Patent Application: US-81044407-A

Abstract:
in a first exemplary embodiment of the present invention , an automated , computerized method is provided for determining an illumination flux condition in a scene . the method comprises the steps of generating and storing a sequence of images of the scene , each one of the sequence of images comprising an array of pixels and corresponding to the scene photographed in a preselected polarization direction , different from the polarization direction of other ones of the sequence of images , determining a polarization sequence vector for at least one pixel in the array , as a function of color information for the pixel in the array , among the sequence of images ; and utilizing the polarization sequence vector to determine one of a shadowed and lit illumination condition for the at least one pixel .

Description:
referring now to the drawings , and initially to fig1 , there is shown a block diagram of a computer system 10 arranged and configured to perform operations related to images . a cpu 12 is coupled to a device such as , for example , a digital camera 14 via , for example , a usb port . the digital camera 14 operates to download images stored locally on the camera 14 , to the cpu 12 . the cpu 12 stores the downloaded images in a memory 16 as image files 18 . the image files 18 can be accessed by the cpu 12 for display on a monitor 20 , or for print out on a printer 22 . alternatively , the cpu can be implemented as a microprocessor embedded in a device such as , for example , the digital camera 14 or a robot . the cpu can also be equipped with a real time operating system for real time operations related to images , in connection with , for example , a robotic operation or an interactive operation with a user . as shown in fig2 , each image file 18 comprises an n × m pixel array . each pixel , p , is a picture element corresponding to a discrete portion of the overall image . all of the pixels together define the image represented by the image file 18 . each pixel comprises a digital value corresponding to a set of color bands , for example , red , green and blue color components ( rgb ) of the picture element . the present invention is applicable to any multi - band image , where each band corresponds to a piece of the electromagnetic spectrum . the pixel array includes n rows of m columns each , starting with the pixel p ( 1 , 1 ) and ending with the pixel p ( n , m ). when displaying or printing an image , the cpu 12 retrieves the corresponding image file 18 from the memory 16 , and operates the monitor 20 or printer 22 , as the case may be , as a function of the digital values of the pixels in the image file 18 , as is generally known . in an image operation , the cpu 12 operates to analyze the rgb values of the pixels of a stored image file 18 to achieve various objectives , such as , for example , shadow detection in the subject image . a fundamental observation underlying a basic discovery of the present invention , is that an image comprises two components , material and illumination . all spectral variations in pixel values are caused by one or the other or both of these components . a method for detecting of one of these components , for example , illumination , provides a mechanism for distinguishing material or object geometry , such as object edges , from shadows caused by illumination . what is visible to the human eye upon display of a stored image file 18 by the cpu 12 , is the pixel color values caused by the interaction between specular and body reflection properties of material objects in , for example , a scene photographed by the digital camera 14 and illumination flux present at the time the photograph was taken . the illumination flux comprises an ambient illuminant and an incident illuminant . the incident illuminant is light that causes a shadow and is found outside a shadow perimeter . the ambient illuminant is light present on both the bright and dark sides of a shadow , but is more perceptible within the dark region . based upon the fundamental observation of the present invention that an image comprises two components , material and illumination , the computer system 10 can be operated to differentiate between material aspects of the image such as , for example , object edges , and illumination flux through recognition of physical properties of the illumination flux . one such physical property is a spectral shift caused by the interplay between the incident illuminant and the ambient illuminant in the illumination . when one of material and illumination is known in an image , the other can be readily deduced . the spectrum for the incident illuminant and the ambient illuminant can be different from one another . a spectral shift caused by a shadow , i . e ., a decrease of the intensity of the incident illuminant , will be substantially invariant over different materials present in a scene depicted in an image . pursuant to a feature of the present invention , this spectral shift information is detected by determination of an illuminant ratio , or a spectral ratio formed by the interplay of the incident illuminant and the ambient illuminant . a spectral ratio is a ratio based upon a difference in color or intensities between two areas of a scene depicted in an image , which may be caused by different materials , an illumination change or both . an illuminant ratio provides a basis for identifying illumination change in an image . moreover , in accordance with a further discovery relevant to the present invention , another physical property of illumination flux comprises polarization characteristics of the incident illuminant and the ambient illuminant . the polarization characteristics can be used to identify shadowed areas of a subject image . direct sunlight is typically not polarized but becomes partially polarized upon reflection from a material surface . pursuant to a feature of the present invention , an analysis is made regarding differences in polarization in light reflected from various regions of a recorded image , due to variations of the interplay of the incident illuminant and the ambient illuminant , to determine shadowed and unshadowed regions of the image . the variations of the interplay , according to a feature of the present invention , comprise differences between the polarization of the reflected incident illuminant and the polarization of the reflected ambient illuminant . referring now to fig3 , there is shown a flow chart for identifying shadowed regions of an image depicted in a sequence of image files 18 of the type shown in fig2 , as a function of polarization characteristics , according to a feature of the present invention . in step 100 , an image of a scene is recorded in a sequence of image files 18 , each one of the sequence of image files 18 corresponding to the scene photographed in a different polarization direction . the camera 14 comprises a linear response camera that is utilized to capture each of the images through a polarizer mounted on the lens of the camera 14 . for example , camera 14 comprises a canon 20d digital , 8 . 