Abstract:
A method for scene classification of a digital image comprising the steps of: (a) extracting predetermined camera metadata tags from the digital image; (b) obtaining estimates of image class based on the extracted metadata tags; (c) obtaining estimates of image class based on image content; and (d) producing a final estimate of image class based on a combination of metadata-based estimates and image content-based estimates.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention is related to image processing, and in particular to image classification using camera and content-based cues.  
       BACKGROUND OF THE INVENTION  
       [0002]     Automatically determining the semantic classification (e.g., indoor, outdoor—sunset, picnic, beach) of an arbitrary image is a difficult problem. Much research has been done recently, and a variety of classifiers and feature sets have been proposed. The most common design for such systems has been to use low-level features (e.g., color, texture) and statistical pattern recognition techniques. Such systems are exemplar-based, relying on learning patterns from a training set. Examples are M. Szummer and R. W. Picard, “Indoor-outdoor image classification”, in  Proceedings of IEEE Workshop on Content - based Access of Image and Video Databases,  1998, and A. Vailaya, M. Figueiredo, A. Jain, and H. J. Zhang, “Content-based hierarchical classification of vacation images”, in  Proceedings of IEEE International Conference on Multimedia Computing and Systems,  1999.  
         [0003]     Semantic scene classification can improve the performance of content-based image organization and retrieval (CBIR). Many current CBIR systems allow a user to specify an image and search for images similar to it, where similarity is often defined only by color or texture properties. This so-called “query by example” has often proven to be inadequate due to its simplicity. Knowing the category of a scene a priori helps narrow the search space dramatically. For instance, knowing what constitutes a party scene allows us to consider only party scenes in our search to answer the query “Find pictures of Mary&#39;s birthday party”. This way, the search time is reduced, the hit rate is higher, and the false alarm rate is expected to be lower.  
         [0004]     Classification of unconstrained consumer images in general is a difficult problem. Therefore, it can be helpful to use a hierarchical approach, in which classifying images into indoor or outdoor images occurs at the top level and is followed by further classification within each subcategory, as suggested by Vailaya et al.  
         [0005]     Still, current scene classification systems often fail on unconstrained image sets. The primary reason appears to be the incredible variety of images found within most semantic classes. Exemplar-based systems must account for such variation in their training sets. Even hundreds of exemplars do not necessarily capture all of the variability inherent in some classes.  
         [0006]     Consequently, a need exists for a method that overcomes the above-described deficiencies in image classification.  
         [0007]     While the advent of digital imaging created an enormous number of digital images and thus the need for scene classification (e.g., for use in digital photofinishing and in image organization), it also brings with it a powerful source of information little-exploited for scene classification: camera metadata embedded in the digital image files. Metadata (or “data about data”) for cameras includes values such as date/time stamps, presence or absence of flash, exposure time, and aperture value. Most camera manufacturers today store metadata using the EXIF (EXchangeable Image File Format) standard (http://www.exif.org/specifications.html).  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, the invention resides in a method for using of camera metadata for scene classification, where the method comprises the steps of: (a) extracting pre-determined camera metadata tags from a digital image; (b) obtaining estimates of image class based on the extracted metadata tags, thereby providing a metadata-based estimate; (c) obtaining estimates of image class based on image content, thereby providing an image content-based estimate; and (d) producing a final estimate of image class based on a combination of the metadata-based estimate and the image content-based estimate.  
         [0009]     The present invention provides a method for image classification having the advantage of (1) robust image classification by combining image content and metadata when some or all of the useful metadata is available using a Bayesian inference engine, and (2) extremely fast image classification by using metadata alone (which can be retrieved and processed using negligible computing resources) and without any content-based cues.  
         [0010]     These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a diagram illustrating elements of a method for practicing the present invention.  
         [0012]      FIG. 2  shows an example distribution of exposure times (ET) of indoor and outdoor scenes, where exposure times over 1/45 (0.022) second are more likely to be indoor scenes, because of lower lighting.  
         [0013]      FIG. 3  shows an example distribution of aperture (AP) of indoor and outdoor scenes.  
         [0014]      FIG. 4  shows an example distribution of scene energy (SE) of indoor and outdoor scenes.  
         [0015]      FIG. 5  shows an example distribution of subject distance (SD) of indoor and outdoor scenes, where the large peak for outdoor scenes occurs at infinity (long-range scenery images).  
