Abstract:
A method operable in a digital image acquisition system having no photographic film is provided. The method comprises receiving a relatively low resolution image of a scene from an image stream, wherein the scene potentially includes one or more faces. At least one high quality face classifier is applied to the image to identify relatively large and medium sized face regions and at least one relaxed face classifier is applied to the image to identify relatively small sized face regions. A relatively high resolution image of nominally the same scene is received and at least one high quality face classifier is applied to the identified small sized face regions in the higher resolution version of said image.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention provides an improved method and apparatus for image processing in acquisition devices. In particular, the invention provides improved face tracking in a digital image acquisition device, such as a camera phone. 
       BACKGROUND 
       [0002]      FIG. 1  illustrates digital image acquisition apparatus, for example a camera phone. The apparatus  10  comprises an Image Signal Processor, ISP,  14 , which is in general, a general purpose CPU with relatively limited processing power. Typically, the ISP  14  is a dedicated chip or chip-set with a sensor interface  20  having dedicated hardware units that facilitate image processing including image pipeline  22 . Images acquired by an imaging sensor  16  are provided to the ISP  14  through the sensor interface  20 . 
         [0003]    The apparatus further comprises a relatively powerful host processor  12 , for example, an ARM9, which is arranged to receive an image stream from the ISP  14 . 
         [0004]    The apparatus  10  is equipped with a display  18 , such as an LCD, for displaying preview images, as well as any main image acquired by the apparatus. Preview images are generated automatically once the apparatus is switched on or only in a pre-capture mode in response to half pressing a shutter button. A main image is typically acquired by fully depressing the shutter button. 
         [0005]    Conventionally, high level image processing, such as face tracking, is run on the host processor  12  which provides feedback to the pipeline  22  of the ISP  14 . The ISP  14  then renders, adjusts and processes subsequent image(s) in the image stream based on the feedback provided by the host processor  12 , typically through an I2C interface  24 . Thus, acquisition parameters of the subsequent image in the stream may be adjusted such that the image displayed to the user is enhanced. 
         [0006]    Such acquisition parameters include focus, exposure and white balance. 
         [0007]    Focus determines distinctness or clarity of an image or relevant portion of an image and is dependent on a focal length of a lens and a capture area of the imaging sensor  16 . Methods of determining whether an image is in-focus are well known in the art. For example, if a face region is detected in an image, then given that most faces are approximately the same size and the size of the face within an acquired image, an appropriate focal length can be chosen for a subsequent image to ensure the face will appear in focus in the image. Other methods can be based on the overall level of sharpness of an image or portion of an image, for example, as indicated by the values of high frequency DCT coefficients in the image. When these are highest in the image or a region of interest, say a face region, the image can be assumed to be in-focus. Thus, by adjusting the focal length of the lens to maximize sharpness, the focus of an image may be enhanced. 
         [0008]    Exposure of an image relates to an amount of light falling on the imaging sensor  16  during acquisition of an image. Thus an under-exposed image appears quite dark and has an overall low luminance level, whereas an overexposed image appears quite bright and has an overall high luminance level. Shutter speed and lens aperture affect the exposure of an image and can therefore be adjusted to improve image quality and the processing of an image. For example, it is well known that face detection and recognition are sensitive to over or under exposure of an image and so exposure can be adjusted to optimize the detection of faces within an image stream. 
         [0009]    Due to the fact that most light sources are not 100% pure white, objects illuminated by a light source will be subjected to a colour cast. For example, a halogen light source illuminating a white object will cause the object to appear yellow. In order for a digital image acquisition apparatus to compensate for the colour cast, i.e. perform white balance, it requires a white reference point. Thus, by identifying a point in an image that should be white, for example the sclera of an eye, all other colours in the image may be compensated accordingly. This compensation information may then be utilised to determine the type of illumination under which an image should be acquired. 
