Not able to make libvlc_video_set_output_callbacks work with libVLC 4.0

[gsibaldi]

Hello everyone,

I am working on a mixed libVLC / openFrameworks project.
Until now I used libvLC 3.0.9 and libvlc_video_set_callbacks to get pixel frame buffers, and it works very well except for performance issues when playing many videos simultaneously.
So I decided to try libVLC 4.0 which AFAIK natively supports openGL, hoping to get some performance boost.
I downloaded and set up the last nightly build, then in order to have a starting reference point I had a look at some early Qt and SDL code using openGL at https://git.videolan.org.
I tried some openFrameworks customization and started testing, but I'm not able to get any useful result from setting libvlc_video_set_output_callbacks, not even a crash or a nasty error.
The only thing I can see in the console is that VLC media player starts parsing an .mp4 file normally and as soon as the video decoding process begins I can hear a good audio but I get a continuous stream of these errors:

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[0000000101705ea0] main video output error: video output display creation failed [h264 @ 0x1018b3e00] get_buffer() failed [h264 @ 0x1018b3e00] thread_get_buffer() failed [h264 @ 0x1018b3e00] decode_slice_header error [h264 @ 0x1018b3e00] no frame!

So I start putting some debug feedback text into the callbacks, supposing there are video out setup problems or GL related problems.
And I discovered that no one of the callbacks set with libvlc_video_set_output_callbacks gets actually called.
From debug text I also see that libvlc_video_set_output_callbacks returns 1, which is a "true" value and so it seems that callbacks are successfully set.

I'm on macOS 10.14.6, Xcode 10.1 and after a whole day of trial and error and googling around I have no idea about what's happening here.
I include below a link to the "core" class of my code: any suggestion, clue or advice is warmly welcome.

Thank you in advance.

https://pastebin.com/fEX8ybc7

[gsibaldi]

UPDATE: rising the "verbose" debug level now I get some more information:

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[0000600000ca82d0] iokit_inhibit inhibit debug: Inhibiting display sleep [000000010144a090] main video output debug: deinterlace -1, mode auto, is_needed 0 [000000010144a090] main video output debug: Opening vout display wrapper [00007000089eb568] main generic debug: looking for vout display module matching "gl": 16 candidates [00007000089eb568] main generic debug: no vout display modules matched [000000010144a090] main video output error: video output display creation failed [0000600000ca82d0] iokit_inhibit inhibit debug: Releasing previous IOPMAssertion [0000600000ca82d0] iokit_inhibit inhibit debug: Removed previous inhibition [0000000101161260] main player debug: saving a free vout [h264 @ 0x105000000] get_buffer() failed [h264 @ 0x105000000] thread_get_buffer() failed [h264 @ 0x105000000] decode_slice_header error [h264 @ 0x105000000] no frame! [h264 @ 0x102097400] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 2 [h264 @ 0x102097400] Frame num gap 17 13

The problem appears to be "video output display creation failed" because gl module has not been found.
But how to fix that?...