         [0016]      FIG. 6  shows an example of the Bayesian Network. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]     The present invention will be described as implemented in a programmed digital computer. It will be understood that a person of ordinary skill in the art of digital image processing and software programming will be able to program a computer to practice the invention from the description given below. The present invention may be embodied in a computer program product having a computer readable storage medium such as a magnetic or optical storage medium bearing machine readable computer code. Alternatively, it will be understood that the present invention may be implemented in hardware or firmware.  
         [0018]     The present invention describes the use of camera metadata for scene classification, and in particular a preferred embodiment for solving the problem of indoor-outdoor scene classification. It is also demonstrated that metadata alone (which can be retrieved and processed using negligible computing resources) can be used as an “Ultra-Lite” version of the indoor-outdoor scene classifier, and can obtain respectable results even when used alone (without any content-based cues). A preferred inference engine (a Bayesian network) is used to combine evidence from a content-based classifier and from the metadata, which is especially useful when some or all of the metadata tags are missing.  
         [0019]     Classification of unconstrained consumer images in general is a difficult problem. Therefore, it can be helpful to use a hierarchical approach, in which classifying images into indoor or outdoor images occurs at the top level and is followed by further classification within each subcategory (See A. Vailaya, M. Figueiredo, A. Jain, and H. J. Zhang, “Content-based hierarchical classification of vacation images”, in  Proceedings of IEEE International Conference on Multimedia Computing and Systems,  1999). In the present invention, a baseline content-based classifier in-house for indoor/outdoor classification ( 10 C) is implemented as described by Serreno at al. (See N. Serrano, A. Savakis, and J. Luo, “A Computationally Efficient Approach to Indoor/Outdoor Scene Classification”, in  Proceedings of International Conference on Pattern Recognition,  2002). Briefly summarized, a plurality of color and textures features are first extracted from image sub-blocks in a 4×4 tessellation and then used as the input to a Support Vector Machine which generates estimates for individual sub-blocks, and these estimates are combined to provide an overall classification for the entire image as either an indoor or outdoor image.  
         [0020]     In general, most digital cameras encode metadata in the header of the Exif file. Among the metadata tags, and of potential interest to scene classification, are DateTime, FlashUsed, FocalLength (FL), ExposureTime (ET), ApertureFNumber (AP), (Subject) Distance, ISOequivalent, BrightnessValue (BV), SubjectDistanceRange (SD), and Comments. A large body of research is concerned with the combination of text (e.g., Comments and key word annotations) and image retrieval (See, for example, Y. Lu, C. Hu, X. Zhu, H. J. Zhang, and Q. Yang, “A unified framework for semantics and feature based relevance feedback in image retrieval systems”, in  ACM Multimedia Conference , Los Angeles, Calif., October 2000), which, however, are not the subject of the present invention.  
         [0021]     Other metadata fields appear to discern certain scene types, even if weakly. For example, flash tends to be used more frequently on indoor images than on outdoor images. Because sky is brighter than indoor lighting, the exposure time on outdoor images is often shorter than on indoor images. In general, only outdoor images can have a large subject distance. Sunset images tend to have a brightness value within a certain range, distinct from that of mid-day sky or of artificial lighting. It is clear that some tags will be more useful than others for a given problem. In a preferred embodiment of the present invention, the tags that are most useful for the problem of indoor-outdoor scene classification are identified through statistical analysis.  
         [0022]     Other metadata can be derived from the recorded metadata. For instance, Moser and Schroder (See S. Moser and M. Schroder, “Usage of DSC meta tags in a general automatic image enhancement system”, in  Proceedings of International Symposium on Electronic Imaging,  2002) defined scene pseudo-energy to be proportional to  
         ln   ⁡     (     t     f   2       )       ,       
 
 for exposure time t and aperture f-number f. Scene energy was proposed as a metric highly correlated with scene types and different illuminations. Note that Moser and Schroder do not teach scene classification in general using metadata. They use metadata, and metadata only, to decide what proper image enhancement process to apply. 
 
         [0023]     Three families of tags useful for scene classification in general and indoor-outdoor scene classification in particular are categorized in the following: 
        Distance (subject distance, focal length). With few exceptions, only outdoor scenes contain large distances. While less direct and less intuitive than subject distance, focal length is related to distance (in the camera&#39;s auto-focus mode); however, it would be expected to be far less reliable, because although the zoom-in function is more likely to be used for distant outdoor objects, it is also used for close-ups in indoor pictures; zoom-out is used with equal likelihood for both indoor and outdoor occasions to expand the view.     Scene Brightness (exposure time, aperture, brightness value, shutter speed). Overall, outdoor scenes are brighter than indoor scenes, even under overcast skies, and therefore have a shorter exposure time, a smaller aperture, and a larger brightness value. The exception to this, of course, is night outdoor scenes (which arguably should be treated as indoor scenes for many practical applications).     Flash. Because of the lighting differences described above, (automatic) camera flash is used on a much higher percentage of images of indoor scenes than of outdoor scenes.        
