled images (not in the training data) are subjected successively to the classifiers in the classifier chain in order to determine whether those images probably contain faces. For a given classifier in the classifier chain, a determination is made as to whether the value observed for that classifier&#39;s corresponding feature in the unlabeled image falls within the previously machine-learned numerical range associated with that classifier&#39;s corresponding feature. As soon as an unlabeled image (or a selected portion thereof) fails to pass a particular classifier in the classifier chain (due to a value of a feature in the image falling outside of the corresponding classifier&#39;s numerical range), it is concluded that the image (or the selected portion thereof) probably does not contain a face. Subsequent classifiers in the chain do not thereafter need to be applied to the image (or the selected portion thereof). In order to increase face-detection speed, face-omitting images are eliminated from consideration as early as possible. In one implementation, an image (or a selected portion thereof) is only determined to be likely to contain a face if that image (or selected portion thereof) passes all of the classifiers in the classifier chain. 
     In order to process images extremely rapidly while achieving high facial feature detection rates, automated face-detection techniques may generate and use an “integral image.” Generation and use of an integral image is described in “Robust Real-Time Face Detection” by Paul Viola and Michael J. Jones, in  International Journal of Computer Vision  57(2), 137-154 (2004), which is incorporated by reference herein. An integral image is automatically generated based on a source image (e.g., an image captured by a digital camera). The integral image can be computed from the source image using a few operations per pixel. After the integral image has been computed, image features (called “Haar-like” features by Viola and Jones due to those features&#39; conceptual relatedness to Haar Basis functions) within the corresponding source image can be detected rapidly. Based on the values of certain image features in certain regions of the source image, portions of the source image can be determined to probably represent either facial portions or non-facial portions. If a sufficient quantity of various different Haar-like feature values fall within specified ranges for those features (each such feature&#39;s value possibly being compared to a different feature-corresponding range), then it can be reasonably concluded that the area of the image in which all of those Haar-like features occur contains a face. Generation and characteristics of an integral image are discussed in greater detail below, but Haar-like features are briefly discussed first. 
     Viola and Jones propose the use of three different types of features: two-rectangle features, three-rectangle features, and four-rectangle features. Regarding two-rectangle features, some rectangular region of the source image is divided into two adjacent rectangles. These rectangles may be side-by-side or one on top of the other. The value of the two-rectangle feature is equal to the difference between (a) the sum of the pixel intensities within one of the rectangles and (b) the sum of the pixel intensities within the other of the rectangles. Regarding three-rectangle features, some rectangular region of the source image is divided into three rectangles of equal area; again, these may be side-by-side in a row or one above the other in a column. One of the three rectangles—the center rectangle—will be positioned between the other two outer rectangles. The value of the three-rectangle feature is equal to (a) the sum of the pixel intensities within the center rectangle minus (b) the sum of the pixel intensities within the two outer rectangles. Regarding four-rectangle features, some rectangular region of the source image is divided into four rectangular quadrants. The upper-left quadrant and the lower-right quadrant make up one diagonal quadrant pair, while the upper-right quadrant and the lower-left quadrant make up another diagonal quadrant pair. The value of the four-rectangle feature is the difference between (a) the sum of the pixel intensities within one of these diagonal quadrant pairs and (b) the sum of the pixel intensities within the other of these diagonal quadrant pairs. 
     The process of determining features of the source image typically involves calculating various sums of pixel intensity values within various different rectangular regions of the source image. A less effective approach for computing these sums might involve scanning all of the pixels of each such rectangular region in the source image separately for each such rectangular region. Since such rectangular regions may overlap, this approach would detrimentally involve the probable repeated scanning of certain source image pixels multiple times—once for each separate rectangular region in which that source image pixel occurred. Fortunately, after the integral image has been generated, such repetitive scanning of source image pixels can largely be avoided. 
     The rectangular features discussed above can be computed more rapidly using an integral image than using a source image directly. Each pixel of the integral image corresponds to a similarly located (in the same column and row) pixel in the source image, such that the integral image has the same pixel height and width as the source image. However, for each particular pixel in the integral image, the intensity value of that particular pixel is equal to the sum of the intensity values of all of the source image&#39;s pixels occurring within the rectangular region that occurs above and to the left of, and including, the particular pixel&#39;s corresponding position. For each particular pixel in the integral image, that particular pixel&#39;s corresponding rectangular region&#39;s upper-left corner is the source image&#39;s pixel at the upper-left corner of the source image, and that particular pixel&#39;s corresponding rectangular region&#39;s lower-right corner is the source image&#39;s pixel that is located, in the source image, at the same position in which the particular pixel is located in the integral image. Expressed mathematically, 
                 ii   ⁡     (     x   ,   y     )       =       ∑         x   ′     ≤   x     ,       y   ′     ≤   y         ⁢           ⁢     i   ⁡     (       x   ′     ,     y   ′       )           ,         
where ii(x,y) is the integral image, and i(x,y) is the source image.
