[XviD-devel] [RFC] Decoder speedup

[skal]

	Hi Edouard and all,

On Sun, 2004-06-13 at 22:32, Edouard Gomez wrote:

> BENCHMARKs: VC: 255,768s VO:   0,189s A: 0,000s Sys:  12,763s =  268,720s <-- 1.0.1
> BENCHMARKs: VC: 249,709s VO:   0,193s A: 0,000s Sys:  13,287s =  263,189s <-- 1.0.1 + get_coeff optimization
> BENCHMARKs: VC: 222,033s VO:   0,196s A: 0,000s Sys:  12,269s =  234,498s <-- head
> BENCHMARKs: VC: 217,613s VO:   0,187s A: 0,000s Sys:  12,141s =  229,940s <-- head + get_coeff optimization
> 
> So now the reason why i call for comments...
> 

	What is the average quant of your bitstream??

	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 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...

	bye!
Skal