Blocking, often called macroblocking, is the breakup of a picture into visible squares, as if reassembled from coarse tiles. It is the most recognizable compression artifact because the grid is real and countable - every modern codec compresses each frame on a lattice of blocks. The cause is coarse quantization: each block is transformed by the DCT and rounded independently, and because the transform never models correlation across block boundaries, neighboring blocks collapse to slightly different flat tones, opening a visible seam exactly on the grid. It shows first in smooth, low-detail regions, where a small step has nothing to hide behind, and is masked in busy texture. Codecs fight it with an in-loop deblocking filter (in H.264, HEVC, and AV1), trading visible tiles for mild softness. PSNR, SSIM, and VMAF all detect blocking reasonably well since it is a spatial structural error, but PSNR under-weights a visible seam as small pixel error.
The handbook · 7 volumes
Fora Soft Video Engineering Handbook
Blocking is the most recognizable compression artifact, and it usually signals a bitrate, preset or deblocking problem rather than bad luck. We wrote seven volumes covering the whole video stack, and for blocking start with Volume 1: transforms and quantization, deblocking filters, rate control and artifact metrics in H.264, HEVC and AV1. Written by Nikolay Sapunov, CEO at Fora Soft.
Seeing blocking in fast motion or dark scenes on your streams?
Send us clips where blocking appears and your encoder settings: we find the cause and give you rate control and deblocking settings to remove it. 250+ video products shipped since 2005.