2 - megapixel auto focus , single lens reflex filter , 67 mm , quantaray circular polarizer . a linear polarizer can be used in place of a circular polarizer . a linear response is achieved by capturing a raw image for each polarization with the camera 14 , in our exemplary embodiment , the canon 20d digital . pursuant to a feature of the present invention , the polarizer is rotated through preselected angular orientations and an image file 18 is recorded for each angular orientation of the polarizer . for example , the polarizer can be oriented from 0 ° to 180 ° in increments of 10 ° with an image file 18 corresponding to each 10 ° incremental orientation . in general , overall image intensities are modulated as a function of polarizer direction . the modulation varies spatially and spectrally . in step 102 , the cpu 12 is operated such that for each pixel location ( p ( 1 , 1 ) to p ( n , m ) ( see fig2 )), the set of color values for a respective pixel location , throughout a sequence of image files 18 corresponding to a scene , as recorded at the various angular orientations of the polarizer , is organized as a vector in rgb space . rgb space corresponds to a three dimensional graph wherein the three axes define the red , green and blue values of a pixel , as discussed in respect of fig4 , below . the vector in rgb space is a polarization sequence vector p → with the intensity of a pixel location traveling along the direction of p → , as a function of the intensity variations for the respective pixel location , throughout the sequence of image files 18 corresponding to the scene . the polarizer has the effect of modulating the intensity of polarized light with a sinusoidal multiplicative factor , thus the intensity value will travel in both positive and negative directions along p → . moreover , the direction of the vector p → is a function of the illumination characteristics of the pixel location , that is , lit or shadow . thus , the direction of a polarization sequence vector p → can be used to determine whether the corresponding pixel location is in a lit portion of the scene , or a portion of the scene in shadow . estimation of the direction of p → is a three dimensional line fitting problem . an estimation of a vector direction for a set of pixel color values corresponding to a pixel location in a sequence of image files 18 of a scene at different angular orientations of the polarizer , can be achieved with standard mathematical tools such as singular value decomposition and random sample consensus ( ransac ). abdi , h . “[ 2 ] (( 2007 ). singular value decomposition ( svd ) and generalized singular value decomposition ( gsvd ). in n . j . salkind ( ed . ): encyclopedia of measurement and statistics . thousand oaks ( calif . ): sage .”. demmel , j . and kahan , w . 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( 1987 ). the truncated svd as a method for regularization . bit , 27 , 534 - 553 . horn , roger a . and johnson , charles r ( 1985 ). “ matrix analysis ”. section 7 . 3 . cambridge university press . isbn 0 - 521 - 38632 - 2 . horn , roger a . and johnson , charles r ( 1991 ). topics in matrix analysis , chapter 3 . cambridge university press . isbn 0 - 521 - 46713 - 6 . strang g ( 1998 ). “ introduction to linear algebra ”. section 6 . 7 . 3rd ed ., wellesley - cambridge press . isbn 0 - 9614088 - 5 - 5 . m . a . fischler and r . c . bolles ( june 1981 ). “ random sample consensus : a paradigm for model fitting with applications to image analysis and automated cartography ”. comm . of the acm 24 : 381 - 395 . doi : 10 . 1145 / 358669 . 358692 david a . forsyth and jean ponce ( 2003 ). computer vision , a modern approach . prentice hall . isbn isbn 0 - 13 - 085198 - 1 richard hartley and andrew zisserman ( 2003 ). multiple view geometry in computer vision , 2nd edition , cambridge university press . fig4 shows a graph plotting pixel color values in an rgb space , the color values coming from selected points of a scene upon which the graph is superimposed , three in shadow and three in lit portions of the scene . the pixel color values of each selected point form a polarization sequence vector as a function of a sequence of image files 18 containing the respective point , at different angular orientations of the polarizer , as described above . the points illustrated in the graph of fig4 where selected manually . a direction in the rgb color space for each selected point was estimated using one of the mathematical tools described above . the color of each point changes as a function of the angular orientation of the polarizer , but the direction of change is different for lit points as opposed to points in shadow . in the graph of fig4 , the polarization sequence vectors corresponding to lit points are indicated by an arrow from a lit portion of the scene , while each of the points in shadow are indicated by an arrow from a shadowed portion of the scene . referring back to fig3 , in step 104 , the cpu 12 operates , with respect to each polarization sequence vector p → to generate a normalized value of the vector | p → |. the normalized value | p → | is interpreted as an rgb value that reflects a significant difference between pixel locations in lit and shadowed areas of a scene . the cpu 12 further operates to classify each pixel location as in shadow or lit , as a function of the normalized values for the respective pixel locations . in the preceding specification , the invention has been described with reference to specific exemplary embodiments and examples thereof . it will , however , be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the claims that follow . the specification and drawings are accordingly to be regarded in an illustrative manner rather than a restrictive sense .