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Methods and apparatus including computer program products for combining source images to form a final image, each image being a digital image defined by color component values and a transparency value. The invention performs the steps of receiving two or more source images, each defined by color com

Methods and apparatus including computer program products for combining source images to form a final image, each image being a digital image defined by color component values and a transparency value. The invention performs the steps of receiving two or more source images, each defined by color component and alpha information; assembling a new, mixed image in per-component format by applying a selection operation on the color and alpha information of the source images to define color component and alpha information of the new image according to a source selection definition; and converting the new per-component image into a shared-alpha format to generate the final image in shared-alpha format. In particular embodiments, the selection selects a color value and alpha pair from exactly one source image for each color component of the combined image. The mixed image in per-component format can be converted to a shared-alpha format through use of a matting color.

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1. A method for combining source images to form a final image, each image being a digital image defined by color component values and a transparency value, the method comprising:converting each of two or more shared-alpha source images into corresponding per-component source images in which each col

1. A method for combining source images to form a final image, each image being a digital image defined by color component values and a transparency value, the method comprising:converting each of two or more shared-alpha source images into corresponding per-component source images in which each color component value is paired with a per-component alpha value, the alpha value indicating to what extent color exists in a range of fully transparent to fully opaque;assembling a new image in per-component format by selecting each color and alpha pair for each component of the new image from exactly one of the source images in per-component format according to a source selection definition; andconverting the new per-component image into shared-alpha format to generate the final image in shared-alpha format. 2. The method of claim 1, wherein:the new per-component image is a matted image having color and alpha pairs at each location of the image <<X 0 , A 0 >, . . . , <Xn, An>>, the Xi being the color value of the new image of the i-th color component, the Ai being the alpha value paired with the i-th color component, and the image being matted with a matting color M having color component values Mi; andthe final image is in a matted shared-alpha format <C 0 , . . . , Cn, A>, the Ci being the color value of the i-th color component and the A being the shared alpha value, the Ci and A satisfying the two relationships A≧ Max( Ai ), where i ranges over the color components, and Ci=Xi. 3. The method of claim 2, wherein:A=Max(Ai); andthe source images are converted into a matted format with the matting color M before the assembling of the new per-component image occurs. 4. The method of claim 1, wherein:the new per-component image is a matted image having color and alpha pairs at each location of the image <<X 0 , A 0 >, . . . , <Xn, An>>, the Xi being the matted color value of the i-th color component, the Ai being the alpha value paired with the i-th color component, and the image being matted with a matting color M having color component values Mi; andthe final image is in an unmatted shared-alpha format <C 0 , . . . , Cn, A>, the Ci being the color value of the i-th color component and the A being the shared alpha value, the Ci and A satisfying the two relationships A≧ Max( Ai ), where i ranges over the color components, and Ci= ( Xi−Mi* (1 −A ))/ A. 5. The method of claim 4, wherein the source images are converted into a matted format with the matting color M before the assembling of the new per-component image occurs. 6. The method of claim 5, wherein the source images are converted from an unmatted format. 7. The method of claim 5, wherein the source images are converted from a matted format with a matting color different from M. 8. The method of claim 4, wherein A=Max(Ai). 9. The method of claim 8, wherein the source selection definition is location independent. 10. The method of claim 8, wherein the number of source images is two. 11. The method of claim 10, wherein the color components are cyan, magenta, yellow, and black and the matting color M is white. 12. A method for combining source images to form a final image, each image being a digital image defined by color component values and a transparency value, the method comprising:converting each of two or more unmatted, shared-alpha source images into a corresponding unmatted per-component source image in which each color component value is paired with a per-component alpha value, the alpha value indicating to what extent color exists in a range of fully transparent to fully opaque;assembling a new image in per-component format <<P 0 , A 0 >, . . . , <Pn, An>> by selecting each color and alpha pair for each component of the new image from exactly one of the per-component source images according to a source selection d efinition; andconverting the new per-component image into a final unmatted shared-alpha image <C 0 , . . . , Cn, A>, the Ci being the color value of the i-th color component of the final image and the A being the shared alpha value of the final image, the Ci and A satisfying the two relationships A= Max( Ai ), where i ranges over the color components, and Ci=Pi* ( Ai/A )+ Mi* (1−( Ai/A )), where Mi is the color value of the i-th color component of a matting color. 13. A method for converting an image in a per-component format into an image in a shared-alpha format, the method comprising:receiving a matted per-component image having color and alpha pairs at each location representable as <<X 0 , A 0 >, . . . , <Xn, An>>, the Xi being the matted color value of the i-th color component and the Ai being the alpha value paired with the i-th color component of the per-component image; andcalculating the color values and alpha of a shared-alpha image representable as <C 0 , . . . , Cn, A>, the Ci being the color value of the i-th color component and the A being the shared alpha value, the Ci and A being calculated to satisfy the relationships A≧ Max( Ai ), where i ranges over the color components, and Ci −( Xi−Mi* (1 −A ))/ A,  where Mi is the color value of the i-th color component of a matting color. 