 
     The integral image may be viewed as a two-dimensional array of intensity values in which one dimension&#39;s size is equal to the integral image&#39;s width, and in which the other dimension&#39;s size is equal to the integral image&#39;s height. For any rectangular region of the source image, the sum of the pixel intensity values in that rectangular region can be determined mathematically using just a few values from the array representation of the corresponding integral image. For example, for any given rectangular region of having an upper-left corner at position A, an upper-right corner at position B, a lower-left corner at position C, and a lower-right corner at position D, the sum of the source image&#39;s pixel intensity values in that rectangular region can be computed quickly by adding the values at positions A and D in the array to generate a first sum, adding the values at positions B and C in the array to generate a second sum, and then subtracting the second sum from the first sum. 
     A face-recognition algorithm may behave differently when applied to the same face under different illumination conditions. When a face is illuminated uniformly, a face-recognition algorithm may detect facial features (e.g., eyes) more correctly than when the same face is illuminated only partially—such as when one side of the face is illuminated by a light source to the side of that face, leaving the other side of the face in relative shadow. Extreme cases such as underexposure (caused by lowlight and backlight) and overexposure will decrease contrast on the face. Self-shadows caused by a directional illuminant or shadows introduced by foreign objects are more unpredictable because they can change the appearance of facial features. As is discussed above, when Haar-like features are used to detect faces within a source image, the geometric relationships and contrast information between adjacent rectangular regions are extracted. The Haar-like feature function will translate the illumination and geometric information to a numerical value. If the same Haar-like feature function is evaluated under different lighting conditions, the numerical result will be at least slightly different. 
       FIGS. 2A and 2B  illustrate an example of the same face under two different lighting conditions. A left face  202  is uniformly illuminated, having little contrast between the left and right sides of face  202 . In contrast, a right face  204  is illuminated from the right side, leaving the left side of face  204  in shadow, thus producing a high and non-uniform contrast between the left and right sides of face  204 ; right face  204  becomes progressively darker proceeding from the right side toward the left side. Thus, in left face  202 , the difference computed between (a) the sum of pixel intensity values of rectangle  206  and (b) the sum of pixel intensity value of rectangle  208  will be relatively small; the illumination difference between rectangles  206  and  208  is mostly due to the fact that rectangle  208  contains an eye while rectangle  206  does not. In contrast, in right face  204 , the difference computed between (a) the sum of pixel intensity values of rectangle  210  and (b) the sum of pixel intensity values of rectangle  212  will be relatively larger; the larger difference in illumination between rectangles  210  and  212  is due not only to the fact that rectangle  212  contains an eye while rectangle  210  does not, but also due to the fact that the rectangle  210  is generally darker than rectangle  212 . This may be the reverse of the case with left face  202 , in which the presence of the eye within a rectangle caused that rectangle&#39;s average pixel luminance to be darker, not lighter, than that of the rectangle adjacent to it. The values of the Haar-like features of the same face, under different lighting conditions, will be different, causing a face-detecting algorithm potentially to produce different results even though the same person&#39;s face is being evaluated in both images. Clearly, a face should be detected in both images. 
     Ideally, under circumstances in which the same face is being evaluated in different lighting conditions, the values of the same Haar-like features (e.g., two-rectangle features positioned over the ocular region of the face, as in  FIGS. 2A and 2B ), would be similar. In order to minimize the variance in Haar-like feature values that is caused by differing lighting conditions, one corrective approach divides each source image&#39;s feature value by the statistical variance of all of the pixel intensity values within the source image&#39;s entire facial region (which might, in some cases, include all of the source image&#39;s pixels). If this statistical variance is low, as would be the case with a uniformly illuminated, low contrast face such as face  202 , then the division will have a relatively minor effect on the feature values. In contrast, if this statistical variance is high, as would be the case with a non-uniformly illuminated, high contrast face such as face  204 , then the division will have a relatively major effect on the feature values. The division is performed both during the machine-learning procedure, relative to labeled images, and during the classifier-applying procedure, relative to unlabeled images. The division has a normalizing effect on the feature values, so that feature values in non-uniformly illuminated faces will tend to fall within the same numerical ranges as corresponding feature values in uniformly illuminated faces; without such normalization, the learned numerical ranges would probably be so broad as to reduce greatly their discriminatory ability when applied to unlabeled images. 
     Unfortunately, this corrective approach, involving division by statistical variance, suffers from the fact that the computation of the statistical variance of all of the pixel values within the source image&#39;s entire facial region is memory resource-expensive, processing time-consuming, and not very conducive to real-time face detection applications. This is the case even when the integral image is used to reduce the quantity of computations needed to calculate the statistical variance. The integral image itself can be viewed as an element of the process that consumes significant memory resources. The generation of the integral image can be viewed as an element of the process that consumes significant processing time. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows the primary subsystems of a face tracking system in accordance with certain embodiments. 
         FIGS. 2A and 2B  illustrate an example of the same face under two different lighting conditions. 
         FIG. 3  is a flow diagram that illustrates an example of a ratio-based Haar-like feature value computation technique, according to an embodiment of the invention. 
         FIG. 4  shows a block diagram of a special purpose computing device for carrying out techniques of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring the present invention. 
