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

[John Schmiederer]

> > > > > > > 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

Oops - I misapplied those parentheses to fix a "parentheses around + or - inside shift" warning.
Attached patch does it right this time.

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

Agreed.

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

Thanks again for all the help.

-John

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