Abstract:
A system and method for illumination invariant change detection are provided, the system including a processor, an energy ranking unit in signal communication with the processor for extracting block coefficients for the first and second images and computing an energy difference responsive to the coefficients for a frequency energy between the first and second images, and a change detection unit in signal communication with the processor for analyzing the energy difference and detecting a scene change if the energy difference is indicative of change; and the method including receiving first and second images, extracting block coefficients corresponding to frequency energies for the first and second images, computing an energy difference for at least one of the frequency energies between the first and second images, analyzing the at least one energy difference, and detecting a scene change if the energy difference is indicative of change.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Application Ser. No. 60/549,457, filed Mar. 2, 2004 and entitled “Illumination Invariant Change Detection in the Feature Space, Illustration in DCT domain”, which is incorporated herein by reference in its entirety. 

   BACKGROUND 
   In image processing and surveillance applications, for example, change detection is often desired for automatically detecting object changes in a scene. Unfortunately, changes in illumination may be misinterpreted as object changes by the automated systems, thus requiring human intervention and additional time. Accordingly, what is desired is illumination invariant change detection. 
   SUMMARY 
   These and other drawbacks and disadvantages of the prior art are addressed by an exemplary system and method for illumination invariant change detection. 
   An exemplary system for illumination invariant change detection includes a processor, an energy ranking unit in signal communication with the processor for extracting block coefficients for the first and second images and computing an energy difference responsive to the coefficients for a frequency energy between the first and second images, and a change detection unit in signal communication with the processor for analyzing the energy difference and detecting a scene change if the energy difference is indicative of change. 
   An exemplary method for illumination invariant change detection includes receiving first and second images, extracting block coefficients corresponding to frequency energies for the first and second images, computing an energy difference for at least one of the frequency energies between the first and second images, analyzing the at least one energy difference, and detecting a scene change if the energy difference is indicative of change. 
   These and other aspects, features and advantages of the present disclosure will become apparent from the following description of exemplary embodiments, which is to be read in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present disclosure teaches a system and method for illumination invariant change detection in accordance with the following exemplary figures, in which: 
       FIG. 1  shows a schematic diagram of a system for illumination invariant change detection in accordance with an illustrative embodiment of the present disclosure; 
       FIG. 2  shows a flow diagram of a method for illumination invariant change detection in accordance with an illustrative embodiment of the present disclosure; 
       FIG. 3  shows a schematic diagram of a coefficient transformation matrix in accordance with an illustrative embodiment of the present disclosure; 
       FIG. 4  shows graphical diagrams of image data with illumination and actual scene changes in accordance with an illustrative embodiment of the present disclosure; 
       FIG. 5  shows graphical diagrams of exemplary rankings for DC coefficients in accordance with an illustrative embodiment of the present disclosure; 
       FIG. 6  shows graphical diagrams of results for different energy rankings in accordance with an illustrative embodiment of the present disclosure; 
       FIG. 7  shows graphical diagrams of night image data with illumination and actual scene changes in accordance with an illustrative embodiment of the present disclosure; and 
       FIG. 8  shows a flow diagram of a method for illumination invariant change detection in accordance with an illustrative embodiment of the present disclosure. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Embodiments of the present disclosure determine whether an apparent change in imagery is merely due to illumination or due to an actual change within the scene. Exemplary method embodiments for illumination invariant change detection work directly in the discrete cosine transform (DCT) or other compressed domain to save the cost of decompression. The illumination change may be treated as a local contrast change, treated with a nonparametric ranking of the DCT coefficients, and/or treated by ranking only DCT coefficient extremes. 
   As shown in  FIG. 1 , a system for illumination invariant change detection, according to an illustrative embodiment of the present disclosure, is indicated generally by the reference numeral  100 . The system  100  includes at least one processor or central processing unit (CPU)  102  in signal communication with a system bus  104 . A read only memory (ROM)  106 , a random access memory (RAM)  108 , a display adapter  110 , an I/O adapter  112 , a user interface adapter  114 , a communications adapter  128 , and an imaging adapter  130  are also in signal communication with the system bus  104 . A display unit  116  is in signal communication with the system bus  104  via the display adapter  110 . A disk storage unit  118 , such as, for example, a magnetic or optical disk storage unit is in signal communication with the system bus  104  via the I/O adapter  112 . A mouse  120 , a keyboard  122 , and an eye tracking device  124  are in signal communication with the system bus  104  via the user interface adapter  114 . An imaging device  132  is in signal communication with the system bus  104  via the imaging adapter  130 . 
   An energy ranking unit  170  and a change detection unit  180  are also included in the system  100  and in signal communication with the CPU  102  and the system bus  104 . While the energy ranking unit  170  and the change detection unit  180  are illustrated as coupled to the at least one processor or CPU  102 , these components are preferably embodied in computer program code stored in at least one of the memories  106 ,  108  and  118 , wherein the computer program code is executed by the CPU  102 . 