     Overview 
     According to one embodiment of the invention, instead of calculating a Haar-like feature value by subtracting the average pixel intensity value in one rectangular region from the average pixel intensity value in another, adjacent rectangular region, a face-detection system calculates that Haar-like feature value by dividing the average pixel intensity value in one such rectangular region by the average pixel intensity value in the other such adjacent rectangular region. Thus, in one embodiment of the invention, each Haar-like value is calculated as a ratio of average pixel intensity values rather than as a difference between such average pixel intensity values. 
     The feature values may be calculated using this ratio-based technique both during the machine-learning procedure, in which the numerical ranges for features in known face-containing images are learned based on labeled training data, and during the classifier-applying procedure, in which an unlabeled image&#39;s feature values are calculated and compared to the previously machine-learned numerical ranges. However, alternative techniques, such as the statistical variance-based approach discussed previously, may still be during the machine-learning procedure; such alternative techniques still will produce numerical ranges that can be mathematically adapted to work in conjunction with feature values determined using the ratio-based feature value computation techniques disclosed herein. 
     It has been discovered that the ratio-based computation of Haar-like feature values already produces adequate feature value normalization to compensate for differences in uniformity of illumination in face-containing images. As a result, the computation of the statistical variance of a source image&#39;s pixel intensity values is not required, and, in one embodiment of the invention, is not performed. Furthermore, the generation and storage of an integral image is unnecessary when the ratio-based feature computation technique is used. The avoidance of the generation and storage of an integral image conserves memory resources and reduces processing time during the face detection process. 
     Various embodiments of the invention also include a digital image acquisition system, having no photographic film, comprising means for carrying out one or more steps of the methods described in this application. Alternative embodiments of the invention include one or more machine-readable non-transitory storage media storing instructions which when executed by one or more computing devices cause the performance of one or more steps of the methods described in this application. 
     Before aspects of the ratio-based feature computation technique are described in detail, a system in which the ratio-based feature computation technique may be implemented and used is discussed below. 
     Digital Image Acquisition System 
       FIG. 1  shows the primary subsystems of a face tracking system in accordance with certain embodiments. The solid lines indicate the flow of image data; the dashed lines indicate control inputs or information outputs (e.g. location(s) of detected faces) from a module. In this example, an image processing apparatus can be a digital still camera (DSC), a video camera, a cell phone equipped with an image capturing mechanism or a hand help computer equipped with an internal or external camera, or a combination thereof. 
     A digital image, i(x, y), is acquired in raw format from an image sensor  105  such as a charged coupled device (CCD) sensor or complimentary metal oxide semiconductor (CMOS) sensor. An image subsampler  112  generates a smaller copy of the main image. Most digital cameras already contain dedicated hardware subsystems to perform image subsampling, for example to provide preview images to a camera display. Typically, the subsampled image is provided in bitmap format (RGB or YCC). In the meantime, the normal image acquisition chain performs post-processing on the raw image  110  which typically includes some luminance and color balancing. In certain digital imaging systems, the subsampling may occur after such post-processing, or after certain post-processing filters are applied, but before the entire post-processing filter chain is completed. 
     Optionally, the subsampled image may be passed to an integral image generator  115  which creates an integral image from the subsampled image. The integral image, ii(x,y), at location (x, y) contains the sum of the pixel values above and to the left of point (x, y) from image i(x,y). However, in at least some embodiments of the invention, no integral image is generated or stored, and the subsampled image is passed to the next component instead. In embodiments in which the integral image is not generated, integral image generator  115  may be omitted from the system. 
     This integral image, if generated (or the subsampled image, if the integral image is not generated) is next passed to a fixed size face detector  120 . If the integral image is generated, then the face detector is applied to the full integral image, but as this is an integral image of a subsampled copy of the main image, the processing involved in the face detection is proportionately reduced. If the subsampled image is ¼ of the main image, e.g., has ¼ the number of pixels and/or ¼ the size, then the processing time involved is only about 25% of that for the full image. Alternatively, the face detector may be applied to the subsampled image under circumstances in which the integral image is not generated. 
     This approach is particularly amenable to hardware embodiments where the subsampled image memory space can be scanned by a fixed size direct memory access (DMA) window and digital logic to implement a Haar-feature classifier chain can be applied to this DMA window. Several sizes of classifiers may alternatively be used (in a software embodiment), or multiple fixed-size classifiers may be used (in a hardware embodiment). 
     After application of the fast face detector  280 , any newly detected candidate face regions  141  are passed onto a face tracking module  111 , where any face regions confirmed from previous analysis  145  may be merged with new candidate face regions prior to being provided  142  to a face tracker  290 . 
     The face tracker  290  provides a set of confirmed candidate regions  143  back to the tracking module  111 . Additional image processing filters are preferably applied by the tracking module  111  to confirm either that these confirmed regions  143  are face regions or to maintain regions as candidates if they have not been confirmed as such by the face tracker  290 . A final set of face regions  145  can be output by the module  111  for use elsewhere in the camera or to be stored within or in association with an acquired image for later processing either within the camera or offline. Set  145  can also be used in a next iteration of face tracking. 
