How can I correctly provide a mock webcam video to Chrome?

Question

I'm trying to run end-to-end testing in Chrome for a product that requires a webcam feed halfway through to operate. From what I understand this means providing a fake webcam video to Chrome using the --use-file-for-fake-video-capture="/path/to/video.y4m" command line argument. It will then use that as a webcam video.

However, no matter what y4m file I provide, I get the following error from Chrome running under these conditions:

DOMException: Could not start video source
{
  code: 0,
  message: "Could not start video source",
  name: "NotReadableError"
}

Notably I can provide an audio file just fine using --use-file-for-fake-audio-capture and Chrome will work with it well. The video has been my sticking point.

This error comes out of the following straightforward mediaDevices request:

navigator.mediaDevices.getUserMedia({ video: true, audio: true })
  .then(data => {
    // do stuff
  })
  .catch(err => {
    // oh no!
  });

(This always hits the “oh no!” branch when a video file is provided.)

What I've tried so far

I've been running Chrome with the following command line arguments (newlines added for readability), and I'm using a Mac hence the open command:

open -a "Google Chrome" --args
  --disable-gpu
  --use-fake-device-for-media-stream
  --use-file-for-fake-video-capture="~/Documents/mock/webcam.y4m"
  --use-file-for-fake-audio-capture="~/Documents/mock/microphone.wav"

webcam.y4m and microphone.wav were generated from a video file I recorded.

I first recorded a twenty-second mp4 video using my browser's MediaRecorder, downloaded the result, and converted it using the following command line commands:

ffmpeg -y -i original.mp4 -f wav -vn microphone.wav
ffmpeg -y -i original.mp4 webcam.y4m

When this didn't work, I tried the same using a twenty-second movie file I recorded in Quicktime:

ffmpeg -y -i original.mov -f wav -vn microphone.wav
ffmpeg -y -i original.mov webcam.y4m

When that also failed, I went straight to the Chromium file that explains fake video capture, went to the example y4m file list it provided, and downloaded the grandma file and provided that as a command line argument to Chrome instead:

open -a "Google Chrome" --args
  --disable-gpu
  --use-fake-device-for-media-stream
  --use-file-for-fake-video-capture="~/Documents/mock/grandma_qcif.y4m"
  --use-file-for-fake-audio-capture="~/Documents/mock/microphone.wav"

Chrome provides me with the exact same error in all of these situations.

The only time Chrome doesn't error out with that mediaDevices request is when I omit the video completely:

open -a "Google Chrome" --args
  --disable-gpu
  --use-fake-device-for-media-stream
  --use-file-for-fake-audio-capture="~/Documents/mock/microphone.wav"

Accounting for C420mpeg2

TestRTC suggests Chrome will “crash” if I give it a C420mpeg2 file, and recommends that simply replacing the metadata fixes the issue. Indeed the video file I generate from ffmpeg gives me the following header:

YUV4MPEG2 W1280 H720 F30:1 Ip A1:1 C420mpeg2 XYSCSS=420MPEG2

Chrome doesn't actually crash when run with this file, I just get the error above. If I edit the video file to the following header though per TestRTC's recommendations I get the same situation:

YUV4MPEG2 W1280 H720 F30:1 Ip A1:1 C420 XYSCSS=420MPEG2

The video file still gives me the above error in these conditions.

What can/should I do?

How should I be providing a video file to Chrome for this command line argument?

How should I be recording or creating the video file?

How should I convert it to y4m?

Answer 1

After reading the link you provided I noticed that we can also provide an mjpeg.

Depending on what your test requirements - this may be sufficient for you.

ffmpeg -i oldfile.mp4 newfile.mjpeg

then I tested using:

google-chrome --use-fake-device-for-media-stream --use-file-for-fake-video-capture=newfile.mjpeg

After navigating to Tracking JS I could see the video being played back.

I hope that works for you!