Understanding Chroma Subsampling in MozJPEG
A deep-dive into how YCbCr color channels and subsampling ratios (4:2:0 vs 4:4:4) affect compression.
MV
Marcus Vance
Contributing Author · Squoosh Next BlogChroma subsampling is a spatial compression technique that exploits the human visual system's lower sensitivity to color than to brightness. By reducing the resolution of chrominance channels compared to the luma channel, JPEG encoders achieve significant compression gains. In MozJPEG, selecting 4:2:0 subsampling halves chrominance resolution both horizontally and vertically, reducing encoded size by up to 50% with minimal perceived quality change.
For text-heavy images, screenshots, or designs with sharp color boundaries, 4:4:4 subsampling is strongly recommended to avoid color bleeding at edges. The 4:2:2 mode offers a balanced middle ground: horizontal chroma is halved while vertical resolution is preserved, making it ideal for portrait photography where vertical gradients matter most.
Key Takeaways
Chroma subsampling is a spatial compression technique that exploits the human visual system's lower sensitivity to color than to brightness.
By reducing the resolution of chrominance channels compared to the luma channel, JPEG encoders achieve significant compression gains.
In MozJPEG, selecting 4:2:0 subsampling halves chrominance resolution both horizontally and vertically, reducing encoded size by up to 50% with minimal perceived quality change.
For text-heavy images, screenshots, or designs with sharp color boundaries, 4:4:4 subsampling is strongly recommended to avoid color bleeding at edges.
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