[XviD-devel] [RFC] Decoder speedup

[Edouard Gomez]

skal (skal at planet-d.net) wrote:
> 	What is the average quant of your bitstream??

Frames: 33205
Average Qp: 6.396356
Width: 640
Height: 480
Duration: 1386s
FPS: 23.976
MPEG4 Features: 1 bframe

> 	Because there's a big difference between ffmpeg and xvid:
> 	ffmpeg is dequantizing the coeffs on-the-fly, so that
> 	this operation scales as the number of non-zero coeffs.
> 	XviD, contrary, dequantizes every 64 coeffs with SIMD.
> 	It's faster, but inefficient if only few coeffs are
> 	non-zero.

I'll check what is the ratio of zero coeffs vs non zero
coeffs. If the ratio is huge then i'll give a try at on teh
fly dequantizing. Are they using plain C for on the fly
dequant ?


> > I think that if we want to speed up the decoder we have only
> > some choices left now (profiling the code doesn't show any
> > really good candidate now):
> >  - Merging some MPEG4 decoding operations (like DCT+zigzag, or
> >    zigzag + dequant, memtransfer+interpolation etc etc)
> 
> 	A hint: Idct+Add-pixel is the best candidate i can
> 	think of...

Ok, i'll have a look. Thx !

-- 
Edouard Gomez