Chroma Subsampling
A compression technique that reduces chrominance channel resolution relative to luminance, exploiting human vision's lower sensitivity to color detail.
Chroma subsampling reduces the spatial resolution of chrominance components (Cb, Cr) relative to luminance (Y) in YCbCr color space. It exploits the human visual system's greater sensitivity to brightness changes than color changes.
Notation uses J:a:b format. 4:4:4 means no subsampling (highest quality), 4:2:2 halves horizontal chroma resolution, and 4:2:0 halves both horizontal and vertical. JPEG defaults to 4:2:0, reducing data by approximately 50% compared to 4:4:4 with negligible visual difference in photographs.
However, images containing text or thin lines exhibit color fringing at 4:2:0. Screenshots and logos benefit from 4:4:4 or PNG format. When adjusting JPEG quality in the compression tool, chroma subsampling settings change internally.