The cairoblend transition has an optimization that if the b frame is opaque, it will never request the a frame imagehttps://github.com/mltframework/mlt/blob/master/src/modules/frei0r/transition_frei0r.c#L61 When this optimization is triggered, get_frame is never called on the a frame. As a result, the field order filter never has a chance to set the top_field_first flag on the a frame.https://github.com/mltframework/mlt/blob/master/src/modules/core/filter_fieldorder.c#L115
As a test, you can defeat the optimization in a couple of ways:* Set invert=1 on the transition* Set the color to be slightly transparent* Comment out the optimization in the code What should be done in this optimization case? I am not sure. Maybe copy the key image properties from the b frame to the a frame. There are many others that could be included. This might be a pretty good list here: https://github.com/mltframework/mlt/blob/master/src/framework/mlt_tractor.c#L379 Or maybe there could be a different trick like all the contents of the a_frame and b_frame could be swapped when performing this optimization. ~Brian On Thursday, July 20, 2023 at 02:42:47 AM CDT, Jean-Baptiste Mardelle <j...@kdenlive.org> wrote: Hi all, I was recently made aware of an issue in interlaced rendering that seems like a bug but I am not exactly sure where the problem lies. The problem is that in some cases, MLT renders an interlaced video with bottom field first, even when asked for a top field first result. I tracked down the issue to some transitions. For example with a simple 2 track color sample : melt color:red out=20 -track color:blue out=20 -consumer avformat:test.mp4 f=mxf progressive=0 real_time=-1 threads=0 top_field_first=1 vcodec=mpeg2video Produces a correct top field first video. Now if you add a transition but keep the exact same rendering parameters: melt color:red out=20 -track color:blue out=20 -transition frei0r.cairoblend a_track=0 b_track=1 out=20 -consumer avformat:test.mp4 f=mxf progressive=0 real_time=-1 threads=0 top_field_first=1 vcodec=mpeg2video You get an interlaced video with bottom field first! Not all transition seem to have this problem, but at least frei0r.cairoblend and qtblend do break the field order. FYI, I used mediainfo to detect the field order of the rendered videos. Do you have any hint on where / how to fix this ? Thanks a lot, Jean-Baptiste_______________________________________________ Mlt-devel mailing list Mlt-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/mlt-devel
_______________________________________________ Mlt-devel mailing list Mlt-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/mlt-devel
