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.
