blockiness

Blockiness is a visual artifact in digital images and videos characterized by the appearance of block-shaped discontinuities, typically aligned to an underlying grid. It arises most often in images and video produced by block-based compression schemes, where the picture is divided into fixed-size blocks before processing.

During compression, each block is transformed, quantized, and encoded separately. At low bitrates or with aggressive quantization, neighboring blocks may diverge abruptly in brightness or color, producing visible square boundaries known as blocking or block artifacts. Blocking is most noticeable in smooth gradients, large flat areas, and low-frequency regions, and can be amplified by chroma subsampling or motion estimation errors in video.

Blockiness is common in JPEG images and in many lossy video codecs such as MPEG-2/4, AVC/H.264, HEVC, and AV1, especially at low bitrates or when scaling is applied after compression. It is separate from other artifacts like ringing, blur, or noise, though multiple artifacts can interact in a compressed frame.

Mitigation strategies include deblocking filters, higher bitrates, larger transform blocks, and more advanced codecs that use improved intra- and inter-frame prediction. In post-processing, artifact-reduction or super-resolution techniques can reduce visible block boundaries, albeit with potential loss of detail. Awareness of blockiness informs encoding decisions and quality assessment.