Alpha channel prediction

Abstract

Image coding using alpha channel prediction may include generating a reconstructed image using alpha channel prediction and outputting the reconstructed image. Generating the reconstructed image using alpha channel prediction may include decoding reconstructed color channel values for a current pixel expressed with reference to first color space, obtaining color space converted color channel values for the current pixel by converting the reconstructed color channel values to a second color space, obtaining an alpha channel lower bound for an alpha channel value for the current pixel using the color space converted color channel values, generating a candidate predicted alpha value for the current pixel, obtaining an adjusted predicted alpha value for the current pixel using the candidate predicted alpha value and the alpha channel lower bound, generating a reconstructed pixel for the current pixel using the adjusted predicted alpha value, and including the reconstructed pixel in the reconstructed image.

What is claimed is: 
     
         1 . A non-transitory computer-readable storage medium storing an encoded bitstream, wherein the encoded bitstream is configured for decoding by operations comprising:
 generating a reconstructed image, wherein generating the reconstructed image includes generating the reconstructed image using alpha channel prediction, wherein generating the reconstructed image using alpha channel prediction includes:
 obtaining reconstructed color channel values for a current pixel of the current image expressed with reference to first color space; 
 obtaining color space converted color channel values for the current pixel by converting the reconstructed color channel values to a second color space; 
 obtaining an alpha channel lower bound for an alpha channel value for the current pixel using the color space converted color channel values; 
 generating a candidate predicted alpha value for the current pixel; 
 obtaining an adjusted predicted alpha value for the current pixel using the candidate predicted alpha value and the alpha channel lower bound; 
 generating a reconstructed pixel for the current pixel using the adjusted predicted alpha value; and 
 including the reconstructed pixel in the reconstructed image; and 
   outputting the reconstructed image.   
     
     
         2 . The non-transitory computer-readable storage medium of  claim 1 , wherein:
 the first color space is the YUV color space;   the reconstructed color channel values include a luminance channel value, a first chrominance channel value, and a second chrominance channel value; and   obtaining the reconstructed color channel values includes decoding residual color channel values from an encoded bitstream.   
     
     
         3 . The non-transitory computer-readable storage medium of  claim 1 , wherein the second color space is the RGB color space and the color space converted color channel values include a red color channel value, a green color channel value, and a blue color channel value. 
     
     
         4 . The non-transitory computer-readable storage medium of  claim 3 , wherein obtaining the alpha channel lower bound includes:
 obtaining a normalized red color channel value by dividing the red color channel value by a defined maximum value for the red color channel;   obtaining a normalized green color channel value by dividing the green color channel value by a defined maximum value for the green color channel;   obtaining a normalized blue color channel value by dividing the blue color channel value by a defined maximum value for the blue color channel;   identifying a maximum value among the normalized red color channel value, the normalized green color channel value, and the normalized blue color channel value; and   identifying, as the alpha channel lower bound, a product of multiplying the maximum value by a defined maximum value for the alpha channel.   
     
     
         5 . The non-transitory computer-readable storage medium of  claim 1 , wherein generating the candidate predicted alpha value includes:
 identifying a previously reconstructed context pixel for predicting the candidate predicted alpha value; and   obtaining the candidate predicted alpha value using the previously reconstructed context pixel.   
     
     
         6 . The non-transitory computer-readable storage medium of  claim 1 , wherein obtaining the adjusted predicted alpha value includes:
 identifying, as the adjusted predicted alpha value, a maximum value among the candidate predicted alpha value and the alpha channel lower bound.   
     
     
         7 . A non-transitory computer-readable storage medium storing an encoded bitstream, the encoded bitstream generated by performing operations comprising:
 generating an encoded image, wherein generating the encoded image includes generating the encoded image using alpha channel prediction, wherein generating the encoded image using alpha channel prediction includes:
 identifying a current pixel from an input image, wherein the current pixel includes input color channel values, wherein the input color channel values are expressed with reference to first color space, and wherein the input color channel values include an input alpha channel value; 
 obtaining pre-multiplied color channel values for the pixel using the input color channel values; 
 obtaining reconstructed color values for the pixel using the pre-multiplied color channel values, wherein the reconstructed color channel values are expressed with reference to second color space; 
 obtaining color space converted color channel values for the current pixel by color space converting the reconstructed color channel values to the first color space; 
 obtaining an alpha channel lower bound for a reconstructed alpha channel value for the current pixel using the color space converted color channel values; 
 generating a candidate predicted alpha value for the current pixel; 
 obtaining an adjusted predicted alpha value for the current pixel using the candidate predicted alpha value and the alpha channel lower bound; 
 obtaining a residual alpha value as a difference of subtracting the adjusted predicted alpha value from the input alpha channel value; and 
 including the residual alpha value in an output bitstream; and 
   outputting the output bitstream.   
     
     
         8 . The non-transitory computer-readable storage medium of  claim 7 , wherein the second color space is the YUV color space and the reconstructed color channel values include a luminance channel value, a first chrominance channel value, and a second chrominance channel value. 
     
