[FFmpeg-devel] [PATCH] rotate filter

[Michael Niedermayer]

On Thu, May 05, 2011 at 01:31:26AM +0200, Stefano Sabatini wrote:
> On date Wednesday 2011-05-04 14:56:12 +0200, Michael Niedermayer encoded:
> > On Wed, May 04, 2011 at 12:31:29PM +0200, Stefano Sabatini wrote:
> [...]
> > > > frac_x >>= 6;
> > > > frac_y >>= 6;
> > > > for (i = 0; i < src_linestep; i++) {
> > > >     int s00 = src[src_linestep * int_x  + i + src_linesize * int_y ];
> > > >     int s01 = src[src_linestep * int_x1 + i + src_linesize * int_y ];
> > > >     int s10 = src[src_linestep * int_x  + i + src_linesize * int_y1];
> > > >     int s11 = src[src_linestep * int_x1 + i + src_linesize * int_y1];
> > > >     int s0 = (((1<<10) - frac_x)*s00 + frac_x*s01);
> > > >     int s1 = (((1<<10) - frac_x)*s10 + frac_x*s11);
> > > > 
> > > >     dst_color[i] = (((1<<10) - frac_y)*s0 + frac_y*s1)>>20;
> > > > }
> > > > 
> > > > you can also shift by just 4 instead of 6 and use unsigned values
> > > 
> > > Isn't this losing precision (and indeed producing a different output)?
> > > Which are the pros/cons?
> > 
> > 64bit arithmetic is slow on hw that does not have native 64bit support
> > and with infinite precisse coeffs you will have +-0.5 errors
> > with 16bit ~0.502
> > with 12bit ~0.531
> > 
> > these worst case errors happen on black white transitions. in flatly
> > colored areas the difference is significantly smaller that is its all
> > pretty much 0.5 there
> > 
> > also mmx/sse optimized code would not use 64bit arithmetic so it would
> > produce different output when C is using 64bit arithmetic
> 
> Yes I see the point but the code changed like that as a rather visible
> quality loss.

well ive not tested, it could contain bugs.
you could commit the int64 version and i could test and change to
int32 later

maybe you could publish your work in a branch or something this way we
could easier both work on the code.

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

It is dangerous to be right in matters on which the established authorities
are wrong. -- Voltaire
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