[FFmpeg-devel] [PATCH v2 1/2] lavu/display: add av_display_rotation_hflip_get and bump version

[Michael Niedermayer]

On Tue, May 14, 2019 at 10:36:44PM -0700, Jun Li wrote:
> Add av_display_rotation_hflip_get to get information whether the
> matrix indicates horizontal flip.
> ---
>  doc/APIchanges            |  3 +++
>  libavutil/display.c       | 14 ++++++++++++++
>  libavutil/display.h       | 14 ++++++++++++++
>  libavutil/tests/display.c |  8 ++++++++
>  libavutil/version.h       |  2 +-
>  tests/ref/fate/display    |  4 ++++
>  6 files changed, 44 insertions(+), 1 deletion(-)
> 
> diff --git a/doc/APIchanges b/doc/APIchanges
> index e75c4044ce..73c6809563 100644
> --- a/doc/APIchanges
> +++ b/doc/APIchanges
> @@ -15,6 +15,9 @@ libavutil:     2017-10-21
>  
>  API changes, most recent first:
>  
> +2019-05-13 - XXXXXXXXXX - lavu 56.28.100 - display.h
> +  Add av_display_rotation_hflip_get
> +
>  2019-04-20 - 3153a6502a - lavc 58.52.100 - avcodec.h
>    Add AV_CODEC_FLAG_DROPCHANGED to allow avcodec_receive_frame to drop
>    frames whose parameters differ from first decoded frame in stream.
> diff --git a/libavutil/display.c b/libavutil/display.c
> index a0076e067b..f5a6a4a08d 100644
> --- a/libavutil/display.c
> +++ b/libavutil/display.c
> @@ -71,3 +71,17 @@ void av_display_matrix_flip(int32_t matrix[9], int hflip, int vflip)
>          for (i = 0; i < 9; i++)
>              matrix[i] *= flip[i % 3];
>  }
> +
> +double av_display_rotation_hflip_get(const int32_t matrix[9], int *hflip)
> +{
> +    int32_t m[9];
> +    *hflip = 0;
> +    memcpy(m, matrix, sizeof(m));
> +    

> +    if (m[0] > 0 && m[4] < 0 || m[0] < 0 && m[4] > 0 ||
> +        m[1] > 0 && m[3] > 0 || m[1] < 0 && m[3] < 0) {
> +        *hflip = 1;
> +        av_display_matrix_flip(m, 1, 0);
> +    }

This is not correct
consider the identity transform matrix
1   0
0   1

now if the 0 elements are only so slightly off 0 this could set hflip, this
does not make the matrix flip anything, its still basically a identity matrix.

What you want to detect here, i _think_, is the direction in which the matrix
rotates the vector. Not sure what the technical term is but the "z" sign of
the cross product of the 2 basis vectors should be that if iam not mistaken.

I think that results in something like m[0]*m[4] < m[1]*m[3] but ive not
checked this so please check before using it 

thx

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Those who would give up essential Liberty, to purchase a little
temporary Safety, deserve neither Liberty nor Safety -- Benjamin Franklin
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