 
         [0027]     Statistics of various metadata tags, comparing distributions over indoor images with those over outdoor images, are described here. The statistics are presented as probabilities: proportions of images of each type that take on a given certain metadata value.  FIG. 5  shows the distribution of subject distance (SD). Most indoor scenes have a distance of between 1-3 meters, while outdoor scenes have a relatively flat distribution of distances, except for a peak at infinity, corresponding to long-range scenery images.  
         [0028]      FIG. 2  shows the distributions of exposure times (ET). Those over 1/45 (0.022) second are more likely to be indoor scenes, because of lower lighting. However, extremely long exposure times (over 1 second) are usually night scenes with the exposure time set manually.  FIG. 3  shows the distribution of aperture values (AP), which appear to be less discriminatory than other tags.  FIG. 4  shows the distribution of scene energy (SE) as a function of exposure time and f-number (defined by Moser and Schroder). Note that scene energy does not appear to be as good a feature for discriminating indoor scenes from outdoor scenes as, for example, exposure time.  
         [0029]     Table 1 presents typical camera flash statistics. It is clear that flash is a strong cue for indoor-outdoor scene classification.  
                                 TABLE 1                           Distribution of flash.                Class   P(on | scene class)   P(off | scene class)                       Indoor   0.902   0.098           Outdoor   0.191   0.809                      
 
         [0030]     Scene brightness and exposure time, in particular, are highly correlated to the illuminants present in the captured scenes. The choice of metadata tags in the preferred embodiment is largely motivated by this physical property of illuminant and the apparent separabilities shown by these plots.  
         [0031]     A Bayesian network is a robust method for combining multiple sources of probabilistic information (See, for example, J. Luo and A. Savakis, “Indoor vs. outdoor classification of consumer photographs using low-level and semantic features”, in  IEEE International Conference on Image Processing , Thessaloniki, Greece, October 2001). In the preferred embodiment of the present invention, a Bayesian net of the topology shown in  FIG. 6  is used to fuse low-level image cues  610  and metadata cues  630 . The low-level input is pseudo-probabilistic, generated by applying a sigmoid function to the output of the low-level scene classifier (e.g., a Support Vector Machine Classifier, see Serrano at al.). The metadata input is either binary (e.g., flash fired) or discrete (e.g., exposure time is divided into discrete intervals, and the exposure time for a single test image falls into exactly one of those intervals).  
         [0032]     Referring again to  FIG. 6 , scene classification of an image into either indoor or outdoor is achieved at the root node  600  once the Bayesian network is settled after belief propagation. There are three types of potential evidences (cues), namely low-level cues  610 , semantic cues  620 , and metadata cues  630 , that can contribute to the final scene classification. Examples of low-level image features  610  include “color”  611  and “texture”  612 . Examples of semantic cues  620  include “sky”  621  and “grass”  622 , which are strong indicators of outdoor scenes. The corresponding broken lines related to semantic features  621  and  622  simply indicate that semantic features are not used in the preferred embodiment of the present invention because it would be a natural extension.  FIG. 6  shows only a few of the potential input cues that could be used for metadata, i.e., “subject distance”  631 , “flash fired”  632 , and “exposure time”  633 . For indoor-outdoor scene classification, they are the best of the categories discussed previously. If used, nodes for other metadata, such as the aforementioned “brightness value” or “scene energy”, would be siblings of the existing metadata nodes.  
         [0033]     Bayesian networks are very reliable in the presence of (either partially or completely) missing evidence. This is ideal when dealing with metadata, because some tags, e.g., subject distance, are often not given a value by many camera manufacturers.  
         [0034]     There are a few issues related to the proper combination of multiple cues. First, combining multiple cues of the same category (e.g. brightness value, exposure time, and scene energy) would hurt the classifiers&#39; accuracy due to the violation of the conditional independence necessary for Bayesian networks. Second, the most reliable cues, when used in combination, appear to be exposure time, flash, and subject distance, in that order. Third, combining multiple cues from different categories (e.g., exposure time and flash) does improve accuracy. In practice, the highest accuracy is achieved when using exactly one (the best) of each of the cue types (exposure time, flash, and subject distance).  
