[FFmpeg-devel] [PATCH] Use tkhd matrix for proper display in mov

[Michael Niedermayer]

On Wed, May 28, 2008 at 05:02:12PM -0400, John Schmiederer wrote:
> > > > > > On Tue, May 27, 2008 at 02:49:24PM -0400, John Schmiederer
> > wrote:
> > > > > > > > > Attached is a patch to account for the transformation
> > > > > > > > > matrix
> > > > > > contained in the tkhd atom for proper display width/height.
> > > > > > > >
> [...]
> 
> > > > > +    //transform the display width/height according to the matrix
> > > > > +    if (width && height) {
> > > > > +        for (i = 0; i < 2; i++)
> > > > > +            disp_transform[i] =
> > > > > +                (int64_t) width * display_matrix[0][i] +
> > > > > +                (int64_t) height * display_matrix[1][i] +
> > > > > +                display_matrix[2][i];
> > > >
> > > > This is not what the original patch did.
> > >
> > > It may look a little different, but the functionality has not
> > changed.
> >
> > You removed the scaling by 1<<16 and 1<<30 the code is no longer doing
> > the same. The relative scaling of w*matrix[0] and  matrix[2] has
> > changed They are added together so the result is more than just wrong
> > by a constant scale factor.
> 
> Argh!  You're right, the math is off.
> Although I maintain that the functionality didn't change from the first patch - that was wrong, too. =)
> I forgot that width and height were 16.16 fixed, so instead of multiplying by [width height 1] it should have always been [width height 1<<16]
> 
> The 1<<16 vs 1<<30 scaling is no longer an issue though, as all the 1<<30 scaled terms were in that last column of the display matrix that I no longer read in or use (display_matrix[*][2], not display_matrix[2][*]).
> 
> So in this updated patch all the multiplied terms are in the same scale.

[...]

> +    //transform the display width/height according to the matrix
> +    // to keep the same scale, use [width height 1<<16]
> +    if (width && height) {
> +        for (i = 0; i < 2; i++)
> +            disp_transform[i] =
> +                (int64_t) width * display_matrix[0][i] +
> +                (int64_t) height * display_matrix[1][i] +

> +                (int64_t) (display_matrix[2][i] << 16);

with that order of operations display_matrix[2][i] << 16 can overflow

also things can be vertically aligned like:
                (int64_t) width  * display_matrix[0][i] +
                (int64_t) height * display_matrix[1][i] +

looks more pretty ...

anyway iam mostly ok with the patch after this, maybe baptiste has some
further comments though ...

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

Freedom in capitalist society always remains about the same as it was in
ancient Greek republics: Freedom for slave owners. -- Vladimir Lenin
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