     After the main image acquisition chain is completed, a full-size copy of the main image  130  will normally reside in the system memory  140  of the image acquisition system. This may be accessed by a candidate region extractor  125  component of the face tracker  290 , which selects image patches based on candidate face region data  142  obtained from the face tracking module  111 . In one embodiment of the invention, these image patches for each candidate region are optionally passed to an integral image generator  115 , which passes the resulting integral images to a variable sized detector  121 —as one possible example a Viola-Jones detector—which then applies a classifier chain—preferably at least a 32-classifier chain—to the integral image (or subsampled image) for each candidate region across a range of different scales. 
     The range of scales  144  employed by the face detector  121  is determined and supplied by the face tracking module  111  and is based partly on statistical information relating to the history of the current candidate face regions  142  and partly on external metadata determined from other subsystems within the image acquisition system. 
     As an example of the former, if a candidate face region has remained consistently at a particular size for a certain number of acquired image frames, then the face detector  121  is applied at this particular scale and/or perhaps at one scale higher (i.e. 1.25 times larger) and one scale lower (i.e. 1.25 times lower). 
     As an example of the latter, if the focus of the image acquisition system has moved to approximately infinity, then the smallest scalings will be applied in the face detector  121 . Normally these scalings would not be employed, as they would be applied a greater number of times to the candidate face region in order to cover it completely. It is worthwhile noting that the candidate face region will have a minimum size beyond which it should not decrease—this is in order to allow for localized movement of the camera by a user between frames. In some image acquisition systems which contain motion sensors, such localized movements may be tracked. This information may be employed to further improve the selection of scales and the size of candidate regions. 
     The candidate region tracker  290  provides a set of confirmed face regions  143  based on full variable size face detection of the image patches to the face tracking module  111 . Clearly, some candidate regions will have been confirmed while others will have been rejected, and these can be explicitly returned by the tracker  290  or can be calculated by the tracking module  111  by analyzing the difference between the confirmed regions  143  and the candidate regions  142 . In either case, the face tracking module  111  can then apply alternative tests to candidate regions rejected by the tracker  290  to determine whether these should be maintained as candidate regions  142  for the next cycle of tracking or whether these should indeed be removed from tracking. 
     Once the set of confirmed candidate regions  145  has been determined by the face tracking module  111 , the module  111  communicates with the sub-sampler  112  to determine when the next acquired image is to be sub-sampled, and so provided to the detector  280 , and also to provide the resolution  146  at which the next acquired image is to be sub-sampled. 
     Where the detector  280  does not run when the next image is acquired, the candidate regions  142  provided to the extractor  125  for the next acquired image will be the regions  145  confirmed by the tracking module  111  from the last acquired image. On the other hand, when the face detector  280  provides a new set of candidate regions  141  to the face tracking module  111 , these candidate regions are preferably merged with the previous set of confirmed regions  145  to provide the set of candidate regions  142  to the extractor  125  for the next acquired image. 
     Zoom information may be obtained from camera firmware. Using software techniques which analyze images in camera memory  140  or image store  150 , the degree of pan or tilt of the camera may be determined from one image to another. 
     In one embodiment, the acquisition device is provided with a motion sensor  180 , as illustrated at  FIG. 1 , to determine the degree and direction of pan from one image to another, and avoiding the processing involved in determining camera movement in software. 
     Such motion sensor for a digital camera may be based on an accelerometer, and may be optionally based on gyroscopic principals within the camera, primarily for the purposes of warning or compensating for hand shake during main image capture. U.S. Pat. No. 4,448,510, to Murakoshi, which is hereby incorporated by reference, discloses such a system for a conventional camera, and U.S. Pat. No. 6,747,690, to Molgaard, which is also incorporated by reference, discloses accelerometer sensors applied within a modern digital camera. 
     Where a motion sensor is incorporated in a camera, it may be optimized for small movements around the optical axis. The accelerometer may incorporate a sensing module which generates a signal based on the acceleration experienced and an amplifier module which determines the range of accelerations which can effectively be measured. The accelerometer may allow software control of the amplifier stage which allows the sensitivity to be adjusted. 
     The motion sensor  180  could equally be implemented with MEMS sensors of the sort which will be incorporated in next generation consumer cameras and camera-phones. 
     In any case, when the camera is operable in face tracking mode, i.e., constant video acquisition as distinct from acquiring a main image, shake compensation would typically not be used because image quality is lower. This provides the opportunity to configure the motion sensor  180  to sense large movements by setting the motion sensor amplifier module to low gain. The size and direction of movement detected by the sensor  180  is preferably provided to the face tracker  111 . The approximate size of faces being tracked is already known, and this enables an estimate of the distance of each face from the camera. Accordingly, knowing the approximate size of the large movement from the sensor  180  allows the approximate displacement of each candidate face region to be determined, even if they are at differing distances from the camera. 
