Block-based Learned Image Compression without Blocking Artifacts

Abstract

Learned image compression (LIC) outperforms traditional codecs but suffers from excessive peak memory usage when handling high-resolution images. Consequently, block-based LIC has been studied to reduce peak memory and peak computational cost, but it often introduces blocking artifacts that degrade visual quality. To mitigate this, the JPEG-AI standard introduced a patch-based scheme in which overlapped blocks are coded independently using empirically determined overlap sizes. However, experimentally searching for optimal overlaps is time-consuming and does not guarantee blocking-free reconstruction. In this paper, we propose an analytic framework that models overlap propagation through convolution and transposed convolution layers to precisely determine the minimal overlaps required for blocking-free reconstruction. Based on the calculated minimum overlaps, we provide a block-based implementation methodology applicable to most CNN-based LIC models. Applied to four CNN-based LIC models on 4K images partitioned into various block sizes (256 x 256, 512 x 512), our method achieves rate-distortion performance identical to full-image coding while reducing average peak memory usage to 13.94% (encoder) and 13.33% (decoder), and average peak computational cost to 2.6% and 1.24%, respectively. Notably, the proposed block-based framework does not require any retraining of the original model. Furthermore, it can also be applied to most CNN-based image-processing neural networks without performance degradation.