[Ffmpeg-devel] rounding question
Marc Hoffman
mmh
Mon Mar 12 16:59:20 CET 2007
On Mar 9, 2007, at 8:02 PM, Marc Hoffman wrote:
> Looking at the set of pixel functions which do xy corner averaging
> I'm confused by the reference math. There are two functions one
> with rounding and the other with out. The one with rounding adds 4
> samples and then adds 1/2 and truncates. The one that is titled
> no_rnd adds 1/4 and then truncates. I'm a little confused by the
> semantics and am not sure why this is.
>
> excerpt from _pixels_xy2_c
> b= LD64(pixels+1);\
> l0= (a&0x0303030303030303ULL)\
> + (b&0x0303030303030303ULL)\
> + 0x0202020202020202ULL;\
> h0= ((a&0xFCFCFCFCFCFCFCFCULL)>>2)\
>
> excerpt from _no_rnd_pixels_xy2_c
> b= LD64(pixels+1);\
> l0= (a&0x0303030303030303ULL)\
> + (b&0x0303030303030303ULL)\
> + 0x0101010101010101ULL;\
> h0= ((a&0xFCFCFCFCFCFCFCFCULL)>>2)\
>
>
> I guess any direction would be great here I might just be missing
> something like the no_rnd means something other than truncate.
>
> Thanks
> Marc
Does this question make sense to anyone? I really want to get an
understanding of what is meant by no_rnd vs the default which is to
add .5 and then truncate.
Thanks
Marc
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