[XviD-devel] Re: [Ffmpeg-devel] Trellis quant (fwd)

[Christoph Lampert]

Hi,

as I assumed: ffmpeg quant does coefficient reduction for Pframes 
by default. With trellis this is replaced by R-D-optimization, so I-frames
get smaller with Trellis, P-frames get larger. 

gruel 

----------------------------------------------------------
To: ffmpeg-devel at lists.sourceforge.net
Subject: Re: [Ffmpeg-devel] Trellis quant

Hi

On Friday 18 April 2003 19:04, Christoph Lampert wrote:
> Hi,
>
> I'm confused about Trellis-Quantization (or just too stupid to understand
> the code):
>
> My impression was that trellis quant sometimes lowers values of quantized
> DCT coefficients in order to safe bits (using a lagrangian).
> Therefore fixed quant files with trellis quant should become smaller at
> a reduced PSNR compared to encoding at the same quant without trellis.
>
> But the opposite is true: With trellis, my test files got _bigger_ at a
> _higher_ PSNR, but still at fixed quant. From what I see, I-VOPs indeed
> do get smaller, but all P-VOPs get larger.
>
> How is that possible? Or is there some other less efficient
> thresholding mechanism enabled by default which I overlooked?
the default quantization code adds a bias (-1/4 for P and 0 for I) like some 
h263 doc recommands, u can change the bias with the ibias & pbias options 
(for mencoder)

[...]

-- 
Michael
screen[y][x] ^= cursor[cy][cx];			(violates patent #4,197,590)
median(mv[y-1][x], mv[y][x-1], mv[y+1][x+1]);	(violates patent #5,905,535)
buf[i]= qp - buf[i-1];				(violates patent #?)
for more examples, see http://mplayerhq.hu/~michael/patent.html
stop it, see http://petition.eurolinux.org & http://petition.ffii.org/eubsa/en


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