Predictive Image Encoding and Decoding with Pixel Group Based Quantization

A method for encoding a first image of which pixel values have a first bit depth, comprising:

• a) obtaining a second image that corresponds to the first image, wherein pixel values of the second image have a second bit depth lower than the first bit depth;
• b) encoding the second image using a second encoder;
• c) predicting the first image based on the second image;
• d) calculating a residual image based on a difference between the predicted first image and the first image;
• characterized by:
• e) grouping pixels in the residual image into a plurality of pixel groups, wherein said grouping comprises:
• for each pixel in the residual image, comparing the pixel value of the corresponding pixel in the second image, in the decoded encoded second image, or in the predicted first image, to a plurality of pixel value ranges respectively corresponding to the plurality of pixel groups and assigning said each pixel to a given pixel group among said plurality pixel groups if the pixel value of its corresponding pixel falls in the range attributed to that pixel group;
• f) quantizing the pixels in each respective pixel group by applying a respective quantizing function to the pixels in each respective pixel group, wherein the bit depth of pixel values of pixels in the quantized residual image is smaller than the bit depth of pixel values of pixels in the non-quantized residual image, wherein each quantizing function maps the range of pixel values of the pixels in a given pixel group to substantially the full range offered by the bit depth of pixel values in the quantized residual image;
• and
• g) encoding the quantized residual image using a first encoder.

An encoder for encoding a first image of which pixel values have a first bit depth, comprising:

• a first input unit for inputting the first image;
• a second input unit for inputting or generating a second image that corresponds to the first image, wherein pixel values of the second image have a second bit depth that is lower than the first bit depth;
• a second encoder for encoding the second image;
• a predicting unit for predicting the first image based on the second image;
• a comparator for comparing the inputted first image to the predicted first image, and for outputting a residual image based on a difference between the predicted first image and the inputted first image;
• a quantizing unit for quantizing the residual image, said quantizing unit comprising a grouping unit for grouping pixels in the residual image into a plurality of pixel groups wherein said grouping comprises:
• for each pixel in the residual image, comparing the pixel value of the corresponding pixel in the second image, in the decoded encoded second image, or in the predicted first image, to a plurality of pixel value ranges respectively corresponding to the plurality of pixel groups and assigning said each pixel to a given pixel group among said plurality pixel groups if the pixel value of its corresponding pixel falls in the range attributed to that pixel group;
• a quantizing function application unit for quantizing the pixels in each respective pixel group by applying a respective quantizing function to the pixels in each respective pixel group, wherein the bit depth of pixel values of pixels in the quantized residual image is smaller than the bit depth of pixel values of pixels in the non-quantized residual image, wherein each quantizing function maps the range of pixel values of the pixels in a given pixel group to substantially the full range offered by the bit depth of pixel values in the quantized residual image;
• and
• a first encoder for encoding the quantized residual image.