[FFmpeg-devel] [PATCH] mpeg2: fix block_last_index when mismatch control modifies last coeff

[Michael Niedermayer]

On Mon, Jun 21, 2010 at 07:12:44PM +0100, M?ns Rullg?rd wrote:
> Michael Niedermayer <michaelni at gmx.at> writes:
> 
> > On Mon, Jun 21, 2010 at 08:00:33PM +0200, Michael Niedermayer wrote:
> >> On Mon, Jun 21, 2010 at 05:12:46PM +0100, M?ns Rullg?rd wrote:
> >> > Michael Niedermayer <michaelni at gmx.at> writes:
> >> > 
> >> > >> Having an incorrect value in block_last_index just seems wrong to me.
> >> > >
> >> > > i dont think a completely correct value would be very efficient speedwise
> >> > > as 75% of the cases where the last coeff is set have no effect on the post
> >> > > dct output for dc blocks (that is with an assumtation of dc values all being
> >> > > equally likely)
> >> > 
> >> > The last coeff is always manipulated, not only for dc-only blocks.  Do
> >> > you know for sure exactly in which cases it makes a difference?
> >> 
> >> I dont think anything except the idct is using that coeff but i 
> >> i cannot formally proof that it is so or is bug free ...
> >> If you have doubts, its surely possible to perform some tests to see
> >> if its value makes a difference outside the idcts
> >
> > Also to everyone to whom things feel semantically wrong
> > one can think of mismatch control to be part of the idct, which isnt
> > far fetched semantically. In that case it makes sense not to update
> > block_last_index. And we then would just be messing with he 64th coeff
> > in the coeff decoder because thats faster there than seperately later.
> 
> Sure, but it's hard to optimise the idct properly when the input
> parameters are wrong.  Choosing various shortcuts based on
> block_last_index is a natural thing to do, but it's not possible when
> this value cannot be trusted.

That specific idct can be disabled or replaced by something else for mpeg2

Setting block_last_index "correctly" just means setting it to 63
that will just disable the optimisations

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Why not whip the teacher when the pupil misbehaves? -- Diogenes of Sinope
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