[FFmpeg-devel] [PATCH] mov: Evaluate the movie display matrix

[Vittorio Giovara]

On Fri, Oct 7, 2016 at 8:38 PM, Michael Niedermayer
<michael at niedermayer.cc> wrote:
> On Fri, Oct 07, 2016 at 03:31:46PM -0400, Vittorio Giovara wrote:
>> This matrix needs to be applied after all others have (currently only
>> display matrix from trak), but cannot be handled in movie box, since
>> streams are not allocated yet.
>>
>> So store it in main context and if not identity, apply it when appropriate,
>> handling the case when trak display matrix is identity and when it is not.
>>
>> Signed-off-by: Vittorio Giovara <vittorio.giovara at gmail.com>
>> ---
>
>> +        } else { // otherwise multiply the two and store the result
>> +            int64_t val = 0;
>> +            for (i = 0; i < 3; i++) {
>> +                for (j = 0; j < 3; j++) {
>> +                    int sh = j == 2 ? 30 : 16;
>> +                    for (e = 0; e < 3; e++) {
>> +                        val += CONV_FP2INT(display_matrix[i][e], sh) *
>> +                               CONV_FP2INT(c->movie_display_matrix[e][j], sh);
>
> This does not work (you are dividing the 32bit numbers down to 2 bit)

I don't understand this comment, are you referring to the fact that
the first two columns of the display matrix are 16.16, while the third
column is 2:30?

> also its not tested by the fate testcase
> i can just replace it by 0 and fate-mov-movie-display-matrix still
> passes

Yes, the sample I shared only has the movie display matrix, not
trak+movie. I can create another sample if you insist, but a fate test
would test little more than the matrix multiplication loops.

> the macros also lack some () protection giving probably unintended
> results

Can you tell me how many more () would they need?

>From the subsequent email

> to explain a bit more how to do it with int64 instead of double floats
> int32 can be multiplied together to form int64 with a new fixed point
> without rounding or shifts
> then added up and then at the end shifted down with rounding to get
> back to the fixed point needed for the final destination

The problem is that the display matrix fields have different mantissa
length, the first two columns are 16.16, the last one is 2:30. I think
that method would work if all elements were the same length no?
-- 
Vittorio