     Thus, when a large movement is detected, the face tracker  111  shifts the locations of candidate regions as a function of the direction and size of the movement. Alternatively, the size of the region over which the tracking algorithms are applied may also be enlarged (and the sophistication of the tracker may be decreased to compensate for scanning a larger image area) as a function of the direction and size of the movement. 
     When the camera is actuated to capture a main image, or when it exits face tracking mode for any other reason, the amplifier gain of the motion sensor  180  is returned to normal, allowing the main image acquisition chain  105 ,  110  for full-sized images to employ normal shake compensation algorithms based on information from the motion sensor  180 . 
     An alternative way of limiting the areas of an image to which the face detector  120  is to be applied involves identifying areas of the image which include skin tones. U.S. Pat. No. 6,661,907, which is hereby incorporated by reference, discloses one such technique for detecting skin tones and subsequently only applying face detection in regions having a predominant skin color. 
     In one embodiment, skin segmentation  190  is preferably applied to a sub-sampled version of the acquired image. If the resolution of the sub-sampled version is not sufficient, then a previous image stored in image store  150  or a next sub-sampled image can be used as long as the two images are not too different in content from the current acquired image. Alternatively, skin segmentation  190  can be applied to the full size video image  130 . 
     In any case, regions containing skin tones are identified by bounding rectangles. In one embodiment of the invention, the bounding rectangles are optionally provided to the integral image generator  115 , which produces integral image patches corresponding to the rectangles in a manner similar to that used by the tracker integral image generator  115 . 
     Not only does this approach reduce the processing overhead associated with face detection, but in one embodiment of the invention, it also allows the face detector  120  to apply more relaxed face detection to the bounding rectangles, as there is a higher chance that these skin-tone regions do in fact contain a face. So, for a Viola-Jones detector  120 , a shorter classifier chain can be employed to more effectively provide similar quality results to running face detection over the whole image with longer Viola-Jones classifiers required to positively detect a face. 
     Further improvements to face detection are also contemplated in other embodiments. For example, based on the fact that face detection can be very dependent on illumination conditions, such that small variations in illumination can cause face detection to fail and cause somewhat unstable detection behavior, in another embodiment, confirmed face regions  145  are used to identify regions of a subsequently acquired sub-sampled image on which luminance correction may be performed to bring regions of interest of the image to be analyzed to the desired parameters. One example of such correction is to improve the luminance contrast within the regions of the sub-sampled image defined by confirmed face regions  145 . 
     Contrast enhancement may be used to increase local contrast of an image, especially when the usable data of the image is represented by close contrast values. Through this adjustment, intensities of pixels of a region when represented on a histogram, which would otherwise be closely distributed, can be better distributed. This allows for areas of lower local contrast to gain a higher contrast without affecting global contrast. Histogram equalization accomplishes this by effectively spreading out the most frequent intensity values. 
     The method is useful in images with backgrounds and foregrounds that are both bright or both dark. In particular, the method can lead to better detail in photographs that are over-exposed or under-exposed. 
     Alternatively, this luminance correction can be included in the computation of an “adjusted” integral image in the generators  115 . However, in one embodiment of the invention, no integral image is generated or stored. 
     In another improvement, when face detection is being used, the camera application is set to dynamically modify the exposure from the computed default to a higher values (from frame to frame, slightly overexposing the scene) until the face detection provides a lock onto a face. 
     Further embodiments providing improved efficiency for the system described above are also contemplated. For example, face detection algorithms typically employ methods or use classifiers to detect faces in a picture at different orientations: 0, 90, 180 and 270 degrees. The camera may be equipped with an orientation sensor  170 , as illustrated at  FIG. 1 . This can include a hardware sensor for determining whether the camera is being held upright, inverted or tilted clockwise or counter-clockwise. Alternatively, the orientation sensor can comprise an image analysis module connected either to the image acquisition hardware  105 ,  110  or camera memory  140  or image store  150  for quickly determining whether images are being acquired in portrait or landscape mode and whether the camera is tilted clockwise or counter-clockwise. 
     Once this determination is made, the camera orientation can be fed to one or both of the face detectors  120 ,  121 . The detectors may apply face detection according to the likely orientation of faces in an image acquired with the determined camera orientation. This feature can either significantly reduce the face detection processing overhead, for example, by avoiding the employment of classifiers which are unlikely to detect faces, or increase its accuracy by running classifiers more likely to detects faces in a given orientation more often. 
     Example Ratio-Based Feature Value Computation Technique 
       FIG. 3  is a flow diagram that illustrates an example of a ratio-based Haar-like feature value computation technique, according to an embodiment of the invention. Alternative embodiments of the invention may involve fewer, more, or different steps than those illustrated in  FIG. 3 . The technique illustrated in  FIG. 3  begins with the assumption that a machine-learning mechanism has already evaluated a set of training data and automatically determined, for each Haar-like feature in a set of Haar-like features, a normalized numerical range corresponding to that feature; thus, the illustrated technique involves the determination of whether an unlabeled image&#39;s feature values, calculated based on ratios, fall within those numerical ranges. Additional optional enhancements to the technique illustrated in  FIG. 3  will be described afterward. 
