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In case of an orthogonal transformation av_display_rotation_get() returns the (anticlockwise) degree that the unit vector in x-direction gets rotated by; get_rotation in cmdutils.c makes a clockwise degree out of this. So if one inserts a transpose filter corresponding to this degree, then the x-vector gets mapped correctly and there are two possibilities for image of the y-vector, namely the two unit vectors orthogonal to the image of the x-vector. E.g. if the x-vector gets rotated by 90° clockwise, then the two possibilities for the y-vector are the unit vector in x direction or its opposite. The latter case is a simple 90° rotation for both vectors* whereas the former is a simple 90° clockwise rotation followed by a horizontal flip. These two cases can be distinguished by looking at the x-coordinate of the image of the y-vector, i.e. by looking at displaymatrix[3]. Similarly for the case of a 270° clockwise rotation. These two cases were previously wrong (they were made to match wrongly parsed exif rotation tag values). *: For display matrices, the y-axis points downward. Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>