Various earlier work in MPEG/JCTVC have shown that out-loop reshaping, which modifies the video signal in preprocessing before encoding and post-processing after decoding in an end-to-end video compression workflow, can improve subjective quality of coded High Dynamic Range (HDR) and Wider Color Gamut (WCG) content compressed using HEVC. However, the requirement of not making normative changes to the HEVC specification has significantly constrain the design and optimization of the reshaper. In April 2018, The Joint Video Experts Team (JVET) has launched a project to develop a new video coding standard to be known as Versatile Video Coding (VVC). This opens the door to exploit possibilities of the reshaper design inside of the core video codec. In this paper, an in-loop architect of reshaper is presented. Preliminary results suggest that the in-loop reshaping architect can retain the functionality of out-loop reshaper. In addition, the in-loop design can resolve many limitations of the out-loop design and can be used as a general coding tool for general video content not limited to HDR.
KEYWORDS: Video processing, Computer programming, High dynamic range imaging, Video compression, Video, Signal processing, Visualization, Distortion, Quantization, Color difference
High Dynamic Range (HDR) and Wider Color Gamut (WCG) content represents a greater range of luminance levels and
a more complete reproduction of colors found in real-world scenes. The characteristics of HDR/WCG content are very
different from the SDR content. It poses a challenge to the compression system which is originally designed for SDR
content. Recently in MPEG/VCEG, two directions have been taken to improve compression performances for
HDR/WCG video using HEVC Main10 codec. The first direction is to improve HDR-10 using encoder optimization.
The second direction is to modify the video signal in pre/post processing to better fit compression system. The process
therefore is out of coding loop and does not involve changes to the HEVC specification. Among many proposals in the
second direction, reshaper is identified to be the key component. In this paper, a novel luma reshaper is presented which
re-allocates the codewords to help codec improve subjective quality. In addition, encoder optimization can be performed
jointly with reshaping. Experiments are conducted with ICtCp color difference signal. Simulation results show that if
both joint optimization of reshaper and encoder are carried out, there is evidence that improvement over the HDR-10
anchor can be achieved.
KEYWORDS: High dynamic range imaging, Video, Video compression, RGB color model, Video coding, Signal processing, Computer programming, Colorimetry, Visual process modeling, Human vision and color perception
High Dynamic Range (HDR) and Wider Color Gamut (WCG) content represents a greater range of luminance levels and a more complete reproduction of colors found in real-world scenes. The current video distribution environments deliver Standard Dynamic Range (SDR) signal. Therefore, there might be some significant implication on today's end-to-end ecosystem from content creation to distribution and finally to consumption. For SDR content, the common practice is to apply compression on Y'CbCr 4:2:0 using gamma transfer function and non-constant luminance 4:2:0 chroma subsampling. For HDR and WCG content, it is desirable to examine if such signal format still works well for compression, and it is interesting to know if the overall system performance can be further improved by exploring different signal formats and processing workflows. In this paper, we will provide some of our insight into those problems.
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