   Turning to  FIG. 2 , a method for illumination invariant change detection, according to an illustrative embodiment of the present disclosure, is indicated generally by the reference numeral  200 . The method  200  includes a start block  210  that passes control to an input block  212 . The input block  212  receives a first image and passes control to a function block  214 . The function block  214  extracts the 8 by 8 block coefficients for the first image and passes control to an input block  216 . The input block  216  receives a second image and passes control to a function block  218 . The function block  218  extracts the 8 by 8 block coefficients for the second image and passes control to a function block  220 . 
   The function block  220  computes the energy scale between the two images using the first DCT coefficients, and passes control to a function block  222 . The function block  222 , in turn, computes the map of the sum of the energy difference of each radial frequency, and passes control to a decision block  224 . The decision block  224  determines whether the frequency structure has changed, and if so, passes control to a function block  226 . If not, the decision block  224  passes control to an end block  228 . The function block  226  detects a scene change and passes control to the end block  228 . 
   Turning now to  FIG. 3 , an 8 by 8 block coefficient transformation matrix is indicated generally by the reference numeral  300 . The matrix  300  has an origin at the upper left, with vertical frequency increasing towards the lower portion and horizontal frequency increasing towards the right portion. The matrix  300  includes block coefficients  0  through  63 , where the coefficients  15  through  20  form are arranged in a zigzag pattern as used in the Joint Photographic Experts Group (JPEG) standard, for example, which improves Run Length Coded (RLC) compression. 
   As shown in  FIG. 4 , image data is indicated generally by the reference numeral  400 . In a first image  410 , a light is switched off. The first image  410  includes a floppy box  412  and a cup  414 . In a second image  420 , the light is switched on. The second image  420  includes a floppy box  422  showing only a difference in illumination compared to the floppy box  412 . There is no cup present in the second image  420  in order to show removal of that object from the scene. The image  430  shows the computed energy differences between the images  410  and  420 , including higher energy rankings  434  for the cup. Thus, embodiments of the present disclosure can detect the scene change comprising the missing cup while recognizing that the floppy box  422  is the same floppy box  412  with a change in illumination. 
   Turning to  FIG. 5 , exemplary rankings of the DC coefficients are indicated generally by the reference numeral  500 . Here, a ranking  510  results from a sum-squared difference of ranks with higher rankings  514  for the cup, a ranking  520  results from a Spearman rank-order correlation with higher rankings  524  for the cup, and a ranking  530  results from Kendall&#39;s Tau ranking with higher rankings  534  for the cup. 
   Turning now to  FIG. 6 , results of different energy rankings are indicated generally by the reference numeral  600 . A first ranking  610  results when an energy comparison is done for all of the energies, and includes results  612  for the box and  614  for the cup. A second ranking  620  results when an energy comparison is done between energies with the same radial frequency and includes results  622  for the box and  624  for the cup. Here, note that ranking of all the energies, as in the ranking  610 , is less noisy than ranking inside each radial frequency, as in the ranking  620 . 
   A third ranking  630  results when an energy comparison is done between radial frequency energies and includes results  612  for the box and  614  for the cup. Note that due to the quantification, only the first frequencies are not null. This provides a very fast algorithm because the energy comparison is done for much less than the pixel number. For example, in most of the test cases only the first four radial energies are not null, which leads to less than 10 energy comparisons. 
   As shown in  FIG. 7  night images are indicated generally by the reference numeral  700 . In a first image  710 , a car  714  is present at dusk. In a second image  720 , the car is missing after dark. The car  714  of the first image is missing in the second image  420  in order to show removal of that object from the scene. The image  730  shows the computed map of energy differences between the images  710  and  720  using Kendall&#39;s Tau operator, including higher energy rankings  734  for the missing car. Due to the high level of gain, the camera noise is very high. Therefore, ranking all the energies is too noisy. The image  740  shows the computed map of energy differences between the images  710  and  720  using extremes ranking, including higher energy rankings  744  for the missing car. Thus, embodiments of the present disclosure can detect the scene change comprising the missing car while recognizing that other apparent changes are merely due to a change in illumination. 
   Turning to  FIG. 8 , another method embodiment for illumination invariant change detection, according to an illustrative embodiment of the present disclosure, is indicated generally by the reference numeral  800 . The method  800  includes a start block  810  that passes control to an input block  812 . The input block  812  receives a first image and passes control to a function block  814 . The function block  814  extracts the 8 by 8 block coefficients for the first image and passes control to an input block  816 . The input block  816  receives a second image and passes control to a function block  818 . The function block  818  extracts the 8 by 8 block coefficients for the second image and passes control to a function block  820 . 
   The function block  820  sorts the DCT energy coefficients for each of the two images, and passes control to a function block  822 . The function block  822  ranks the energy, all or partially, between the images. For example, the ranking may be for all energies or just for extremes in alternate embodiments. The function block  822 , in turn, passes control to a decision block  824 . The decision block  824  determines whether the frequency structure has changed, and if so, passes control to a function block  826 . If not, the decision block  824  passes control to an end block  828 . The function block  826  detects a scene change and passes control to the end block  828 . 
   In operation, an exemplary method embodiment works in the Discrete Cosine Transformation (DCT) domain, where the DCT formula is given by Equation 1. 
   
