For months I was unable to render out video when Movit was enabled, by accident I finally found the cause-and the fix. When I disabled VDPAU support in testing an unrelated bug, I also accidently removed the old --disable-qt option I had used to keep builds in the early days of QT5/kdenlive 15.04 from loading QT4 and segfaulting. The QT module is required to create the OpenGL context used in rendering according to MLT devs. It turns out that OpeGL/Movit is incompatable with VDPAU support in MLT. When I was investigating an issue with the new wipe transitions in December 2015, I had corrupted video from one track, blaned VDPAU, and rebuilt MLT with VDPAU disabled. No help with the transitions, so I tried OpenGL and the transitions worked perfectly but I thought i would not be able to render. Turned out rendering worked perfectly, a little slower than without OpenGL/Movit but now I can use the Movit effects as well. This sucessful test was a little less than a week ago, using Kdenlive built from GIT master, I think it was my Jan 21 build. Apparently when I reconfigured MLT I cut this entire string off my build options: --disable-qt --enable-movit --avformat-vdpau and the one causing the trouble was --disable-qt, NOT the --avformat-vdpau switch.

If MLT is build without the QT module, the OpenGL render context cannot be created. so enabling openGL/Movit then causes Kdenlive to render to the screen and either a 0 byte file or none. If MLT is built without VDPAU support, rendering proceeds normally to a file. Apparently VDPAU is forcing the output to the screen and preventing it from being returned from the video card back to system RAM and finally a file. I sucessfully tested VDPAU with Movit on March 29, 2016 but got no advantage in performance.

I just got builds of MLT 6.1.0 that work with each of these sets of build options:

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./configure --prefix=/usr --enable-gpl --enable-gpl3

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./configure --prefix=/usr --enable-gpl 

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./configure --prefix=/usr --enable-gpl --enable-gpl3 --avformat-no-vdpau


Anything using the old --disable-qt that used to be needed in KF5 builds to prevent segfaults when KDE4 was also installed will cause rendering to divert to the screen. According to the MLT devs the QT module is used to create the openGL context so I don't know why any set of build options disabling it ever worked-but it did when I shortened my build options cutting off --avformat-vdpau and --enable-movit. Perhaps I cut off --disable-qt as well and never realized it?

https://github.com/mltframework/mlt/issues/105 was based on incomplete results and closed but contains much discussion of this, including the tips from MLT devs that told me the QT module being disabled was the culprit