PAR — Pixel Aspect Ratio — is the shape of a single pixel. In modern digital video PAR is virtually always 1:1, meaning pixels are square. But in older standards and a few specialised production formats, pixels are deliberately non-square — rectangular or even slightly skewed — and that's where PAR becomes a thing to worry about.
Why non-square pixels ever existed: in analog and early-digital broadcast, the relationship between the number of horizontal samples and the active picture width was set for backwards compatibility, not for square pixels. Standard-definition NTSC stored 720×480 pixels but those pixels were taller than they were wide, producing a 4:3 image when displayed correctly. PAL did similar with 720×576 and slightly different pixel shapes. Anamorphic widescreen cinema squeezes a 2.39:1 image horizontally into a 1.78:1 frame using non-square optics; later it's stretched back during projection. Get the PAR wrong and faces come out either too narrow or too wide.
For a product team in 2026, PAR is a legacy concern. Anything captured on a modern camera, phone, screencast tool or computer is 1:1 — actual pixel dimensions equal display dimensions. The only places PAR causes problems are ingest of old SD broadcast material, certain DVD content, and anamorphic cinema masters being repurposed for streaming. The fix: detect non-square PAR on ingest, apply a one-time rescale to square pixels, and store the corrected file. Modern players, codecs and CDNs all assume square pixels — sending non-square content through anywhere modern is asking for stretched faces somewhere down the line.
The book · Volume 1 of 7
Video Encoding: A Complete Guide to Video Compression — From Pixels and Color to H.264, HEVC, AV1, Quality Metrics, and AI
Pixel aspect ratio errors stretch or squash the picture, usually after legacy or anamorphic sources pass through a pipeline. Volume 1 explains pixel aspect ratio in detail: square versus non-square pixels, how PAR, SAR and display aspect ratio relate, and how containers and encoders signal them. Written by Nikolay Sapunov, CEO at Fora Soft.
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