[XviD-devel] Re: Luma vs Chroma Quant

[Steffen Schulze]

Michael Militzer wrote:

> Quoting "Hans K. Rosbach" <hk at circlestorm.org>:

>>Okay, this is the fourth time I'm sending this mail since jan 8.
>>Hopefully it'll arrive this time..

By the way: I did not get the original mail from Hans and I could not 
find it on http://list.xvid.org/pipermail/xvid-devel/ . Is there already 
trouble with the mailing list?

>>During a discussion with some of my friends, one of them
>>came up with an interesting question..
>>
>>Is it possible to encode chroma/luma components of the
>>picture with different quant values? Thus one would for
>>example be able to down-prioritize the chroma in favor of
>>the luma..

Michael is right, it is not possible with MPEG4 to encode luma and 
chroma with different quant. But it is possible with MPEG4/AVC (H.264). 
So just wait for the first H.264 codec ;)

The H.264 guys defined a table (H.264 spec table 8-13) to map the Luma 
Quantizer QPY to a Chroma Quantizer QPC. But they do not 
"down-prioritize the chroma in favor of the luma" - they "up-prioritize" 
it :) I think the degration in chroma-quality for big quantizers was too 
much without this mapping.

Just for interest: Table 8-13 from G050r1.doc (actual H.264 spec)
Note that H.264 has 52 quantization steps.

qPI = Clip3( 0, 51, QPY + chroma_qp_index_offset )

qPI .. 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51
QPC .. 29 30 31 32 32 33 34 34 35 35 36 36 37 37 37 38 38 38 39 39 39 39

bye,

Steffen