[Ffmpeg-devel] few remarks for h264 decoder

Michael Niedermayer michaelni
Fri Dec 30 23:59:38 CET 2005


On Fri, Dec 30, 2005 at 09:16:16PM +0100, G?bor Kov?cs wrote:
> First of all sorry for not making patches or more detailed descriptions, 
> but I don't
> have more time for these problems and I'am not depending on ffmpeg's h264 
> decoding accuracy. Here are the isseus I think(!) I found:
> 1.
> pred_direct_motion() uses 8bit width fill_rectangle() with ref[0],ref[1] 
> which can be negative numbers causing trouble with val*0x0101 in 
> fill_rectangle()

val is unsigned!


> 5.
> I didn't check it throughly but mmx h264_qpel4_hv_lowpass macro uses
> approximation (comment sais ((a-b)/4-b)/4). I know a proper solution
> needs 32bit range, but this is why God created "pmaddwd" :) I think the
> performance loss needed for accurate calculation would be minimal.

i dont see why you claim this to be an approximation, please elaborate /
proof your claim


do you have any sample files which are decoded differently in lavc then
with the reference sw due to any of the possible issues you posted? these
would be quite usefull if anyone wants to investigate&fix these


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