Integrated image reshaping and video coding
Given a sequence of images in a first codeword representation, methods, processes, and systems are presented for integrating reshaping into a next generation video codec for encoding and decoding the images, wherein reshaping allows part of the images to be coded in a second codeword representation which allows more efficient compression than using the first codeword representation. A variety of architectures are discussed, including: an out-of-loop reshaping architecture, an in-loop-for intra pictures only reshaping architecture, an in-loop architecture for prediction residuals, and a hybrid in-loop reshaping architecture. Syntax methods for signaling reshaping parameters, and image-encoding methods optimized with respect to reshaping are also presented.
1 . A method for encoding images, the method comprising:
accessing an input image in a first codeword representation;
generating a forward reshaping function mapping pixels of the input image to a second codeword representation;
generating an inverse reshaping function based on the forward reshaping function, wherein the inverse reshaping function maps pixels from the second coding representation to the first coding representation;
generating coded pixel regions of the input image based on input pixel regions in the input image, the forward reshaping function and the inverse reshaping function;
generating reshaping metadata which characterize the forward reshaping function based on a piecewise-linear representation; and
generating an output bitstream based on the coded pixel regions of the input image and the reshaping metadata.
2 . The method of claim 1 , wherein generating the coded pixel regions of the input image comprises applying in-loop reshaping to the input image.
3 . The method of claim 1 , wherein for an input pixel region in the input image, generating a coded pixel region comprises:
computing a predicted region based on pixel data in a reference frame buffer or previously coded spatial neighbors;
generating a reshaped residual region based on the input pixel region, the predicted region, and the forward reshaping function, wherein a reshaped residual sample in the reshaped residual region is derived at least in part from forward reshaping a respective predicted sample in the predicted region;
generating a quantized residual region based on the reshaped residual region;
generating a dequantized residual region based on the quantized residual region;
generating a reconstructed pixel region based on the dequantized residual region, the predicted region, the forward reshaping function, and the inverse reshaping function; and
generating a reference pixel region to be stored on the reference frame buffer based on the reconstructed pixel region.
4 . The method of claim 3 , wherein generating the quantized residual region comprises:
applying a forward coding transform to the reshaped residual region to generate transformed data; and
applying a forward coding quantizer to the transformed data to generate quantized data.
5 . The method of claim 4 , wherein generating the dequantized residual region comprises:
applying an inverse coding quantizer to the quantized data to generate inverse-quantized data; and
applying an inverse coding transform to the inverse-quantized data to generate the dequantized residual region.
6 . The method of claim 3 , wherein generating the reference pixel region to be stored on the reference frame buffer comprises applying a loop filter to the reconstructed pixel region.
7 . A method for decoding a coded bitstream to generate an output image in a first codeword representation, the method comprising:
receiving a coded image partially coded in a second codeword representation;
receiving reshaping metadata for the coded image;
generating based on the reshaping metadata a forward reshaping function mapping pixels from the first codeword representation to the second codeword representation;
generating based on the reshaping metadata an inverse reshaping function, wherein the inverse reshaping function maps pixels from the second codeword representation to the first codeword representation; and
decoding coded regions of the coded image based on the forward reshaping function and the inverse reshaping function to generate output pixel regions.
8 . The method of claim 7 , wherein decoding the coded regions of the coded image is based on in-loop reshaping.
9 . The method of claim 7 , wherein decoding a coded region further comprises:
generating a decoded reshaped residual region;
generating a predicted region based on pixels in a reference pixel buffer or previously decoded spatial neighbors;
generating a reconstructed pixel region based on the decoded reshaped residual region, the predicted region, the forward reshaping function, and the inverse reshaping function, wherein a reconstructed sample in the reconstructed pixel region is derived at least in part from forward reshaping a respective predicted sample in the predicted region;
generating an output pixel region for the output image based on the reconstructed pixel region; and
storing the output pixel region in the reference pixel buffer.
10 . The method of claim 7 , wherein the metadata characterize the forward reshaping function based on a piecewise linear representation, and for each segment of the piecewise linear representation of the forward reshaping function, the reshaping metadata comprise a delta absolute value and a sign of the delta absolute value.