     In block  302 , a source image is produced. For example, a digital camera may capture an image through a system of lenses and sensors. The digital camera may store this image in memory as an array of pixels. Each such pixel may have a set of hue component values, such as red, green, and blue values, and an intensity value which generally indicates the brightness of that pixel regardless of its hue. 
     In block  304 , a first Haar-like feature in a list of Haar-like features is selected to be the currently calculated feature. For example, the first Haar-like feature might be the two-rectangle feature shown as adjacent rectangles  206  and  208  in  FIG. 2A . This is the first feature of several features for which values will be determined. Other features in the list of features may include adjacent rectangles of different quantities, sizes, dimensions, and positions within the source image. The features in the list may be ordered based on the order in which classifiers corresponding to those features occur in the previously generated classifier chain, such that the first feature in the list is the feature that is evaluated by the first classifier in the classifier chain, and so on. The quantity of features in the list may correspond to the quantity of classifiers in the classifier chain. 
     In block  306 , the average pixel intensity values of each of the currently calculated feature&#39;s rectangles are determined. In one embodiment of the invention, such a rectangle&#39;s average pixel intensity value is calculated by summing the intensity values of each of the source image&#39;s pixels that occurs within that rectangle, and then dividing that sum by the quantity of those pixels contained within the rectangle. In an alternative embodiment of the invention, a rectangle&#39;s average pixel intensity value may be calculated based on an integral image that was generated for the source image; however, in one embodiment of the invention, no integral image is generated or stored. The number of average pixel intensity values calculated in block  306  depends on the quantity of rectangles in the currently calculated feature. For example, the currently calculated feature might be a two-rectangle, three-rectangle, or four-rectangle feature. 
     In block  308 , a ratio of the average intensity values determined in block  306  is calculated. If the currently calculated feature is a two-rectangle feature, then the ratio is determined by dividing the average pixel intensity value of one of the feature&#39;s rectangles (e.g., the left rectangle) by the average pixel intensity value of the other of the feature&#39;s rectangles (e.g., the right rectangle). If the currently calculated feature is a three-rectangle feature, then the ratio is determined by dividing the average pixel intensity value of the feature&#39;s center rectangle by the sum of the average pixel intensity values of the feature&#39;s two outer rectangles. If the currently calculated feature is a four-rectangle feature, then the ratio is determined by dividing (a) the sum of one of the feature&#39;s quadrant pairs&#39; (e.g., the quadrant pair consisting of the feature&#39;s upper-left and lower-right rectangles) average pixel intensity values by (b) the sum of the other of the feature&#39;s quadrant pairs&#39; (e.g., the quadrant pair consisting of the feature&#39;s upper-right and lower-left rectangles) average pixel intensity values. 
     In one embodiment of the invention, the ratio is calculated by dividing one average pixel intensity value (or sum of two such values) by another average pixel intensity value (or sum of two such values). However, in an alternative embodiment of the invention which will be described further below, the ratio is instead calculated by multiplying one average pixel intensity value by a reciprocal value located in a pre-generated look-up table. 
     In block  310 , the result of the ratio calculated in block  308  is normalized mathematically. For example, the value produced by the ratio&#39;s division may be normalized such that the value lies somewhere within the range of 0 to 64. This may be accomplished, for example, by multiplying the value (which will be some number between 0 and 1) by 64. This normalized value essentially is the currently calculated feature&#39;s value. 
     In block  312 , a determination is made as to whether the normalized value calculated in block  310  falls within a numerical range that is specified by the classifier that corresponds to (and is designed to evaluate) the currently calculated feature&#39;s value. If the normalized value falls within the classifier&#39;s numerical range (thus indicating that the source image passes that classifier), then control passes to block  314 . Otherwise, the normalized value falls outside of the classifier&#39;s numerical range (thus indicating that the source image fails that classifier), and control passes to block  320 . 
     In block  314 , a determination is made as to whether the list of features (discussed above in connection with block  304 ) contains any further features that have not yet been selected to be the currently calculated feature. If the list contains at least one more feature that has not yet been selected to be the currently calculated feature, then control passes to block  316 . Otherwise, all of the features in the list have been selected to the currently calculated feature at one time or another, and control passes to block  318 . 
     In block  316 , the next feature in the ordered list of features, which has not yet been selected to be the currently calculated feature, is selected to be the currently calculated feature. Control passes back to block  306 . 
     Alternatively, in block  318 , a determination is made that the source image contains a face. Further operations appropriate to this determination, such as the display, on the digital camera&#39;s liquid crystal display (LCD), of a bounding box around the portion of the source image that contains the face, may be performed in response to this determination. Additionally, data, which indicates that the source image contains a face, may be stored in the digital camera&#39;s memory. Under such circumstances, the ratio-based feature calculation technique is completed. 
     Alternatively, in block  320 , a determination is made that the source image does not contain a face. Further operations appropriate to this determination may be performed in response to this determination. For example, data, which indicates that the source image does not contain a face, may be stored in the digital camera&#39;s memory. Under such circumstances, the ratio-based feature calculation technique is completed. 