         [0010]    While adjusting acquisition parameters such as those described above is useful and can improve image quality and processing, the feedback loop to the ISP  14  is relatively slow, thereby causing delays in providing the ISP  14  with the relevant information to rectify the focus, exposure and white balance of an image. This can mean that in a fast changing scene, adjustment indications provided by the host processor  12  may be inappropriate when they are made by the ISP  14  to subsequent images of the stream. Furthermore, typically most of the processing power available to the host processor  12  is required to run the face tracker application, leaving minimal processing power available for carrying out value added processing. 
         [0011]    It is desired to have an improved method of face tracking in a digital image acquisition device. 
       SUMMARY OF THE INVENTION 
       [0012]    A method is provided that is operable in a digital image acquisition system having no photographic film. A relatively low resolution image of a scene from an image stream is received. The scene includes one or more faces. At least one high quality face classifier is applied to the image to identify any relatively large sized face regions. At least one relaxed face classifier is applied to the image to identify one or more relatively small sized face regions. A relatively high resolution image of nominally the same scene is also received. At least one high quality face classifier is applied to at least one of said one or more identified small sized face regions in the higher resolution version of the image. 
         [0013]    Steps a) to c) may be performed on a first processor, while steps d) and e) may be separately performed on a second processor. Value-added applications may be performed on the high resolution image on the separate second processor. 
         [0014]    Step b) and/or step c) may include providing information including face size, face location, and/or an indication of a probability of the image including a face at or in the vicinity of the face region. A weighting may be generated based on the information. Image acquisition parameters of a subsequent image in the image stream may be adjusted based on the information. The adjusted image acquisition parameters may include focus, exposure and/or white balance. The subsequent image may be a preview image or a main acquired image, and it may be displayed to a user. 
         [0015]    A high quality face classifier may include a relatively long cascade classifier or a classifier with a relatively high threshold for accepting a face, or both. The relaxed classifier may include a relatively short cascade classifier or a classifier with a relatively low threshold for accepting a face, or both. 
         [0016]    A digital image acquisition apparatus is also provided. A first processor is operably connected to an imaging sensor. A second processor is operably connected to the first processor. The first processor is arranged to provide an acquired image to the second processor and the second processor is arranged to store the image. The first processor is arranged to apply at least one high quality face classifier to a relatively low resolution image of a scene from an image stream, the scene including one or more faces, to identify any relatively large sized face regions, and to apply at least one relaxed face classifier to the image to identify one or more relatively small sized face regions. The second processor is arranged to receive a relatively high resolution image of nominally the same scene and to apply at least one high quality face classifier to at least one identified small sized face region in the higher resolution version of the image. 
         [0017]    One or more processor-readable storage devices are provided with program code embodied therein for programming one or more processors to perform any of the methods described herein above or below. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0018]    Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0019]      FIG. 1  is a block diagram of a conventional digital image acquisition apparatus. However, certain embodiments of the invention may be combined with one or more features illustrated at  FIG. 1 . 
           [0020]      FIG. 2  is a workflow illustrating a preferred embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0021]    Face tracking for digital image acquisition devices include methods of marking human faces in a series of images such as a video stream or a camera preview. Face tracking can be used to indicate to a photographer, locations of faces in an image or to allow post processing of the images based on knowledge of the locations of the faces. Also, face tracker applications can be used in adaptive adjustment of acquisition parameters of an image, such as, focus, exposure and white balance, based on face information in order to produce improved the quality of acquired images. 
         [0022]    In general, face tracking systems employ two principle modules: (i) a detection module for locating new candidate face regions in an acquired image or a sequence of images; and (ii) a tracking module for confirming face regions. 
         [0023]    A well-known method of fast-face detection is disclosed in US 2002/0102024, incorporated by reference, hereinafter Viola-Jones. In Viola-Jones, a chain (cascade) of 32 classifiers based on rectangular (and increasingly refined) Haar features are used with an integral image, derived from an acquired image, by applying the classifiers to a sub-window within the integral image. For a complete analysis of an acquired image, this sub-window is shifted incrementally across the integral image until the entire image has been covered. 
