Droidcam with OctoPrint

This expects that you're using OctoPrint's Raspberry Pi image, for other systems you'll probably have to adapt a bit.

Also, timelapses will not work as mjpeg-server doesn't currently have a way to fetch a single snapshot.

Why?

OctoPrint ships with mjpeg-streamer, which takes a video device (e.g. /dev/video0) as input and runs a HTTP server that serves that video stream as mjpeg stream.
For this, the camera must support mjpeg natively. Most normal cameras do this, but DroidCam doesn't:

List of supported output formats for the DroidCam virtual video device (click to expand)

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

        [0]: 'YU12' (Planar YUV 4:2:0)
                Size: Discrete 1280x720
                        Interval: Discrete 0.033s (30.000 fps)

The only output option is YU12, which we will need to transcode to an MJPEG sequence.

For comparison, this is the list of supported output formats on my Logitech webcam.

sudo v4l2-ctl -d /dev/video0 --list-formats-ext
ioctl: VIDIOC_ENUM_FMT
        Type: Video Capture

        [0]: 'YUYV' (YUYV 4:2:2)
                Size: Discrete 1920x960
                        Interval: Discrete 0.019s (54.000 fps)
                        Interval: Discrete 0.022s (45.000 fps)
                        Interval: Discrete 0.033s (30.000 fps)
                        Interval: Discrete 0.067s (15.000 fps)
        [1]: 'MJPG' (Motion-JPEG, compressed)
                Size: Discrete 1920x960
                        Interval: Discrete 0.019s (54.000 fps)
                        Interval: Discrete 0.022s (45.000 fps)
                        Interval: Discrete 0.033s (30.000 fps)
                        Interval: Discrete 0.067s (15.000 fps)

Note the MJPG option. This is what we need. This camera would work out of the box with OctoPrint's mjpeg-streamer.

What didn't work

So, the solution seemed simple. Right? Just create a v4l2 loopback device (let's call it /dev/video1), run ffmpeg to take the DroidCam camera stream as input (let's say /dev/video0) and transcode and output it to /dev/video1.

In theory, this would work. However, in practice there are 2 issues with this:

1. mjpeg-streamer is garbage. It is written in C, from what I can tell not actively maintained and has weird issues with v4l2 video devices.
   On Linux, real cameras are UVC devices and mjpeg-streamer can work with these just fine. However, emulated video devices (using v4l2loopback) appear to function ever so slightly differently, meaning mjpeg-streamer can't read from our ffmpeg output (or the DroidCam output, even if it did support MJPEG directly).
2. ffmpeg obliterates the Pi 4's CPU. Transcoding a 720p (or even 480p) stream in the background resulted in frequent video freezing and caused OctoPrint to sometimes freeze for a few seconds. This is most likely because ffmpeg doesn't have a hardware encoder for MJPEG that works on the Pi.

The solution

1. Let's just replace mjpeg-streamer. After searching for a bit, I found mjpeg-server which does exactly what I need.
   Instead of using a video device as input, it simply executes a command (for example ffmpeg) and streams the output from that.
   The one downside to using this is that there is no way to capture a single frame, which means that timelapses will no longer be usable. I might look into forking it and adding snapshot support in the future.
2. Instead of ffmpeg, I opted to use gstreamer. After consulting a good friend of mine (ChatGPT), I managed to construct a gstreamer pipeline that transcodes the video to an MJPEG stream - While using a hardware accelerated transcoder!

How to set it up

Disable the existing mjpeg-streamer service so we can run our own instead:

sudo systemctl disable --now webcamd.service

Clone and compile the DroidCam client repository. DroidCam doesn't have arm64 builds so we need to compile it ourselves.

# Install dependencies
sudo apt install libturbojpeg0{,-dev} libavutil-dev libswscale-dev libasound2-dev libspeex-dev libusbmuxd-dev libplist-dev usbmuxd

# Clone and build
cd ~
git clone https://github.com/dev47apps/droidcam
cd droidcam
make droidcam-cli
cp ./droidcam-cli /usr/bin/droidcam-cli

Set up v4l2loopback, which DroidCam requires.

sudo apt install v4l2loopback-dkms

# Configure the module to load on startup
echo "v4l2loopback" | sudo tee -a /etc/modules
echo "options v4l2loopback devices=1 video_nr=1" | sudo tee -a /etc/modprobe.d/v4l2loopback.conf

# Load the module now with the same configuration
sudo modprobe v4l2loopback devices=1 video_nr=1

Note how we configure it to provide the emulated video device /dev/video1, not /dev/video0 - This should prevent it from breaking should a real camera be plugged in at some point.

Next, install mjpeg-server.

sudo apt install golang
go install github.com/blueimp/mjpeg-server@latest

# You'll also need gstreamer
sudo apt install gstreamer1.0-tools gstreamer1.0-plugins-good

# Verify that it was built and installed correctly.
# If you get "No such file or directory", run the `go get` command again.
~/go/bin/mjpeg-server --help

You should now have everything you need. Let's verify:

These instructions are using DroidCam on an iOS device via USB. If you're using an Android phone or want to use it wirelessly, run droidcam-cli --help and adapt the command accordingly.
Unless your setup matches mine exactly, you will also have to modify the DroidCam command in a systemd service file later on.
Tip: You can change the port in DroidCam settings so that it matches my instructions.

In one terminal, run DroidCam. If using the phone wired, you might have to reattach it once.

# Higher resolutions than 1280x720 may work, but resulted in messed up graphics at full refresh rate. You'll need to adapt the gstreamer pipeline to lower the framerate in that case, but I prefer smoothness over resolution.
# /dev/video1 is the v4l2loopback device we created earlier, unless you used a different device number you don't need to change this.
droidcam-cli -nocontrols -dev=/dev/video1 -size=1280x720 ios 57192

While the first command is running, open a second terminal and run this:

# It is important that we use port 8080 since OctoPrint proxies localhost:8080 to `/webcam` on the OctoPrint HTTP server.
# For `-b gaysex` and `multipartmux boundary=gaysex`, "gaysex" can be changed to any other string, as long as they are identical for both sides. This is just used to signal where the next JPEG frame starts.
/home/pi/go/bin/mjpeg-server -a ":8080" -b gaysex -- gst-launch-1.0 -v v4l2src device=/dev/video1 ! videoconvert ! video/x-raw,format=I420 ! v4l2jpegenc ! multipartmux boundary=gaysex ! filesink location=/dev/stdout

Now, if you open the OctoPrint UI, you should see your camera appear! Didn't work? Good luck. Otherwise, let's finish this up.

Follow the instructions in services/ to install the systemd service files. If you've changed the droidcam-cli command above, make sure to also modify it in droidcam.service. You most likely don't need to change droidcam-streamer.service.

Finally, reboot the system for good measure and check if everything works! If using an iPhone as DroidCam client, I recommend setting up Guided Access to make sure that the screen stays on even when it's not actively streaming and to prevent others from messing with the phone. On Android, App Pinning can be used instead.

You should also enable dimming in DroidCam's settings so that the screen doesn't have to remain on all the time.

Extra: Phone mount

Should you be in need of a mount to prop your phone up, I have included my OpenSCAD files for the mount I designed for myself. It works out of the box with my iPhone 11 with my specific rubber case, you'll probably have to configure it differently.

The files and instructions on what values to change are in phone-mount/.