v4l2: Limit resolutions that a webcam offers

Question

I'm using a HDMI capture card to use a regular mirrorless camera as a webcam. This works great at 1080p or 720p resolutions, but the capture card does not only offer resolutions with 16:9 aspect ratio, but also with 4:3. If such a resolution is selected, then the video signal is stretched/distorted. Unfortunately one of the programs I'm using always chooses such a distorted resolution.

Here is the output of v4l2-ctl -d /dev/video0 --list-formats-ext:

ioctl: VIDIOC_ENUM_FMT
    Type: Video Capture

    [0]: 'MJPG' (Motion-JPEG, compressed)
        Size: Discrete 1920x1080
            Interval: Discrete 0.017s (60.000 fps)
            Interval: Discrete 0.033s (30.000 fps)
            Interval: Discrete 0.040s (25.000 fps)
            Interval: Discrete 0.050s (20.000 fps)
            Interval: Discrete 0.100s (10.000 fps)
        Size: Discrete 1600x1200
            Interval: Discrete 0.017s (60.000 fps)
            Interval: Discrete 0.033s (30.000 fps)
            Interval: Discrete 0.040s (25.000 fps)
            Interval: Discrete 0.050s (20.000 fps)
            Interval: Discrete 0.100s (10.000 fps)
        Size: Discrete 1360x768
            Interval: Discrete 0.017s (60.000 fps)
            Interval: Discrete 0.033s (30.000 fps)
            Interval: Discrete 0.040s (25.000 fps)
            Interval: Discrete 0.050s (20.000 fps)
            Interval: Discrete 0.100s (10.000 fps)
        Size: Discrete 1280x1024
            Interval: Discrete 0.017s (60.000 fps)
            Interval: Discrete 0.033s (30.000 fps)
            Interval: Discrete 0.040s (25.000 fps)
            Interval: Discrete 0.050s (20.000 fps)
            Interval: Discrete 0.100s (10.000 fps)
        Size: Discrete 1280x960
            Interval: Discrete 0.017s (60.000 fps)
            Interval: Discrete 0.033s (30.000 fps)
            Interval: Discrete 0.040s (25.000 fps)
            Interval: Discrete 0.050s (20.000 fps)
            Interval: Discrete 0.100s (10.000 fps)
        Size: Discrete 1280x720
            Interval: Discrete 0.017s (60.000 fps)
            Interval: Discrete 0.020s (50.000 fps)
            Interval: Discrete 0.033s (30.000 fps)
            Interval: Discrete 0.050s (20.000 fps)
            Interval: Discrete 0.100s (10.000 fps)
        Size: Discrete 1024x768
            Interval: Discrete 0.017s (60.000 fps)
            Interval: Discrete 0.020s (50.000 fps)
            Interval: Discrete 0.033s (30.000 fps)
            Interval: Discrete 0.050s (20.000 fps)
            Interval: Discrete 0.100s (10.000 fps)
        Size: Discrete 800x600
            Interval: Discrete 0.017s (60.000 fps)
            Interval: Discrete 0.020s (50.000 fps)
            Interval: Discrete 0.033s (30.000 fps)
            Interval: Discrete 0.050s (20.000 fps)
            Interval: Discrete 0.100s (10.000 fps)
        Size: Discrete 720x576
            Interval: Discrete 0.017s (60.000 fps)
            Interval: Discrete 0.020s (50.000 fps)
            Interval: Discrete 0.033s (30.000 fps)
            Interval: Discrete 0.050s (20.000 fps)
            Interval: Discrete 0.100s (10.000 fps)
        Size: Discrete 720x480
            Interval: Discrete 0.017s (60.000 fps)
            Interval: Discrete 0.020s (50.000 fps)
            Interval: Discrete 0.033s (30.000 fps)
            Interval: Discrete 0.050s (20.000 fps)
            Interval: Discrete 0.100s (10.000 fps)
        Size: Discrete 640x480
            Interval: Discrete 0.017s (60.000 fps)
            Interval: Discrete 0.020s (50.000 fps)
            Interval: Discrete 0.033s (30.000 fps)
            Interval: Discrete 0.050s (20.000 fps)
            Interval: Discrete 0.100s (10.000 fps)
    [1]: 'YUYV' (YUYV 4:2:2)
        Size: Discrete 1920x1080
            Interval: Discrete 0.200s (5.000 fps)
        Size: Discrete 1600x1200
            Interval: Discrete 0.200s (5.000 fps)
        Size: Discrete 1360x768
            Interval: Discrete 0.125s (8.000 fps)
        Size: Discrete 1280x1024
            Interval: Discrete 0.125s (8.000 fps)
        Size: Discrete 1280x960
            Interval: Discrete 0.125s (8.000 fps)
        Size: Discrete 1280x720
            Interval: Discrete 0.100s (10.000 fps)
        Size: Discrete 1024x768
            Interval: Discrete 0.100s (10.000 fps)
        Size: Discrete 800x600
            Interval: Discrete 0.050s (20.000 fps)
            Interval: Discrete 0.100s (10.000 fps)
            Interval: Discrete 0.200s (5.000 fps)
        Size: Discrete 720x576
            Interval: Discrete 0.040s (25.000 fps)
            Interval: Discrete 0.050s (20.000 fps)
            Interval: Discrete 0.100s (10.000 fps)
            Interval: Discrete 0.200s (5.000 fps)
        Size: Discrete 720x480
            Interval: Discrete 0.033s (30.000 fps)
            Interval: Discrete 0.050s (20.000 fps)
            Interval: Discrete 0.100s (10.000 fps)
            Interval: Discrete 0.200s (5.000 fps)
        Size: Discrete 640x480
            Interval: Discrete 0.033s (30.000 fps)
            Interval: Discrete 0.050s (20.000 fps)
            Interval: Discrete 0.100s (10.000 fps)
            Interval: Discrete 0.200s (5.000 fps)

Is there a way to disable some of these resolutions, so that software accessing the webcam does not pick a non-16:9 aspect ratio?

Answer 1

One workaround that I found myself is to create a second, virtual device using v4l2-loopback and to stream the desired resolution into that device using ffmpeg.

First, to create the virtual device at /dev/video7:

sudo modprobe v4l2loopback devices=1 video_nr=7 card_label="VirtualCam" exclusive_caps=1

To stream video data from /dev/video0 to /dev/video7:

ffmpeg -f video4linux2 -framerate 25 -video_size 1280x720 -input_format mjpeg -i /dev/video0 -f v4l2 -pix_fmt yuv420p /dev/video7

However that feels like a hack, and it would be much nicer if I could change the advertised capabilities of the original video device directly. Thus, if someone has an answer to that, it would be very much appreciated.