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

[Jun Li]

On Wed, May 15, 2019 at 3:22 PM Michael Niedermayer <michael at niedermayer.cc>
wrote:

> 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
>

Thanks Michael for review, it is very helpful.
Yes, using cross product should be the right way. Negative value of
m[0]*m[4] - m[1]*m[3]  indicates flipped (the vector points into the
screen).
I tested on the content you shared, "47j9R7PXBep.mov" and "bug.jpg", also
on some exif jpg images from the link in the tickets. All works well.
Let me know if you have any more content need me to test. I have the
updated version:
https://patchwork.ffmpeg.org/patch/13130/
https://patchwork.ffmpeg.org/patch/13131/
Thanks!

Best Regards,
Jun

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