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@xxxxxxxxxx> > --- > .../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