     
       
         
           
             
               
                 
                   
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   In the case of JPEG compression, the variable N of Equation 1 is equal to 8, which yields an 8 by 8 block transformation as introduced above in the matrix  300  of  FIG. 3 . In the JPEG standard, the coefficients are arranged in this zigzag pattern. This type of coefficient reorganization improves the RLC compression. 
   One embodiment treats illumination as a local contrast change. For a given diagonal, such as the coefficients  15 - 20  of  FIG. 3 , a constant radial frequency with different orientation is obtained. This property is used to detect whether a change between two images is due to an illumination change or due to a scene change. In the case of an illumination change, the frequency structure does not change. Most of the time, the illumination change is only reflected in the DC coefficient of the transformation and in a resealing of the AC energies. Thus, the radial frequency energy may be used for more robustness. 
   Referring back to  FIG. 2 , the exemplary algorithm includes extraction of the 8 by 8 block coefficients for both image  1  and image  2 , computation of the energy scale between the two images using the first DCT coefficient as set forth by Equation 2, and computation of the map of the sum of the energy difference of each radial frequency as set forth by Equation 3. 
   
     
       
         
           
             
               
                 
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   Referring back to  FIG. 4  showing an exemplary application, the first image  410  has no added lighting while the second image  420  has a light switched on. Here, the floppy box  422  shows a difference in illumination compared to the floppy box  412 . An object, namely the cup  414 , constitutes an actual scene difference since it is absent from the second image  420  to show how the algorithm detects a scene change using the energy map  430 . 
   In operation of another exemplary embodiment, illumination change detection is done with a ranking approach. Such an approach may use a nonparametric correlation. Nonparametric correlation of the energies is used to estimate the correlation between the two images. All nonparametric correlations are applicable. For simplicity of discussion, but without loosing generality, a method is described using a sum-squared difference of ranks, but alternate embodiments may use a Spearman rank-order correlation or Kendall&#39;s Tau ranking. 
   Referring back to  FIG. 5  and the exemplary ranking of the DC coefficients  510 , Ri(1) is the rank of ei1 or the ith energy of the first image, Ri(2) is the rank of ei2 or the ith energy of the second image, and one possible ranking of the 8 by 8 energy matrix can be the zigzag order used in JPEG compression. Then the sum-squared difference of ranks is given by Equation 4. 
                 D   =       ∑   i     ⁢       (       R   i     (   1   )       -     R   i     (   2   )         )     2               (     Equation   ⁢           ⁢   4     )               
A Spearman Rank-Order Correlation Coefficient is given by Equation 5.
 
