deblocking

Deblocking is a post-processing technique used in lossy image and video compression to reduce blocking artifacts—the visible grid-like patterns that result from block-based quantization. In video, deblocking is often implemented as an in-loop filter inside the decoder, so its results contribute to subsequent prediction and encoding steps. Many standards, including MPEG-2, H.264/AVC, and HEVC, rely on a deblocking filter to improve perceived quality without increasing bit rate.

The deblocking process works by examining the boundaries between blocks and smoothing pixel values along those boundaries. The filter uses information about the strength of the boundary, often derived from the quantization parameter and local image activity, to apply directionally adaptive smoothing. The goal is to reduce horizontal and vertical discontinuities while preserving true edges, with filters designed to avoid excessive blurring by limiting how much a boundary can be changed.

Deblocking improves subjective quality and compression efficiency because lower energy is required to encode fewer visible artifacts; however, if applied too aggressively it can blur fine detail and textures. In modern codecs the deblocking filter is often combined with other in-loop filters, such as sample-adaptive offset and SAO, to balance artifact reduction with detail preservation. Implementations vary, but the general approach is to apply small, boundary-specific adjustments along horizontal and vertical edges and then clamp changes to safe ranges.

Overall, deblocking is a key component of modern block-based compression systems, contributing to smoother reconstructed images and better coding efficiency while aiming to minimize loss of detail.