[XviD-devel] Re: Luma vs Chroma Quant

[Christoph Lampert]

On Sun, 25 Jan 2004, Michael Militzer wrote:
> Even though you can't use a different quant value for chroma blocks, it's
> still possible to lower quality and save some bits for chroma by simply
> changing the quantized chroma coefficients (similar to what trellis does).
> However, the chroma planes compress really well already, so saving some
> bits here has no big influence on the overall global bitrate. So it seems
> pretty doubtful if the bitrate drop would justify the lower picture
> quality.

When I tested greyscale mode, completely _removing_ color information 
caused a +0.1-0.4dB higher luma PSNR for fixed bitrate, or 4%-5% 
lower bitrate for fixed quant. Just an example, of course, no scientific
result. 

Anyway, less than 10% is the usual value to estimate for all chroma
information in a bitstream. So just lowering chroma quality won't 
help much. 

chl