[FFmpeg-devel] [PATCH] MMX implementation of VC-1 inverse transforms

Michael Niedermayer michaelni
Mon Jan 14 22:01:38 CET 2008

On Mon, Jan 14, 2008 at 09:12:40PM +0100, Christophe GISQUET wrote:
> > - Am I wrong or you do all the math in 16 bit signed saturation mode?
> > According to vc1 draft in first stage the input is in the range
> > [-2048;2047] the multiply constants  are in range [-16;16], this makes
> > range [-32768;32768] per multiply and you can have 8 of them.
> > Or multiply constants in range [-22;22], that make range
> > [-45056;45056] per multiply and you can have 4 of them.

you are missing a detail here
45056 >> 3 would be > 4096 thus possibly violate the limit for the 2nd stage
input. Still the 512 limit of the output with >>7 before does not look like
the naive implementation will work with 16bit

> > In the second phase the input range is doubled to [-4096,4095]
> > 
> > Are you sure your transforms produce the same result as their _c equivalents?
> I did test bit exactness (with win32 dll output) but albeit on few
> sequences. Everything was perfect.
> The reference I found said it could be done on 16 bits maths.  Maybe it
> needs a bias to correct, but as output is usually in the range
> [-128;127], it's pretty symmetrical. However, indeed, it would be better
> if proof could be given.

theres a difference between "can be done" and "it works with the naive

as random example:

(22*X+17*Y) >> 3 will not work with 16bit and X and or Y =2048

((X + ((X + (Y>>1))>>1))>>1) + 2*(X+Y) should work fine

there are of course many intermediate variants
the key point to keep in mind is that (2*x + y)>>1 == x + (y>>1)

Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Many that live deserve death. And some that die deserve life. Can you give
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