[XviD-devel] vop flag question

[MAILER-DAEMON]

On Wed, 22 Jun 2005, Pascal Massimino wrote:

> > Also looking through the code I couldn't find an option for
> > a 1-channel image so I'm feeding a 3-channel image.
> > I use XVID_CSP_I420  and set Cb & Cr channel to 0.
> > Is there a better option?
> > 
> > Finally I tested the XVID_VOP_GREYSCALE option by the two following 
> > tests.
> > The src images are a sequence of greyscale images using XVID_CSP_I420 
> > option and setting Cb & Cr channel to 0. The image size is 1008x752.
> > All options were the same for both encoding except for the
> > XVID_VOP_GREYSCALE flag.
> 
> 	hmm... try setting the Cb & Cr values to '127' instead.
> 

Thanks. That is a good idea. Slipped my mind that Cb & Cr is centered.
I'll try it and post the results just in case anyone is interested.

I also had another question about encoding a 1 channel greyscale image.

I encoded it two different ways.
First test was encoded by setting the Y to the greyscale value and setting
Cb & Cr to 0, and setting the csp to XVID_CSP_I420 . I also set the 
XVID_VOP_GREYSCALE flag,so the values of Cb & Cr
should not matter.

The second test was encoded by using the XVID_CSP_BGR color space and 
setting the values of BGR to the same greyscale value. I also set the
XVID_VOP_GREYSCALE flag, and when decoding just ask back the Y value.
So this case basically does the YUV conversion on the RGB images and then
just encodes the Y.

The results from the two tests are very interesting. For the same Q factor
the second test(feeding it RGB) frame size is significantly smaller. The 
difference gets smaller as the Q factor goes up, but at Q factor of 7
the difference is about 1KB, and about 0.5KB at Q = 18. The PSNR is better
for the first case, even at the same fr size(so the Q factor will be 
bigger for the first case since it needs a higher Q factor to compress it
more). Another thing about the 2nd test is that you can get a smaller 
frame size that is not possible with the first test(For the examples I 
ran 1st test at Q=31 and 2nd test at Q=22 have about the same fr size).

Which leads to my question, is there something going on in the YUV
conversion that makes it compress better?

thanks.