Lossless 4:2:0 Screen Content Coding Using Luma-Guided Soft Context Formation

2025-08-26

AI Summary

Existing soft-context formation encoders do not support the YCbCr 4:2:0 chroma subsampling format. To address this limitation, this paper proposes the first lossless compression method specifically designed for 4:2:0 screen content images. The method introduces three key innovations: (1) a luminance-guided per-plane soft-context modeling mechanism, where the Y component steers prediction and probability estimation for Cb/Cr; (2) an adaptive plane coding order strategy based on normalized mutual information; and (3) a joint luma-chroma side-information transmission scheme to enhance chroma modeling accuracy. Integrating pattern matching, palette coding, and arithmetic coding, the approach achieves an average bitrate reduction of 5.66% over HEVC-SCC on large-scale screen content datasets. This work marks the first successful extension of soft-context formation to the 4:2:0 chroma format.

Technology Category

• Computer Vision: Learning & Optimization for CV
• Machine Learning: Deep Generative Models & Autoencoders
• Cognitive Modeling & Cognitive Systems: Neural Spike Coding

Application Category

• Graph Algorithms and Modeling for the Web: Representation, reconstruction, and subgraph or motif discovery in Web-related graphs
• Economics, Online Markets and Human Computation: Cost models of using LLMs in production systems
• Search and Retrieval-Augmented AI: Web learning to rank, online learning, and counterfactual learning for ranking

Abstract

The soft context formation coder is a pixel-wise state-of-the-art lossless screen content coder using pattern matching and color palette coding in combination with arithmetic coding. It achieves excellent compression performance on screen content images in RGB 4:4:4 format with few distinct colors. In contrast to many other lossless compression methods, it codes entire color pixels at once, i.e., all color components of one pixel are coded together. Consequently, it does not natively support image formats with downsampled chroma, such as YCbCr 4:2:0, which is an often used chroma format in video compression. In this paper, we extend the soft context formation coding capabilities to 4:2:0 image compression, by successively coding Y and CbCr planes based on an analysis of normalized mutual information between image planes. Additionally, we propose an enhancement to the chroma prediction based on the luminance plane. Furthermore, we propose to transmit side-information about occurring luma-chroma combinations to improve chroma probability distribution modelling. Averaged over a large screen content image dataset, our proposed method outperforms HEVC-SCC, with HEVC-SCC needing 5.66% more bitrate compared to our method.

Problem

• Extend lossless screen content coding to YCbCr 4:2:0 format
• Enhance chroma prediction using luminance plane guidance
• Improve chroma probability modeling with luma-chroma combination data

Innovation

• Luma-guided soft context formation
• Successive coding of Y and CbCr planes
• Side-information transmission for chroma modeling