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

[Michael Niedermayer]

On Wed, May 28, 2008 at 11:14:31AM -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.
> > > >
> > > > > The video
> > > > >
> > http://samples.mplayerhq.hu/mov/tkhd_matrix/white_zombie_scrunch.m
> > > > > ov plays at 160x240 when it should really be scaled to 320x240.
> > > >
> > > > > +    int i;
> > > > > +    int width;
> > > > > +    int height;
> > > > > +    float disp_transform[3];
> > > > > +    float display_matrix[3][3];
> > > >
> > > > please use integers, there is no need for floats.
> > > >
> > > Ok, I changed display_matrix to int, but left disp_transform as float
> > to avoid any possible overflow from multiplying large ints.
> >
> > use a*(int64_t)b
> >
> All right - no more nasty floats - I switched it to int64_t.
> The only caveat is a float cast when calculating the aspect ratio, since multiplying an int_64 by an int_32 won't fit in 64bit precision.
> 
> To Reimar's note "we do have function for multiplying two 16.16 fixed-point numbers correctly"; I couldn't find those functions.  If anyone knows where that is off the top of their head, I would be appreciated.  The closest thing I found was libavutil/integer, and that didn't look used.
[...]

> +    //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.


> +
> +        //the sample aspect ratio is new width/height divided by old width/height
> +        st->codec->sample_aspect_ratio = av_d2q(
> +            ((float) disp_transform[0] * height) /
> +            ((float) disp_transform[1] * width), INT_MAX);

float has insufficient precission to convert it to a 32bit fraction reliably

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

Dictatorship naturally arises out of democracy, and the most aggravated
form of tyranny and slavery out of the most extreme liberty. -- Plato
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