[libcamera-devel] [PATCH 1/4] media: docs: Describe pixel array properties

[Laurent Pinchart]

Hi Jacopo,

Thank you for the patch.

On Wed, Aug 05, 2020 at 12:57:18PM +0200, Jacopo Mondi wrote:
> The V4L2 selection API are also used to access the pixel array
> properties of an image sensor, such as the size and position of active
> pixels and the cropped area of the pixel matrix used to produce images.
> 
> Currently no clear definition of the different areas that compose an
> image sensor pixel array matrix is provided in the specification, and
> the actual meaning of each selection target when applied to an image
> sensor was not provided.
> 
> Provide in the sub-device documentation the definition of the pixel
> matrix properties and the selection target associated to each of them.
> 
> Signed-off-by: Jacopo Mondi <jacopo at jmondi.org>
> ---
>  .../userspace-api/media/v4l/dev-subdev.rst    | 81 +++++++++++++++++++
>  1 file changed, 81 insertions(+)
> 
> diff --git a/Documentation/userspace-api/media/v4l/dev-subdev.rst b/Documentation/userspace-api/media/v4l/dev-subdev.rst
> index 134d2fb909fa4..c47861dff9b9b 100644
> --- a/Documentation/userspace-api/media/v4l/dev-subdev.rst
> +++ b/Documentation/userspace-api/media/v4l/dev-subdev.rst
> @@ -386,6 +386,87 @@ requests on all selection targets, unless specifically told otherwise.
>  ``V4L2_SEL_FLAG_GE`` and ``V4L2_SEL_FLAG_LE`` flags may be used to round
>  the image size either up or down. :ref:`v4l2-selection-flags`
>  
> +.. _v4l2-subdev-pixel-array-properties:
> +
> +Selection targets for image sensors properties
> +----------------------------------------------
> +
> +The V4L2 selection API can be used on sub-devices that represent an image
> +sensor to retrieve the sensor's pixel array matrix properties by using the
> +:ref:`selection <VIDIOC_SUBDEV_G_SELECTION>` ioctls.
> +
> +Sub-device drivers for image sensor usually register a single source pad, but in
> +the case they expose more, the pixel array properties can be accessed from
> +any of them.
> +
> +The ``V4L2_SEL_TGT_NATIVE``, ``V4L2_SEL_TGT_CROP_BOUNDS``,
> +``V4L2_SEL_TGT_CROP_DEFAULT`` and ``V4L2_TGT_CROP`` targets are used to retrieve
> +the immutable properties of the several different areas that compose the sensor
> +pixel array matrix. Each area describes a rectangle of logically adjacent pixel
> +units. The logical disposition of pixels is defined by the sensor read-out
> +starting point and direction, and may differ from the physical disposition of
> +the pixel units in the pixel array matrix.
> +
> +Each pixel matrix portion is contained in a larger rectangle, with the most
> +largest being the one that describes the pixel matrix physical size. This
> +defines a hierarchical positional system, where each rectangle is defined
> +relatively to the largest available one among the ones exposed by the
> +sub-device driver. Each selection target and the associated pixel array portion
> +it represents are below presented in order from the largest to the smallest one.
> +
> +Pixel array physical size
> +^^^^^^^^^^^^^^^^^^^^^^^^^
> +
> +The image sensor chip is composed by a number of physical pixels, not all of
> +them readable by the application processor. Invalid or unreadable lines might
> +not be transmitted on the data bus at all, or in case on CSI-2 capable sensors
> +they might be tagged with an invalid data type (DT) so that the receiver
> +automatically discard them. The size of the whole pixel matrix area is
> +retrieved using the V4L2_SEL_TGT_NATIVE target, which has its top-left corner
> +defined as position (0, 0). All the other selection targets are defined
> +relatively to this, larger, rectangle. The rectangle returned by
> +V4L2_SEL_TGT_NATIVE describes an immutable property of the image sensor, it
> +does not change at run-time and cannot be modified from userspace.

As I think I've mentioned previously (not sure if it was by e-mail or on
IRC), we could also decide to set V4L2_SEL_TGT_NATIVE_SIZE ==
V4L2_SEL_TGT_CROP_BOUNDS by ignoring the non-readable pixels completely.
What's the advantage of exposing them in the API, when the sensors
doesn't provide them to the rest of the pipeline ?

> +Pixel array readable area
> +^^^^^^^^^^^^^^^^^^^^^^^^^
> +
> +The V4L2_SEL_TGT_CROP_BOUNDS targets returns size and position of the readable
> +area of the pixel array matrix, including pixels with valid image data and pixel
> +used for calibration purposes, such as optical black pixels. It is not unlikely
> +that valid pixels and optical black pixels are surrounded by non-readable rows
> +and columns of pixels. Those does not concur in the definition of the
> +V4L2_SEL_TGT_CROP_BOUNDS rectangle. The rectangle returned by
> +V4L2_SEL_TGT_CROP_BOUNDS describes an immutable property of the image sensor, it
> +does not change at run-time and cannot be modified from userspace.
> +
> +Pixel array active area
> +^^^^^^^^^^^^^^^^^^^^^^^
> +
> +The portion of the pixel array which contains valid image data is defined as the
> +active area of the pixel matrix. The active pixel array is is accessed by mean
> +of the V4L2_SEL_TGT_CROP_DEFAULT target, and is contained in the larger
> +V4L2_SEL_TGT_CROP_BOUNDS rectangle. It represents the largest possible frame
> +resolution the sensor can produce and defines the dimension of the full
> +field-of-view. The rectangle returned by V4L2_SEL_TGT_CROP_BOUNDS describes an
> +immutable property of the image sensor, it does not change at run-time and
> +cannot be modified from userspace.
> +
> +Analog crop rectangle
> +^^^^^^^^^^^^^^^^^^^^^
> +
> +The sensor driver might decide, in order to adjust the image resolution to best
> +match the one requested by applications, to only process a part of the active
> +pixel array matrix. The selected area is read-out and processed by the image
> +sensor on-board ISP in order to produce images of the desired size and
> +resolution while possible maintaing the largest possible field-of-view. The
> +cropped portion of the pixel array which is used to produce images is returned
> +by the V4L2_SEL_TGT_CROP target and represent the only information that can
> +change at runtime as it depends on the currently configured sensor mode and
> +desired image resolution. If the sub-device driver supports that, userspace
> +can set the analog crop rectangle to select which portion of the pixel array
> +to read out.
> +
>  
>  Types of selection targets
>  --------------------------

-- 
Regards,

Laurent Pinchart