     Avoiding Division in Ratio Calculation Using a Look-Up Table 
     When performed by a computer, division is typically a more resource-consuming operation than multiplication is. As is discussed above in connection with block  308 , in an alternative embodiment of the invention, the ratio of a feature&#39;s rectangles&#39; average pixel intensity values is instead calculated by multiplying one average pixel intensity value by a reciprocal value located in a pre-generated look-up table. In such an alternative embodiment of the invention, the real-time (i.e., post-image capture) division of such rectangles&#39; average pixel intensity values is avoided in the calculation of the ratio. 
     In one embodiment of the invention, a look-up table, which correlates discrete values (e.g., integer values) within a specified range (e.g., 0 to 255) to associated reciprocal values, is generated and stored. The look-up table only needs to be generated and stored once, but can be used multiple times. 
     In at least some digital systems, pixel intensity values are integer values that fall within the range of 0 to 255. In such digital systems, the average pixel intensity value of any of a feature&#39;s rectangles necessarily also will fall within this range of 0 to 255; an average cannot be higher than the highest value or lower than the lowest value. In one embodiment of the invention, for each key integer value between 1 and 255, inclusive, the look-up table contains an entry that associates that key integer value with a reciprocal value that is equal to 2 19  divided by that key integer value. Thus, the entry for the key integer value 1 is associated with a reciprocal value of 524288 (2 19 /1), while the entry for the key integer value 255 is associated with a reciprocal value of approximately 2056 (2 19 /255). The entry for the key integer value 0 associates that key integer value with the reciprocal value 524288 (2 19 /1), since division by zero is undefined. 
     In one embodiment of the invention, whenever a ratio of two average pixel intensity values later needs to be calculated, instead of dividing one such value by the other, the first average pixel intensity value is multiplied by the reciprocal value that is associated, in the look-up table, with the key integer value to which the second average pixel intensity value is closest (for purposes of simplicity, truncation of any decimal part of the second average may be performed to determine the key integer value to look up in the look-up table). The product of the multiplication, which will be a value in the range of 0 to 2 20 , then may be normalized to be somewhere with the range of 0 to 64, as is discussed above in connection with block  310 . This normalized value essentially is the currently calculated feature&#39;s value. 
     Hardware Overview 
     According to one embodiment, the techniques described herein are implemented by one or more special-purpose computing devices. The special-purpose computing devices can be incorporated into the digital image acquisition device described in  FIG. 1  or might be separate from the digital image acquisition described in  FIG. 1 . 
     The special-purpose computing devices may be hard-wired to perform the techniques, or may include digital electronic devices such as one or more application-specific integrated circuits (ASICs) or field programmable gate arrays (FPGAs) that are persistently programmed to perform the techniques, or may include one or more general purpose hardware processors programmed to perform the techniques pursuant to program instructions in firmware, memory, other storage, or a combination. Such special-purpose computing devices may also combine custom hard-wired logic, ASICs, or FPGAs with custom programming to accomplish the techniques. The special-purpose computing devices may be desktop computer systems, portable computer systems, handheld devices, networking devices or any other device that incorporates hard-wired and/or program logic to implement the techniques. 
     For example,  FIG. 4  is a block diagram that illustrates a computer system  400  upon which an embodiment of the invention may be implemented. Computer system  400  includes a bus  402  or other communication mechanism for communicating information, and a hardware processor  404  coupled with bus  402  for processing information. Hardware processor  404  may be, for example, a general purpose microprocessor. 
     Computer system  400  also includes a main memory  406 , such as a random access memory (RAM) or other dynamic storage device, coupled to bus  402  for storing information and instructions to be executed by processor  404 . Main memory  406  also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor  404 . Such instructions, when stored in storage media accessible to processor  404 , render computer system  400  into a special-purpose machine that is customized to perform the operations specified in the instructions. 
     Computer system  400  further includes a read only memory (ROM)  408  or other static storage device coupled to bus  402  for storing static information and instructions for processor  404 . A storage device  410 , such as a magnetic disk or optical disk, is provided and coupled to bus  402  for storing information and instructions. 
     Computer system  400  may be coupled via bus  402  to a display  412 , such as a cathode ray tube (CRT), for displaying information to a computer user. An input device  414 , including alphanumeric and other keys, is coupled to bus  402  for communicating information and command selections to processor  404 . Another type of user input device is cursor control  416 , such as a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to processor  404  and for controlling cursor movement on display  412 . This input device typically has two degrees of freedom in two axes, a first axis (e.g., x) and a second axis (e.g., y), that allows the device to specify positions in a plane. 
     Computer system  400  may implement the techniques described herein using customized hard-wired logic, one or more ASICs or FPGAs, firmware and/or program logic which in combination with the computer system causes or programs computer system  400  to be a special-purpose machine. According to one embodiment, the techniques herein are performed by computer system  400  in response to processor  404  executing one or more sequences of one or more instructions contained in main memory  406 . Such instructions may be read into main memory  406  from another storage medium, such as storage device  410 . Execution of the sequences of instructions contained in main memory  406  causes processor  404  to perform the process steps described herein. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions. 
