Hello, Lauries video(1) email to misc@ encouraged me to take this information and try to come up with a proposal to enhance the multimedia faq with information about webcam usage.
It's the first time I'm working on a faq article and I don't know about any style guides. I tried to align with the articles already written. Please educate me about the parts I missed. I'm not sure where people working on documentation collaborate. So I'm trying my luck here on tech@. My proposal is online and I'm also inlining it (not as patch for easier consumption): https://codevoid.de/h/wip_webcamfaq13.html I'm sure some parts need refinement / better wording. The part I'm mostly unsure about is how to correctly give the user permissions to the video devices. Most people probably do it the way I described (doas chmod $USER in .xsession). Once it's good enough to get committed, I'll create a proper patch. Any thoughts on it? Thanks in advance, Stefan ---- <h2 id="webcams">Using a Webcam</h2> <h3>Supported Hardware</h3> <p> Most webcams today work according to the USB Video Class (UVC) specification, which is supported by the <a href="https://man.openbsd.org/uvideo";>uvideo(4)</a> device driver and attaches to the <a href="https://man.openbsd.org/video.4";>video(4)</a> device. The manpage lists some supported devices, but there is a good chance that other devices work as well. For example, webcams in Lenovo Thinkpads laptops do usually work. <p> A supported webcam (or other video device) shows up in <code>dmesg</code> like this: <pre class="cmdbox"> uvideo0 at uhub0 port 8 configuration 1 interface 0 "Azurewave Integrated Camera" rev 2.01/69.05 addr 10 video0 at uvideo0 uvideo1 at uhub0 port 8 configuration 1 interface 2 "Azurewave Integrated Camera" rev 2.01/69.05 addr 10 video1 at uvideo1 </pre> <p> You see that an <code>uvideo</code> device was detected and has been attached to <code>video0</code>. This device will be accessible through <code>/dev/video0</code>. <p> Modern laptops sometimes attach a second video device, which is the infrared camera for face recognition. The second camera does not produce a usable video stream. <p> You can find the usable camera with the <a href="https://man.openbsd.org/video";>video(1)</a> command: <pre class="cmdbox"> $ <b>video -q -f /dev/video0</b> video device /dev/video0: encodings: yuy2 frame sizes (width x height, in pixels) and rates (in frames per second): 320x180: 30 320x240: 30 352x288: 30 424x240: 30 640x360: 30 640x480: 30 848x480: 20 960x540: 15 1280x720: 10 controls: brightness, contrast, saturation, hue, gamma, sharpness, white_balance_temperature $ <b>video -q -f /dev/video1</b> video: /dev/video1 has no usable YUV encodings </pre> <p> The usable camera device shows supported resolutions and framerates. Note that the framerates only apply to the uncompressed YUY2 stream. More on that in <a href="#recvideo">Recording a webcam stream</a>. <h3>Using a webcam as user</h3> <p> To use the webcam as regular user, you need to change the device permissions. Only root is allowed to access video devices by default. <p> One way of allowing your user to access the video devices is to change the permissions from <code>~/.xsession</code>. You can configure <a href="https://man.openbsd.org/doas";>doas(1)</a> to perform <a href="https://man.openbsd.org/chmod";>chmod(1)</a> as superuser without asking for a password for your user. <p> Then add the following line to your <code>~/.xsession</code>: <pre class="cmdbox"> doas chown $USER /dev/video0 </pre> <p> If you're not using <a href="https://man.openbsd.org/xenodm";>xenodm(1)</a> and you are starting your X session with <a href="https://man.openbsd.org/startx";>startx(1)</a>, you can accomplish the same with <a href="https://man.openbsd.org/fbtab";>fbtab(5)</a>. <p> Example entry in <code>/etc/fbtab</code>: <pre class="cmdbox"> /dev/ttyC0 0600 /dev/video0 </pre> <h3 id="recvideo">Recording a webcam stream</h3> <p> This section uses <code>ffplay</code> and <code>ffmpeg</code> from graphics/ffmpeg. To find out what your camera is capable of, run the following command: <pre class="cmdbox"> $ <b>ffplay -f v4l2 -list_formats all -i /dev/video0</b> [...] [video4linux2,v4l2 @ 0x921f8420800] Compressed: mjpeg : MJPEG : 1280x720 320x180 320x240 352x288 424x240 640x360 640x480 848x480 960x540 [video4linux2,v4l2 @ 0x921f8420800] Raw : yuyv422 : YUYV : 640x480 320x180 320x240 352x288 424x240 640x360 848x480 960x540 1280x720 </pre> <p> At the end of the output, you'll find two lines similiar to the two above. <p> The first line shows resolutions supported in the uncompressed YUYV format. The frame rates in this format can be very low. <p> The second line shows the supported MJPEG compressed video resolutions, which deliver much higher framerates (usually 30fps or 60fps). <p> Now try to play the webcam stream. Choose one of the MJPEG resolutions and run: <pre class="cmdbox"> $ <b>ffplay -f v4l2 -input_format mjpeg -video_size 1280x720 -i /dev/video0</b> [...] Input #0, video4linux2,v4l2, from '/dev/video0':B sq= 0B f=0/0 Duration: N/A, start: 1599377893.546533, bitrate: N/A Stream #0:0: Video: mjpeg (Baseline), yuvj422p(pc, bt470bg/unknown/unknown), 1280x720, 30 fps, 30 tbr, 1000k tbn, 1000k tbc </pre> The webcam stream should be displayed. Ffplay also shows the used resolution and the framerate (fps) used. <p> If this works, you can go ahead and record the video with ffmpeg: <pre class="cmdbox"> $ <b>ffmpeg -f v4l2 -input_format mjpeg -video_size 1280x720 -i /dev/video0 ~/video.mkv</b> </pre> End the recording with "q". You have now recorded your stream to file <code>~/video.mkv</code>. <h3>Controlling webcam settings</h3> <p> Webcams usually have brightness, contrast and other controls. <a href="https://man.openbsd.org/video.1";>Video(1)</a> allows you to alter these settings. <p> First find out which supported controls your camera has: <pre class="cmdbox"> $ <b>video -c</b> brightness=128 contrast=32 saturation=64 hue=0 gamma=120 sharpness=3 white_balance_temperature=auto </pre> <p> You can change for example the brightness setting to 200: <pre class="cmdbox"> $ <b>video brightness=200</b> brightness: 128 -> 200 </pre> <p> If you would like to reset all settings to their defaults, you can do so with: <pre class="cmdbox"> $ <b>video -d</b> $ <b>video -c</b> brightness=128 contrast=32 saturation=64 hue=0 gamma=120 sharpness=3 white_balance_temperature=auto </pre> <p> Some settings, like the <code>white_balance_temperature</code> support automatic adjustments. You can set them to a fixed value or set them to "auto", which lets the camera decide and use whatever value it thinks is best. <h3>Access webcam in chromium</h3> <p> Chromium has access to <code>/dev/video</code> by default. To allow Chromium to access other video devices, you need to add the device paths to <code>/etc/chromium/unveil.main</code> and <code>/etc/chromium/unveil.utility_video</code> <h3>Access webcam in firefox</h3> <p> Firefox has access to <code>/dev/video</code> and <code>/dev/video0</code> by default. To allow Firefox to access other video devices, you need to add the device paths to <code>/etc/firefox/unveil.main</code>.
