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Stream static image to v4l2loopback using obs with v4l2 plugin, ffmpeg or gstreamer

I want to stream a static image to the android studio emulator. To do that I created a virtual camera using v4l2loopback v.12.5 in linux ubuntu 20.04. The virtual camera is created in /dev/video2. I can stream there using obs and a plugin, ffmpeg or gstreamer but I’ve got different issues with each one. I don’t care which software works… I always use an image with the resolution 1920×1080 to test this.

OBS: I installed the obs plugin following this tutorial here. It explains how to install v4l2loopback and add the plugin to obs. The plugin is called obs-v4l2sink. I start v4l2loopback using this command:

sudo modprobe v4l2loopback exclusive_caps=1 video_nr=2 card_label=virtualCam

and then:

v4l2loopback-ctl set-caps "video/x-raw, format=NV12, width=1920, height=1080" "/dev/video2"

Then I start Obs with the base canvas 1920×1080 and output 1920×1080 too. In the plugin I choose NV12 as the video format and /dev/video2 as the V4L2 Device. The image is successfully sent to the emulator but the resolution is wrong. Using ffplay I get this message:

“Input #0, video4linux2,v4l2, from ‘/dev/video2’:B sq= 0B f=0/0 Duration: N/A, start: 1650.347834, bitrate: 27648 kb/s Stream #0:0: Video: rawvideo (NV12 / 0x3231564E), nv12, 320×240, 27648 kb/s, 30 fps, 30 tbr, 1000k tbn, 1000k tbc”

It seems, that the resolution is scaled down to 320×240 and I have no clue why.

ffmpeg: I can stream to /dev/video2 with ffmpeg using this command:

ffmpeg -re loop 1 -i licensePlate.png -f v4l2 /dev/video2 

But I only get wrong pixels in ffplay, which outputs this:

[video4linux2, v4l2 @ 0x7fb494000bc0] Dequeued v4l2 buffer contains 118784 bytes, but 115200 were expected. Flags: 0x00000001.

Gstreamer: Gstreamer only works if I don’t use the following command after the modprobe (If I use it, the pipeline is blocked for Gstreamer):

v4l2loopback-ctl set-caps "video/x-raw, format=NV12, width=1920, height=1080" "/dev/video2"

Instead I call this command:

gst-launch-1.0 -v multifilesrc location=licensePlate2.png loop=1 caps="image/png,framerate=10/1" ! pngdec ! videoscale ! videoconvert ! "video/x-raw,format=NV12,width=1920,height=1080" ! v4l2sink device=/-dev/video2

Using ffplay I see the image I desire in 1920×1080 and everything seems fine, but in the android studio emulator I only get a weird greenscreen. I got this greenscreen before and I think it has got something to do with a wrong resolution. I don’t really know what is wrong here. The emulator shows these logs many times:

emulator: ERROR: camera_device_start_capturing: Dimensions 0x0 are wrong for pixel format NV12
emulator: ERROR: _camera_client_start: Cannot start camera '/dev/video2' for NV12[0x0]: Inappropriate ioctl for device

The avd I’m using is Pixel2 1920×1080 Android 11 x86 64 and I’m starting it using this command:

./emulator -avd Pixel264 -camera-back webcam1 -no-snapshot
Pixel264 is my avd and webcam1 is the virtual webcam, that is linked to /dev/video2



I just tried it with Android Studio Emulator API 28 (Google X86_ARM) and it works. So it seems that it is related to the CPU Architecture of the emulator and x86 64 does not work. I needed arm though and thankfully Google added Arm translation to an image in Api 28.

It seems that only images with resolution 1920×1080 work, but maybe it is only related to the ratio.

Obs does not work now for some reason but this command works perfectly: ffmpeg -loop 1 -re -i licensePlate2.png -f v4l2 -vcodec rawvideo -pixfmt nv12 -vf transpose=4 /dev/video2

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