     
         9 . The non-transitory computer-readable storage medium of  claim 7 , wherein:
 the first color space is the RGB color space;   the input color channel values include an input red color channel value, an input green color channel value, and an input blue color channel value;   the pre-multiplied color channel values include a pre-multiplied red color channel value, a pre-multiplied green color channel value, and a pre-multiplied blue color channel value; and   the color space converted color channel values include a color space converted red color channel value, a color space converted green color channel value, and a color space converted blue color channel value.   
     
     
         10 . The non-transitory computer-readable storage medium of  claim 9 , wherein obtaining the pre-multiplied color channel values includes:
 obtaining, as the pre-multiplied red color channel value, a product of multiplying the input red color channel value by the input alpha channel value;   obtaining, as the pre-multiplied green color channel value, a product of multiplying the input green color channel value by the input alpha channel value; and   obtaining, as the pre-multiplied blue color channel value, a product of multiplying the input blue color channel value by the input alpha channel value.   
     
     
         11 . The non-transitory computer-readable storage medium of  claim 9 , wherein obtaining the alpha channel lower bound includes:
 obtaining a normalized red color channel value by dividing the color space converted red color channel value by a defined maximum value for the red color channel;   obtaining a normalized green color channel value by dividing the color space converted green color channel value by a defined maximum value for the green color channel;   obtaining a normalized blue color channel value by dividing the color space converted blue color channel value by a defined maximum value for the blue color channel;   identifying a maximum value among the normalized red color channel value, the normalized green color channel value, and the normalized blue color channel value; and   identifying, as the alpha channel lower bound, a product of multiplying the maximum value by a defined maximum value for the alpha channel.   
     
     
         12 . The non-transitory computer-readable storage medium of  claim 7 , wherein obtaining the reconstructed color values includes:
 color space converting the pre-multiplied color channel values to the second color space;   obtaining respective predicted color channel values for the pre-multiplied color channel values;   obtaining respective residual color channel values as respective differences of subtracting the predicted color channel values from the corresponding pre-multiplied color channel values;   lossily encoding the residual color channel values to obtain encoded residual color channel values; and   obtaining, as the reconstructed color values, respective sums of adding the encoded residual color channel values to the corresponding predicted color channel values.   
     
     
         13 . The non-transitory computer-readable storage medium of  claim 7 , wherein generating the candidate predicted alpha value includes:
 identifying a previously reconstructed context pixel for prediction of the candidate predicted alpha value; and   obtaining the candidate predicted alpha value in accordance with the previously reconstructed context pixel.   
     
     
         14 . The non-transitory computer-readable storage medium of  claim 7 , wherein obtaining the adjusted predicted alpha value includes:
 identifying, as the adjusted predicted alpha value, a maximum value among the candidate predicted alpha value and the alpha channel lower bound.   
     
     
         15 . A non-transitory computer-readable storage medium storing an encoded bitstream, wherein the encoded bitstream is configured for decoding by operations comprising:
 generating a reconstructed image, wherein generating the reconstructed image includes generating the reconstructed image using alpha channel prediction, wherein generating the reconstructed image using alpha channel prediction includes:
 obtaining reconstructed color channel values for a current pixel of the current image expressed with reference to first color space; 
 obtaining color space converted color channel values for the current pixel by converting the reconstructed color channel values to a second color space; 
 obtaining an adjusted predicted alpha value for the current pixel using a candidate predicted alpha value for the current pixel and an alpha channel lower bound for an alpha channel value for the current pixel obtained using the color space converted color channel values; 
 generating a reconstructed pixel for the current pixel using the adjusted predicted alpha value; and 
 including the reconstructed pixel in the reconstructed image; and 
   outputting the reconstructed image.   
     
     
         16 . The non-transitory computer-readable storage medium of  claim 15 , wherein:
 the first color space is the YUV color space;   the reconstructed color channel values include a luminance channel value, a first chrominance channel value, and a second chrominance channel value; and   obtaining the reconstructed color channel values includes decoding residual color channel values from an encoded bitstream.   
     
     
         17 . The non-transitory computer-readable storage medium of  claim 15 , wherein the second color space is the RGB color space and the color space converted color channel values include a red color channel value, a green color channel value, and a blue color channel value. 
     
     
         18 . The non-transitory computer-readable storage medium of  claim 17 , wherein the alpha channel lower bound is obtained by:
 obtaining a normalized red color channel value by dividing the red color channel value by a defined maximum value for the red color channel;   obtaining a normalized green color channel value by dividing the green color channel value by a defined maximum value for the green color channel;   obtaining a normalized blue color channel value by dividing the blue color channel value by a defined maximum value for the blue color channel;   identifying a maximum value among the normalized red color channel value, the normalized green color channel value, and the normalized blue color channel value; and   identifying, as the alpha channel lower bound, a product of multiplying the maximum value by a defined maximum value for the alpha channel.   
     
     
         19 . The non-transitory computer-readable storage medium of  claim 15 , wherein the candidate predicted alpha value is generated by:
 identifying a previously reconstructed context pixel for predicting the candidate predicted alpha value; and   obtaining the candidate predicted alpha value using the previously reconstructed context pixel.   
     
     
         20 . The non-transitory computer-readable storage medium of  claim 15 , wherein obtaining the adjusted predicted alpha value includes:
 identifying, as the adjusted predicted alpha value, a maximum value among the candidate predicted alpha value and the alpha channel lower bound.