Chrome Subsampling

The chromasub parameter controls the chroma subsampling rate for JPEG and progressive JPEG output formats. Valid values are 444, 422, and 420. The default is 420.

chromasub is a parameter that should only be used in very specific cases, and you should fully understand how it affects image quality before use. Please read the Wikipedia page on chroma subsampling for how this works. Increasing the chroma subsampling rate to 444 or 422 results in technically higher-quality images, but these improvements can be imperceptible to users. Output files with a higher chroma subsampling rate will be larger than files with a lower chroma subsampling rate.