The libcamera camera sensor model

libcamera defines an abstracted camera sensor model in order to provide a description of the several processing steps that result in image data being sent on the media bus and that form the image stream delivered to applications.

Applications should use the here defined abstracted camera sensor model to precisely control the operations of the camera sensor.

The libcamera camera sensor model targets image sensors producing frames in RAW format, delivered through a MIPI CSI-2 compliant bus implementation.

The abstract sensor model maps libcamera components to the characteristics and operations of an image sensor, and serves as a reference to model the libcamera CameraSensor class and SensorConfiguration classes and operations.

In order to control the configuration of the camera sensor through the SensorConfiguration class, applications should understand the here defined model and map it to the combination of image sensor and kernel driver in use.

The here defined camera sensor model is based on the MIPI CCS specification, particularly on Section 8.2 - Image readout of Chapter 8 - Video Timings.

_images/camera-sensor-model.png

Gloassary

  • Pixel array: The full grid of pixels, active and inactive ones

  • Pixel array active area: The portion(s) of the pixel array that contains valid and readable pixels; corresponds to the libcamera properties::PixelArrayActiveAreas

  • Analog crop rectangle: The portion of the pixel array active area which is read-out and passed to further processing stages

  • Subsampling: Pixel processing techniques that reduce the image size by interpolating (binning) or by skipping adjacent pixels

  • Digital crop: Crop of the sub-sampled image data

  • Digital scaling: Digital scaling of the image data

  • Output crop: Crop of the scaled image data to form the final output image

  • Frame output: The frame (image) as output on the media bus by the camera sensor

Camera Sensor configuration parameters

The libcamera defined camera sensor model defines parameters that allow users to control:

  1. The image format bit depth

  2. The size and position of the Analog crop rectangle

  3. The subsampling factors used to downscale the pixel array readout data to a smaller frame size. Two configuration parameters are made available to control the downscaling factor

    • binning
      • binning reduces the image resolution by combining adjacent pixels

      • reduces the image size without reducing the image field of view

      • a vertical and horizontal binning factor can be specified, the image will be downscaled in its vertical and horizontal sizes by the specified factor

      _images/binning.png

      Figure 39 from the MIPI CCS Specification (Version 1.1)

      Definition: The horizontal and vertical binning factors
      horizontal_binning = xBin;
      vertical_binning = yBin;
      
    • skipping
      • reduces the image resolution by skipping the read-out of a number of adjacent pixels

      • the skipping factor is specified by the ‘increment’ number (number of pixels to ‘skip’) in the vertical and horizontal directions and for even and odd rows and columns

      _images/skipping.png

      Figure 35 from the MIPI CCS Specification (Version 1.1)

      Definition: The horizontal and vertical skipping factors
      horizontal_skipping = (xOddInc + xEvenInc) / 2
      vertical_skipping = (yOddInc + yEvenInc) / 2
      
    • binning and skipping can be generically combined

    • different sensors perform the binning and skipping stages in different orders

    • for the sake of computing the final output image size the order of execution is not relevant.

    • the overall down-scaling factor is obtained by combining the binning and skipping factors

    Definition: The total scaling factor (binning + sub-sampling)
    total_horizontal_downscale = horizontal_binning + horizontal_skipping
    total_vertical_downscale = vertical_binning + vertical_skipping
    
  4. The output data frame size
    • the output size is used to specify any additional cropping on the sub-sampled frame

    • todo Add support for controlling scaling in the digital domain

  5. The total line length and frame height (visibile pixels + blankings) as sent on the MIPI CSI-2 bus

  6. The pixel transmission rate on the MIPI CSI-2 bus

The above parameters are combined to obtain the following high-level configurations

  • frame output size

    obtained by applying to the physical pixel array size a first crop in the analog domain, followed by optional binning and sub-sampling (in any order), followed by an optional crop step in the output digital domain.

    todo Add support for controlling scaling in the digital domain

  • frame rate

    the combination of the total frame size, the image format bit depth and the pixel rate of the data sent on the MIPI CSI-2 bus allows to compute the image stream frame rate. The equation is the well known

    frame_duration = total_frame_size / pixel_rate
    frame_rate = 1 / frame_duration
    

    where the pixel_rate parameter is the result of the sensor’s configuration of the MIPI CSI-2 bus (the following formula applies to MIPI CSI-2 when used on MIPI D-PHY physical protocol layer only)

    pixel_rate = CSI-2_link_freq * 2 * nr_of_lanes / bits_per_sample
    

The SensorConfiguration class

Applications can control the camera sensor configuration by populating the sensorConfig member of the CameraConfiguration class.

Camera applications that specify a SensorConfiguration are assumed to be highly-specialized applications that know what sensor they’re dealing with and which modes are valid for the sensor in use.

todo The sensorConfig instance should be fully specified in all its fields and it is applied in full to the camera sensor. For now only consider bit-depth and output size as the kernel interface doesn’t allow to fully control the sensor configuration.

todo If any of the parameters the application has populated the sensorConfig with cannot be applied as they are to the camera sensor, the CameraConfiguration is considered to be Invalid.

If the application provides a valid SensorConfiguration, its configuration takes precedence over any conflicting StreamConfiguration request.

In example:

  • If the platform cannot upscale, all the processed streams should be smaller or equal in size than the requested sensor configuration

  • If the platform produces RAW streams using the frames from the camera sensor without any additional processing, the RAW stream format should be adjusted to match the one configured by the sensorConfig