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Definition
Chroma subsampling is a compression technique that reduces color information in an image or video while preserving luminance (brightness) detail. Human eyes are more sensitive to brightness changes than color changes, so reducing chroma data saves significant file size with minimal visible quality loss.
Digital images separate pixel data into luminance (Y) and two chrominance channels (Cb and Cr). Chroma subsampling reduces the resolution of the color channels relative to the luminance channel. The notation uses three numbers: 4:4:4 means no subsampling (full color resolution), 4:2:2 halves horizontal color resolution, and 4:2:0 halves both horizontal and vertical color resolution. JPEG images typically use 4:2:0 subsampling, which reduces the data by about 50% compared to 4:4:4.
For photographs of natural scenes, 4:2:0 subsampling is nearly indistinguishable from 4:4:4 because the human visual system has poor spatial resolution for color compared to brightness. However, for images with sharp color transitions — red text on a white background, color charts, fine colored patterns, screenshots — 4:2:0 can introduce visible color fringing along edges. This is why PNG (which uses no chroma subsampling) is better for screenshots and text-heavy images.
Video uses chroma subsampling extensively. Consumer video (Blu-ray, streaming) is almost always 4:2:0. Professional video workflows often use 4:2:2 for better chroma keying (green screen) performance, since clean color edges are critical for accurate key extraction. 4:4:4 video is used in high-end post-production and visual effects but produces very large files. Understanding chroma subsampling helps explain why some JPEG exports of screenshots look worse than expected around colored text.