USB webcam and available formats

Question

I have an ELP usb camera. The available formats are:

$ v4l2-ctl --list-formats-ext -d /dev/video1
ioctl: VIDIOC_ENUM_FMT
Type: Video Capture

[0]: 'MJPG' (Motion-JPEG, compressed)
    Size: Discrete 1920x1080
        Interval: Discrete 0.033s (30.000 fps)
    Size: Discrete 1280x720
        Interval: Discrete 0.017s (60.000 fps)
    Size: Discrete 1024x768
        Interval: Discrete 0.033s (30.000 fps)
    Size: Discrete 640x480
        Interval: Discrete 0.008s (120.101 fps)
    Size: Discrete 800x600
        Interval: Discrete 0.017s (60.000 fps)
    Size: Discrete 1280x1024
        Interval: Discrete 0.033s (30.000 fps)
    Size: Discrete 320x240
        Interval: Discrete 0.008s (120.101 fps)
[1]: 'YUYV' (YUYV 4:2:2)
    Size: Discrete 1920x1080
        Interval: Discrete 0.167s (6.000 fps)
    Size: Discrete 1280x720
        Interval: Discrete 0.111s (9.000 fps)
    Size: Discrete 1024x768
        Interval: Discrete 0.167s (6.000 fps)
    Size: Discrete 640x480
        Interval: Discrete 0.033s (30.000 fps)
    Size: Discrete 800x600
        Interval: Discrete 0.050s (20.000 fps)
    Size: Discrete 1280x1024
        Interval: Discrete 0.167s (6.000 fps)
    Size: Discrete 320x240
        Interval: Discrete 0.033s (30.000 fps)

Suppose I would like to use MJPG 1280x720 @ 20fps, or even something strange like 1280x720 @ 18fps.

This answer says:

Only the listed formats are supported by v4l2, anything else would need to be coded by the user, and RGBs are very seldom provided, despite virtually all CCDs working in Bayer RGGB [...]

What's the way to set custom resolution and framerate? Even more silly question, is there a way to control shutter time with these USB cameras?

EDIT: I'm working with Python. OpenCV cannot access the camera at all, while it works with Webcamoid software.

Were you able to get the desired framerate from the camera? I'm also using opencv and the acquisition process is very slow. — Alex, Jul 09 '22 at 12:01

score 0 · Answer 1 · answered Nov 08 '20 at 08:58

I would just change the frequency that you sample from the camera to the frame rate you desire.

If we look at the MJPEG:

Size: Discrete 1280x720 Interval: Discrete 0.017s (60.000 fps)

Just read from that stream 20 times a second (essentially just getting every 3rd frame), or 18 times a second (getting every third/fourth frame).

IN HTML you'd do this with the getUserMedia:

set your variables of width, height and framerate:

var constraints = { audio: {sampleRate: audiobitrate, 
                            echoCancellation: true},
    video:{
        width: { min: 100, ideal: width, max: 1920 },
        height: { min: 100, ideal: height, max: 1080 },
        frameRate: {ideal: framerate}
    }
};
console.log(constraints);
navigator.mediaDevices.getUserMedia(constraints).then(function(stream) {

// do your thing... }

This code is pulled from a livestream app I built (line 273): https://github.com/dougsillars/browserLiveStream/blob/master/public/index.html

USB webcam and available formats

1 Answers1