         [0024]    In addition to moving the sub-window across the entire integral image, the sub window is also scaled up/down to cover the possible range of face sizes. It will therefore be seen that the resolution of the integral image is determined by the smallest sized classifier sub-window, i.e. the smallest size face to be detected, as larger sized sub-windows can use intermediate points within the integral image for their calculations. 
         [0025]    A number of variants of the original Viola-Jones algorithm are known in the literature, such as disclosed in U.S. patent application Ser. No. 11/464,083, which is assigned to the same assignee and in incorporated by reference. 
         [0026]    In the present embodiment, a face tracking process runs on the ISP  14  as opposed to the host processor  12 . Thus, more processing power of the host processor is available for further value added applications, such as face recognition. Furthermore, parameters of an acquired image, such as focus, exposure and white balance, can be adaptively adjusted more efficiently by the ISP  14 . 
         [0027]    As will be appreciated, face tracking applications carried out on high resolution images will generally achieve more accurate results than on relatively lower resolution images. Furthermore, tracking relatively small size faces within an image generally requires proportionally more processing than for larger faces. 
         [0028]    The processing power of the ISP  14  is of course limited, and so the arrangement of face tracking application according to the present invention is optimized to run efficiently on the ISP  14 . 
         [0029]    In the preferred embodiment, a typical input frame resolution is 160 by 120, and face sizes are categorised as small, medium or large. Medium sized and large sized faces in an image are detected by applying 14×14 and 22×22 high quality classifiers respectively, e.g. relatively long cascade classifiers or classifiers with a relatively high threshold for accepting a face. 
         [0030]    The distance of a subject face from the acquisition apparatus determines a size of the subject face in an image. Clearly, a first subject face located at a greater distance from the acquisition device than a second subject face will appear smaller. Smaller sized faces comprise fewer pixels and thus less information may be derived from the face. As such, detection of smaller sized faces is inherently less reliable even given the proportionally more processing required than for larger faces. 
         [0031]    In the preferred embodiment, small sized faces are detected with a relaxed 7×7 classifier, e.g. a short-cascade classifier or classifier with a lower threshold for accepting a face. Using a more relaxed classifier reduces the processing power which would otherwise be required to detect small sized faces. 
         [0032]    Nonetheless, it is appreciated that the application of such a relaxed classifier results in a larger number of false positives, i.e. non-face regions being classified as faces. As such, the adjustment of image acquisition parameters is applied differently in response to detection of small faces and the further processing of images is different for small faces than medium or large faces as explained below in more detail. 
         [0033]      FIG. 2  shows a workflow illustrating a preferred embodiment. 
         [0034]    On activation, the apparatus  10  automatically captures and stores a series of images at close intervals so that sequential images are nominally of the same scene. Such a series of images may include a series of preview images, post-view images, or a main acquired image. 
         [0035]    In preview mode, the imaging sensor  16  provides the ISP  14  with a low resolution image e.g. 160 by 120 from an image stream, step  100 . 
         [0036]    The ISP  14  applies at least one high quality classifier cascade to the image to detect large and medium sized faces, step  110 . Preferably, both 14×14 and 22×22 face classifier cascades are applied to the image. 
         [0037]    The ISP  14  also applies at least one relaxed face classifier to the image to detect small faces, step  120 . Preferably, a 7×7 face classifier is applied to the image. 
         [0038]    Based on knowledge of the faces retrieved from the classifiers, image acquisition parameters for a subsequent image in the stream may be adjusted to enhance the image provided to the display  18  and/or to improve processing of the image. In the preferred embodiment, knowledge of the faces retrieved from the classifiers is utilised to adjust one or more of focus, exposure and/or white balance of a next image in the image stream, step  130 . 
