36. The Virtual Media Controller Driver (vimc)

The vimc driver emulates complex video hardware using the V4L2 API and the Media API. It has a capture device and three subdevices: sensor, debayer and scaler.

36.1. Topology

The topology is hardcoded, although you could modify it in vimc-core and recompile the driver to achieve your own topology. This is the default topology:

vimc.dot

Media pipeline graph on vimc

36.1.1. Configuring the topology

Each subdevice will come with its default configuration (pixelformat, height, width, …). One needs to configure the topology in order to match the configuration on each linked subdevice to stream frames through the pipeline. If the configuration doesn’t match, the stream will fail. The v4l-utils package is a bundle of user-space applications, that comes with media-ctl and v4l2-ctl that can be used to configure the vimc configuration. This sequence of commands fits for the default topology:

media-ctl -d platform:vimc -V '"Sensor A":0[fmt:SBGGR8_1X8/640x480]'
media-ctl -d platform:vimc -V '"Debayer A":0[fmt:SBGGR8_1X8/640x480]'
media-ctl -d platform:vimc -V '"Sensor B":0[fmt:SBGGR8_1X8/640x480]'
media-ctl -d platform:vimc -V '"Debayer B":0[fmt:SBGGR8_1X8/640x480]'
v4l2-ctl -z platform:vimc -d "RGB/YUV Capture" -v width=1920,height=1440
v4l2-ctl -z platform:vimc -d "Raw Capture 0" -v pixelformat=BA81
v4l2-ctl -z platform:vimc -d "Raw Capture 1" -v pixelformat=BA81

36.2. Subdevices

Subdevices define the behavior of an entity in the topology. Depending on the subdevice, the entity can have multiple pads of type source or sink.

vimc-sensor:

Generates images in several formats using video test pattern generator. Exposes:

  • 1 Pad source
vimc-debayer:

Transforms images in bayer format into a non-bayer format. Exposes:

  • 1 Pad sink
  • 1 Pad source
vimc-scaler:

Scale up the image by a factor of 3. E.g.: a 640x480 image becomes a 1920x1440 image. (this value can be configured, see at Module options). Exposes:

  • 1 Pad sink
  • 1 Pad source
vimc-capture:

Exposes node /dev/videoX to allow userspace to capture the stream. Exposes:

  • 1 Pad sink
  • 1 Pad source

36.3. Module options

Vimc has a few module parameters to configure the driver. You should pass those arguments to each subdevice, not to the vimc module. For example:

vimc_subdevice.param=value

  • vimc_scaler.sca_mult=<unsigned int>

    Image size multiplier factor to be used to multiply both width and height, so the image size will be sca_mult^2 bigger than the original one. Currently, only supports scaling up (the default value is 3).

  • vimc_debayer.deb_mean_win_size=<unsigned int>

    Window size to calculate the mean. Note: the window size needs to be an odd number, as the main pixel stays in the center of the window, otherwise the next odd number is considered (the default value is 3).