Piping video through v4l2loopback or shmdata

It may happen that the capture card does not comply with the Video4Linux2 API. For example, Blackmagic distributes proprietary drivers with their capture card. As such they are not compatible with Splash out of the box, but it is still possible to use them. The trick is to transmit the data through v4l2loopback or shmdata, and read the capture card from FFmpeg or GStreamer.

Piping through v4l2loopback

The goal of v4l2loopback is to simulate a V4L2 device (simular to a webcam or a capture card), which is feed from a software. First, you have to install and load it onto your computer. On Ubuntu:

sudo apt install v4l2loopback-utils
sudo modprobe v4l2loopback

A new device should have appeared as /dev/videoX, X being the id of the device. Let’s say you have no other V4L2 device, the path would then be /dev/video0.

Then you can launch a GStreamer pipeline (provided that it is installed, which is outside of the scope of this documentation), and feed the device a test stream or the stream from a Blackmagic capture card:

# Test stream
gst-launch-1.0 -v videotestsrc ! v4l2sink device=/dev/video0

# Capture from a Blackmagic card
gst-launch-1.0 -v decklinkvideosrc ! v4l2sink device=/dev/video0

Then in Splash you can create a V4L2 media and grab the feed from the /dev/video0 device

Piping through shmdata

Shmdata is a library which gives abilities similar to Syphon on macOS, but more general as it can carry any kind of data. First you have to make sure that it is installed on your system and that Splash has been compiled against it. Grab it here and follow the readme to compile and install it. Then recompile and install Splash as described on the Installation page.

You can then launch a GStreamer pipeline and send any feed through Shmdata:

# Test stream
gst-launch-1.0 -v videotestsrc ! shmdatasink socket-path=/tmp/gst-feed

# Capture from a Blackmagic card
gst-launch-1.0 -v decklinkvideosrc ! shmdatasink socket-path=/tmp/gst-feed

Then in Splash you can create a ‘video through shared memory’ object and set /tmp/gst-feed as the filed path. The video feed should display instantaneously.