         [0035]     While the low-level cues were less accurate in general and the camera metadata cues were more reliable, combining low-level and metadata cues gave the highest accuracy.  
         [0036]     In practice, not all cameras store metadata and among those that do, not all the useful metadata tags are available. Therefore, a more accurate measure of performance of the combined system should take missing metadata into account. Table 2 shows example statistics on the richness of the metadata that is currently typically available in the market.  
                                           TABLE 2                           Availability of metadata tags.                Percentage of Entire Data   Percentage of those images       Category   set   with any metadata                    Any metadata   71%   100%       Exposure time   70%   98%       Flash   71%   100%       Flash (strength)   32%   45%       Subject Distance   22%   30%       Brightness   71%   100%       Date and Time   69%   96%                  
 
         [0037]     Using the same data set but simulating the actual availability of metadata according to Table 2, the overall accuracy increase is about 70% of the best-case scenario (with all tags). This is a more realistic estimate of how the method might do with general consumer images, because metadata is not yet fully supported by all camera manufacturers.  
         [0038]      FIG. 1  shows a diagram of the method for scene classification of a digital image using camera and content-based cues according to the invention. Initially, an input image  10  is provided. The input image is processed  20  to extract metadata and image data. The image data  30  and the metadata  80  will be processed separately in two paths. If it is decided that there is a need to use scene content for image classification in step  40 , a plurality of image features, such as color, texture or even semantic features, are extracted directly from the image data  30  in step  50 . Content-based scene classification is performed in step  60  using the image-based features and a trained classifier such as a support vector machine. Otherwise if there is no need to use scene content for classification, a “null” estimate is generated in step  70 . A “null” estimate has no effect on a subsequent integrating scene classification step  140 . In the meantime, if pre-determined metadata tags are found to be available in step  90  among the extracted metadata  80 , they are extracted in step  100  and then used to generate metadata-based scene classification estimates in step  110 . Otherwise, a “null” estimate is generated in step  120 . Again, a “null” estimate has no effect on the subsequent integrating scene classification step  140 . The estimates from both the image data path and the metadata path are combined to produce an integrated scene classification  150  in the integrating scene classification step  140  according to the invention. In a preferred embodiment of the present invention, a pre-determined (trained) Bayesian network  130  is used to perform the integration. As indicated by the broken lines connecting the “null” estimate generation steps of 70 and 120, the method according to the present invention can allow either one of the processing paths to be missing (e.g., metadata), or turned off (e.g., content-based classification) for speed and accuracy reasons, within a unified system.  
         [0039]     As mentioned in the Background section, scene classification can improve the performance of image-based systems, such as content-based image organization and retrieval. Scene classification can also find application in image enhancement. Rather than applying generic color balancing and exposure adjustment to all scenes, we could customize them to the scene, e.g., retaining or boosting brilliant colors in sunset images while removing warm-colored cast from tungsten-illuminated indoor images. For instance, a method for image enhancement of a digital image according to the present invention could include the steps of: (a) performing scene classification of the digital image into a plurality of scene classes based on image feature and metadata; and (b) applying a customized image enhancement procedure in response to the scene class of the digital image. Thereupon, in a given situation wherein the image enhancement is color balancing, the customized image enhancement procedure could include retaining or boosting brilliant colors in images classified as sunset scenes and removing warm-colored cast from indoor images classified as tungsten-illuminated scenes.  
         [0040]     The invention has been described with reference to a preferred embodiment. However, it will be appreciated that variations and modifications can be effected by a person of ordinary skill in the art without departing from the scope of the invention.  
       Parts List  
       [0000]    
       
           10  original input digital image  
           20  extracting metadata and image step  
           30  image data  
           40  deciding to use scene content for classification step  
           50  extracting image features step  
           60  content-based scene classification step  
           70  generating null estimate step  
           80  metadata  
           90  deciding pre-determined metadata availability step  
           100  extracting metadata step  
           110  metadata-based scene classification step  
           120  generating null estimate step  
           130  Bayesian network  
           140  integrating scene classification step  
           150  final scene classification  
           600  root node of the Bayesian network  
           610  low-level features node  
           611  color feature node  
           612  texture feature node  
           620  semantic features node  
           621  “sky” feature node  
           622  “grass” feature node  
           630  metadata features node  
           631  “subject distance” feature node  
           632  “flash fired” feature node  
           633  “exposure time” feature node