     The term “storage media” as used herein refers to any non-transitory media that store data and/or instructions that cause a machine to operation in a specific fashion. Such storage media may comprise non-volatile media and/or volatile media. Non-volatile, non-transitory media includes, for example, optical or magnetic disks, such as storage device  410 . Volatile media includes dynamic memory, such as main memory  406 . Common forms of non-transitory storage media include, for example, a floppy disk, a flexible disk, hard disk, solid state drive, magnetic tape, or any other magnetic data storage medium, a CD-ROM, any other optical data storage medium, any physical medium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROM, NVRAM, any other memory chip or cartridge. 
     Non-transitory storage media is distinct from but may be used in conjunction with transmission media. Transmission media participates in transferring information between storage media. For example, transmission media includes coaxial cables, copper wire and fiber optics, including the wires that comprise bus  402 . Transmission media can also take the form of acoustic or light waves, such as those generated during radio-wave and infra-red data communications. 
     Various forms of media may be involved in carrying one or more sequences of one or more instructions to processor  404  for execution. For example, the instructions may initially be carried on a magnetic disk or solid state drive of a remote computer. The remote computer can load the instructions into its dynamic memory and send the instructions over a telephone line using a modem. A modem local to computer system  400  can receive the data on the telephone line and use an infra-red transmitter to convert the data to an infra-red signal. An infra-red detector can receive the data carried in the infra-red signal and appropriate circuitry can place the data on bus  402 . Bus  402  carries the data to main memory  406 , from which processor  404  retrieves and executes the instructions. The instructions received by main memory  406  may optionally be stored on storage device  410  either before or after execution by processor  404 . 
     Computer system  400  also includes a communication interface  418  coupled to bus  402 . Communication interface  418  provides a two-way data communication coupling to a network link  420  that is connected to a local network  422 . For example, communication interface  418  may be an integrated services digital network (ISDN) card, cable modem, satellite modem, or a modem to provide a data communication connection to a corresponding type of telephone line. As another example, communication interface  418  may be a local area network (LAN) card to provide a data communication connection to a compatible LAN. Wireless links may also be implemented. In any such implementation, communication interface  418  sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information. 
     Network link  420  typically provides data communication through one or more networks to other data devices. For example, network link  420  may provide a connection through local network  422  to a host computer  424  or to data equipment operated by an Internet Service Provider (ISP)  426 . ISP  426  in turn provides data communication services through the world wide packet data communication network now commonly referred to as the “Internet”  428 . Local network  422  and Internet  428  both use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on network link  420  and through communication interface  418 , which carry the digital data to and from computer system  400 , are example forms of transmission media. 
     Computer system  400  can send messages and receive data, including program code, through the network(s), network link  420  and communication interface  418 . In the Internet example, a server  430  might transmit a requested code for an application program through Internet  428 , ISP  426 , local network  422  and communication interface  418 . 
     The received code may be executed by processor  404  as it is received, and/or stored in storage device  410 , or other non-volatile storage for later execution. 
     Extensions and Alternatives 
     In this description certain process steps are set forth in a particular order, and alphabetic and alphanumeric labels may be used to identify certain steps. Unless specifically stated in the description, embodiments of the invention are not necessarily limited to any particular order of carrying out such steps. In particular, the labels are used merely for convenient identification of steps, and are not intended to specify or require a particular order of carrying out such steps. 
     In the foregoing specification, embodiments of the invention have been described with reference to numerous specific details that may vary from implementation to implementation. Thus, the sole and exclusive indicator of what is the invention, and is intended by the applicants to be the invention, is the set of claims that issue from this application, in the specific form in which such claims issue, including any subsequent correction. Any definitions expressly set forth herein for terms contained in such claims shall govern the meaning of such terms as used in the claims. Hence, no limitation, element, property, feature, advantage or attribute that is not expressly recited in a claim should limit the scope of such claim in any way. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. 
     All references cited above herein, in addition to the background and summary of the invention sections, are hereby incorporated by reference into the detailed description of the embodiments as disclosing alternative embodiments and components, and details relating to certain embodiments. The following are also incorporated by reference for these purposes: U.S. patent application Ser. Nos. 12/479,593; 12/362,399; 12/116,140; 12/354,707; 12/042,104; 11/761,647; 11/554,539; 11/464,083; 11/027,001; 10/842,244; 11/024,046; 11/233,513; 11/460,218; 11/573,713; 11/319,766; 11/464,083; 11/744,020; 11/767,412; 10/763,801; 11/573,713; 12/042,104; 11/859,164; 12/026,484; 11/861,854; and 11/460,218; and U.S. Published Patent Applications Nos. 2008/0013798; 2008/0031498; 2006/0285754; 2006/0140455; 2006/0120599; 2006/0039690; 2008/0285754; 2007/0189748; 2007/0269108; 2008/0175481; and 2005/0068452; and U.S. Pat. Nos. 7,551,755; 7,551,754; 7,471,846; 7,469,055; 7,474,341; 7,466,866; 7,515,740; 7,315,631; 7,403,643; 7,460,694; and 7,460,695.