14. The method of claim 13, wherein A=Max(Ai). 15. The method of claim 14, wherein the number of source images is two. 16. The method of claim 15, wherein the color components are cyan, magenta, yellow, and black and the matting color is white, representable as <0,0,0,0>. 17. A method for converting an image in a per-component format into an image in a shared-alpha format, the method comprising:receiving an unwatted per-component image having color and alpha pairs at each location representable as <<P 0 , A 0 >, . . . , <Pn, An>>, the Pi being the color value of the i-th color component and the Ai being the alpha value paired with the i-th color component of the per-component image; andcalculating the color values and alpha of an unmatted shared-alpha image representable as <C 0 , . . . , Cn, A>, the Ci being the color value of the i-th color component and the A being the shared alpha value, the Ci and A being calculated to satisfy the relationships A≧ Max( Ai ), where i ranges over the color components, and Ci=Pi* ( Ai/A )+ Mi* (1−( Ai/A )), where Mi is the color value of the i-th color component of a matting color. 18. The method of claim 17, wherein A=Max(Ai). 19. The method of claim 18, wherein the number of source images is two. 20. The method of claim 19, wherein the color components are cyan, magenta, yellow, and black and the matting color is white, representable as <0,0,0,0>. 21. A method for overprinting an element into a base image having transparency, the method comprising:marking an element into a base image; andusing component-restricted blending to blend the base image from before the marking operation with the base image after the marking operation to generate a final image in which one or more excluded color components match closely in the base image, whereby only a subset of the color components of the resulting base image appear to have been marked, the component-restricted blending comprising converting shared-alpha source images into corresponding per-component source images, assembling a new, mixed, per-component image by selecting color and alpha pairs from the per-component source images according to a source selection definition, and converting the new, mixed, per-component image into a shared-alpha image. 22. A method for combining source images to form a final image, each image being a digital image defined by color component values and a transparency value, the method comprising:receiving two or more source images, each defined by color component and alpha information;assembling a new, mixed image in per-component format by applying a selection operation on the color and alpha information of the source images to define color component and alpha information of the new image according to a source selection definition; andconverting the new per-component image into a shared-alpha format to generate the final image in shared-alpha format. 23. The method of claim 22, wherein the source selection definition corresponds to a selection matrix with elements S(i, j), where i ranges over the color components, j ranges over the source images, and S(i, j) is the weight given to the contribution to the i-th color component and alpha of the mixed image from the i-th color component and alpha of the j-th source image. 24. A software product tangibly embodied in a machine-readable medium, for combining source images to form a final image, each image being a digital image defined by color component values and a transparency value, the software product comprising instructions operable to cause one or more data processing apparatus to perform operations comprising:converting each of two or more shared-alpha source images into corresponding per-component source images in which each color component value is paired with a per-component alpha value, the alpha value indicating to what extent color exists in a range of fully transparent to fully opaque;assembling a new image in per-component format by selecting each color and alpha pair for each component of the new image from exactly one of the source images in per-component format according to a source selection definition; andconverting the new per-component image into shared-alpha format to generate the final image in shared-alpha format. 25. The software product of claim 24, wherein:the new per-component image is a matted image having color and alpha pairs at each location of the image <<X0, A0>, . . . ,<Xn,An>>, the Xi being the color value of the new image of the i-th color component, the Ai being the alpha value paired with the i-th color component, and the image being matted with a matting color M having color component values Mi; andthe final image is in a matted shared-alpha format <C0, . . . Cn,A>, the Ci being the color value of the i-th color component and the A being the shared alpha value, the Ci and A satisfying the two relationshipsA > Max (Ai), where i ranges over the color components, and Ci=Xi. 26. The software product of claim 25, wherein: A=Max (Ai); and the source images are converted into a matted format with the matting color M before the assembling of the new per-component image occurs. 27. The software product of claim 24, wherein:the new per-component image is a matted image having color and alpha pairs at each location of the image <<X0, A0>, . . . ,<Xn, An>>, the Xi being the matted color value of the i-th color component, the Ai being the alpha value paired with i-th color component, and the image being matted with a matting color M having color component values Mi; andthe final image is in an unmatted shared-alpha format <C0, . . . ,Cn,A>, the Ci being the color value of the i-th color component and the A being the shared alpha value, the Ci and A satisfying the two relationshipsA > Max (Ai), where i ranges over the color components, andCi=(Xi−Mi*(1−A))/A. 28. The software product of claim 27, wherein the source images are converted into a matted format with the matting color M before the assembling of the new per-component image occurs. 29. The software product of claim 28, wherein the source images are converted from an unmatted format. 30. The software product of claim 28, wherein the source images are converted from a matted format with a matting color different from M. 31. The software product of claim 27, wherein A=Max (Ai). 