         [0039]    Knowledge of the faces received from the classifiers comprises information relating to the location of the faces, the size of the faces and the probability of the identified face actually being a face. U.S. patent application Ser. Nos. 11/767,412 and 60/892,883 (FN182/FN232/FN214), which are assigned to the same assignee and the present application and incorporated by reference, discusses determining a confidence level indicating the probability of a face existing at the given location. This information may be utilised to determine a weighting for each face to thereby facilitate the adjustment of the acquisition parameters. 
         [0040]    In general, a large face will comprise more information than a relatively smaller face. However, if the larger face has a greater probability of being falsely identified as a face, and/or is positioned at non-central position of the image, it could be allocated a lower weighting even than that of a relatively smaller face, positioned at a centre of the image and comprising a lower probability of being a false positive. Thus, the information derived from the smaller face could be used to adjust the acquisition parameters in preference to the information derived from the large face. 
         [0041]    In the embodiment, where only small sized faces are detected in the image, knowledge of the small faces is utilised only to adjust exposure of the next image in the stream. It will be appreciated that although the relaxed classifier passes some false positives, these do not severely adversely influence the adjustment of the exposure. 
         [0042]    Focus adjustment is not performed on the next image based on small faces, due to the fact that a lens of the apparatus will be focused at infinity for small faces and there is little to be gained from such adjustment. White balance is not adjusted for small faces because they are considered too small to retrieve any significant white balance information. Nonetheless, each of focus and white balance can be usefully adjusted based on detection of medium and large sized faces. 
         [0043]    In the preferred embodiment, once a user acquires a full-sized main image, e.g. by clicking the shutter, and this is communicated to the host, step  150 , the detected/tracked face regions are also communicated to the host processor  12 , step  140 . 
         [0044]    In alternative embodiments full-sized images may be acquired occasionally without user intervention either at regular intervals (e.g. every 30 preview frames, or every 3 seconds), or responsive to an analysis of the preview image stream—for example where only smaller faces are detected it may be desirable to occasionally re-confirm the information deduced from such images. 
         [0045]    After acquisition of a full-sized main image the host processor  12  retests the face regions identified by the relaxed small face classifier on the larger (higher resolution) main image, typically having a resolution of 320×240, or 640×480, with a high quality classifier, step  160 . This verification mitigates or eliminates false positives passed by the relaxed face classifier on the lower resolution image. Since the retesting phase is carried out on a higher resolution version of the image, the small sized faces comprise more information and are thereby detectable by larger window size classifiers. In this embodiment, both 14×14 and 22×22 face classifiers are employed for verification. 
         [0046]    Based on the verification, the main image can be adjusted for example, by adjusting the luminance values of the image to more properly illuminate a face or by adjusting the white balance of the image. Other corrections such as red-eye correction or blur correction are also improved with improved face detection. 
         [0047]    In any case, the user is then presented with a refined image on the display  18 , enhancing the user experience, step  170 . 
         [0048]    The verification phase requires minimal computation, allowing the processing power of the host processor  12  to be utilised for further value added applications, for example, face recognition applications, real time blink detection and prevention, smile detection, and special real time face effects such as morphing. 
         [0049]    In the preferred embodiment, a list of verified face locations is provided back to the ISP  14 , indicated by the dashed line, and this information can be utilised to improve face tracking or image acquisition parameters within the ISP  14 . 
         [0050]    In an alternative embodiment, the verification phase can be carried out on the ISP  14  as although verification is carried out on a higher resolution image, the classifiers need not be applied to the whole image, and as such little processing power is required. 
         [0051]    The present invention is not limited to the embodiments described above herein, which may be amended or modified without departing from the scope of the present invention as set forth in the appended claims, and structural and functional equivalents thereof. 
         [0052]    In 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. 
         [0053]    In addition, all references cited above herein, in addition to the background and summary of the invention sections themselves, and  FIGS. 1 and 2 , are hereby incorporated by reference into the detailed description of the preferred embodiments as disclosing alternative embodiments and components.