Comment by eonpi | Hacker News Reader

Comment by eonpi | Hacker News Reader

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Not sure the kind of processing you need to do on the video stream, but have you considered giving `ffmpeg` a try if you just need plain pass-thru from video input to output? `ffmpeg` might be built with support for the Mali libraries you mention for the OS you are using. If you are able to run `weston`, `ffmpeg` should be able to output directly to the DRM card thru the use of SDL2 (assuming it was built with it).

If the HDMI-USB capture card that outputs `mjpeg` exposes a `/dev/video` node, then it might be as simple as running:

`SDL_VIDEODRIVER=kmsdrm ffmpeg -f video4linux2 -input_format mjpeg -i /dev/video0 -f opengl "hdmi output"`

An alternative could be if you can get a Raspberry Pi 3 or even 2, and can get a distro where `omxplayer` can still be installed. You can then use `omxplayer` to display your mjpeg stream on the output of your choice, just make sure the `kms/fkms` dtoverlay is not loaded because `omxplayer` works directly with DispmanX/GPU (BTW, not compatible with Pi 4 and above and much less 5), which contains a hardware `mjpeg` decoder, so for the most part, bytes are sent directly to the GPU.

Hope some of this info can be of help.

Looks helpful! I assume ffmpeg needs to be built with SDL for this to work? I couldn't get it to work with my current minimal compile, but I don't think the board I'm working on has SDL, so might need to install that and recompile

That's correct, `ffmpeg` needs to be built with `SDL` (SDL2 really is what is used on all recent versions). When `ffmpeg` is built, and the dev files for SDL2 are present, ffmpeg's build configuration picks it up automatically and it will link against the library unless instructed otherwise by a configuration flag. When you run `ffmpeg` first lines usually show the configuration which it was built, so it might have hints as to what it was built with, but if you want to confirm what it was links against you can do a quick:

$ ldd `which ffmpeg`

And you should get the list of dynamic libraries your build is linked against. If SDL2 is indeed included, you should see a line starting with "libSDL2-2...".

If I remember correctly you should be able to output to the framebuffer even if there is no support for SDL2, you just have to change the previous line provided from using `-f opengl "hdmi output"` to `-pix_fmt bgra -f fbdev /dev/fb0`.

You can also use any other framebuffer device if present and you'd prefer it (e.g. /dev/fb1, /dev/fb2). Also, you might need something different to `bgra` on your board, but usually `ffmpeg` will drop a hint as to what.

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