32. The software produc t of claim 31, wherein the source selection definition is location independent. 33. The software product of claim 31, wherein the number of source images is two. 34. The software product of claim 33, wherein the color components are cyan, magenta, yellow, and black and the matting color M is white. 35. A software product tangibly embodied in a machine-readable medium, for combining source images to form a final image, each image being a digital image defined by color component values and a transparency value, the software product comprising instructions operable to cause one or more data processing apparatus to perform operations comprising:converting each of two or more unmatted, shared-alpha source images into a corresponding unmatted per-component source image in which each color component value is paired with a per-component alpha value, the alpha value indicating to what extent color exists in a range of fully transparent to fully opaque;assembling a new image in per-component format <<P0, A0>, . . . ,<Pn,An>> by selecting each color and alpha pair for each component of the new image from exactly one of the per-component source images according to a source selection definition; andconverting the new per-component image into a final unmatted shared-alpha image <C0, . . . ,Cn,A>, the Ci being the color value of the i-th color component of the final image and the A being the shared alpha value of the final image, the Ci and A satisfying the two relationshipsA=Max (Ai), where i ranges over the color components, andCi=Pi*(Ai/A)+Mi*(1−(Ai/A)), where Mi is the color value of the i-th color component of a matting color. 36. A software product tangibly embodied in a machine-readable medium, for converting an image in a per-component format into an image in a shared-alpha format, the software product comprising instructions operable to cause one or more data processing apparatus to perform operation comprising:receiving a matted per-component image having color and alpha pairs at each location representable as <<X0,A0>, . . . ,<Xn,An>>, the Xi being the matted color value of the i-th color component and the Ai being the alpha value paired with the i-th color component of the per-component image; andcalculating the color values and alpha of a shared-alpha image representable as <C0, . . . ,Cn,A>, the Ci being the color value of the i-th color component and the A being the shared alpha value, the Ci and A being calculated to satisfy the relationshipsA=Max (Ai), where i ranges over the color components, andCi=(Xi−Mi*(1−A))/A, where Mi is the color value of the i-th color component of a matting color. 37. The software product of claim 36, wherein A=Max (Ai). 38. The software product of claim 37, wherein the number of source images is two. 39. The software product of claim 38, wherein the color components are cyan, magenta, yellow, and black and the matting color is white, representable as <0,0,0,0>. 40. A software product tangibly embodied in a machine-readable medium for converting an image in a per-component format into an image in a shared-alpha format, the software product comprising instructions operable to cause one or more data processing apparatus to perform operation comprising:receiving an unmatted per-component image having color and alpha pairs at each location representable as <<P0,A0>, . . . ,<Pn,An>>, the Pi being the color value of the i-th color component and the Ai being the alpha value paired with the i-th color component of the per-component image; andcalculating the color values and alpha of an unmatted shared-alpha image representable as <C0, . . . ,Cn,A>, the Ci being the color component and the A being the shared alpha value, the Ci and A being calculated to satisfy the relationshipsA > Max (Ai), where i ranges over the color components, andCi=Pi*(Ai/A)+Mi*( 1−(Ai/A)), where Mi is the color value of the i-th color component of a matting color. 41. The software product of claim 40, wherein A=Max (Ai). 42. The software product of claim 41, wherein the number of source images is two. 43. The software product of claim 42, wherein the color components are cyan, magenta, yellow, and black and the matting color is white, representable as <0,0,0,0>. 44. A software product tangibly embodied in a machine-readable medium, for overprinting an element into a base image having transparency, the software product comprising instruction operable to cause one or more data processing apparatus to perform operations comprising:marking an element into a base image; andusing component-restricted blending to blend the base image from before the marking operation with the base image after the marking operation to generate a final image in which one or more excluded color components match closely in the base image, whereby only a subset of the color components of the resulting base image appear to have been marked, the component-restricted blending comprising converting shared-alpha source images into corresponding per-component source images, assembling a new, mixed, per-component image by selecting color and alpha pairs from the per-component source images according to a source selection definition, and converting the new, mixed, per-component image into a shared-alpha image. 45. A software product tangibly embodied in a machine-readable medium, for combining source images to form a final image, each image being a digital image defined by color component values and a transparency value, the software product comprising instructions operable to cause one or more data processing apparatus to perform operations comprising:receiving two or more source images, each defined by color component and alpha information;assembling a new, mixed image in per-component format by applying a selection operation on the color and alpha information of the source images to define color component and alpha information of the new image according to a source selection definition; andconverting the new per-component image into a shared-alpha format to generate the final image in shared-alpha format. 46. The software product of claim 45, wherein the source selection definition corresponds to a selection matrix with elements S(i,j), where i ranges over the color components, j ranges over the source images, and S(i,j) is the weight given to the contribution to the i-th color component and alpha of the mixed image from the i-th color component and alpha of the j-the source image.