   
     
       
         
           
             
               
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   Another exemplary embodiment uses extreme ranking. Referring back to  FIG. 7 , night images were described. Due to the high level of gain, the camera noise is very high. Thus, in this case, ranking all of the energies is too noisy. 
   For an application such as determination of whether an apparent change is an illumination change or a scene change, the algorithm can use the fact that it expects the same scene in many embodiments. In those cases, it may rank only the extremes. The two energies that work the most in opposition are used, that is, the two highest energies of opposite sign. These two opposite energies describe a large part of the image structure and are robust to the illumination changes and high frequency noise. The quantity measured is the difference between these two energies. 
   If the image pixels follow a Gaussian distribution, the DCT transformation coefficients also follow a Gaussian distribution as a sum of Gaussians. If e-hat is the observed energy value, the true value e can be approximated by N(e-hat, sigma-squared-sub-N-sub-e-hat), and the difference between the two selected energy at time t is given by Equation 6.
 
 d   t   =e   1   t   −e   2   t    (Equation 6)
 
   Using the observed energy value, the approximation is given by Equation 7. 
   
     
       
         
           
             
               
                 
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 p   1   =P ( d   1 ≧0)
 
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 D =√{square root over ( p   1   p   2 )}+√{square root over ((1− p   1 )(1− p   2 ))}{square root over ((1− p   1 )(1− p   2 ))}  (Equation 9)
 
   D measures the concurrence in ordering. Thus, if D is close to 1, the ordering is highly preserved; while if D is close to 0, the ordering is not consistent between the frames. 
   Referring back to  FIG. 6 , results using different energy rankings are indicated, where the first energy comparison  610  is done for all of the energies; the second ranking  620  is done between energies with the same radial frequency; and the third ranking  630  is done between radial energies. Due to the quantification, only the first frequencies are not null, which gives a very fast algorithm. 
   In an alternate embodiment, a DCT transformation may be used for non-compressed data. The present teachings may then be applied to the transformed data as discussed above. 
   In another alternate embodiment, a multi-scale approach provides great stability and a quick labialization. For non-compressed data, the method builds the image pyramid and process. For compressed data, the pyramid construction can be done in two ways, by uncompressing the data or by building the pyramid from the DCT coefficient. In the DCT coefficient case, the second level is built directly, and the DCT transformation is performed. 
   Building a three-dimensional (3D) DCT, where the three dimensions include 2D DCT space and time, and estimating its statistic is straightforward. The linearity of the DCT transformation leads to a simple way to compute the correlation between the coefficients. Thus, one can estimate its statistic starting from the image pixels statistic. This 3D DCT can be used for applications involving change detection on dynamic backgrounds, for example. 
   In alternate embodiments of the apparatus  100 , some or all of the computer program code may be stored in registers located on the processor chip  102 . In addition, various alternate configurations and implementations of the energy ranking unit  170  and the change detection unit  180  may be made, as well as of the other elements of the system  100 . In addition, the methods of the present disclosure can be performed in color or in gray level. 
   It is to be understood that the teachings of the present disclosure may be implemented in various forms of hardware, software, firmware, special purpose processors, or combinations thereof. Most preferably, the teachings of the present disclosure are implemented as a combination of hardware and software. 
   Moreover, the software is preferably implemented as an application program tangibly embodied on a program storage unit. The application program may be uploaded to, and executed by, a machine comprising any suitable architecture. Preferably, the machine is implemented on a computer platform having hardware such as one or more central processing units (CPU), a random access memory (RAM), and input/output (I/O) interfaces. 
   The computer platform may also include an operating system and microinstruction code. The various processes and functions described herein may be either part of the microinstruction code or part of the application program, or any combination thereof, which may be executed by a CPU. In addition, various other peripheral units may be connected to the computer platform such as an additional data storage unit and a printing unit. 
   It is to be further understood that, because some of the constituent system components and methods depicted in the accompanying drawings are preferably implemented in software, the actual connections between the system components or the process function blocks may differ depending upon the manner in which the present disclosure is programmed. Given the teachings herein, one of ordinary skill in the pertinent art will be able to contemplate these and similar implementations or configurations of the present disclosure. 
   Although the illustrative embodiments have been described herein with reference to the accompanying drawings, it is to be understood that the present disclosure is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one of ordinary skill in the pertinent art without departing from the scope or spirit of the present disclosure. All such changes and modifications are intended to be included within the scope of the present disclosure as set forth in the appended claims.