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+.. -*- coding: utf-8; mode: rst -*-
+
+*************
+Configuration
+*************
+
+Applications can use the :ref:`selection API <VIDIOC_G_SELECTION>` to
+select an area in a video signal or a buffer, and to query for default
+settings and hardware limits.
+
+Video hardware can have various cropping, composing and scaling
+limitations. It may only scale up or down, support only discrete scaling
+factors, or have different scaling abilities in the horizontal and
+vertical directions. Also it may not support scaling at all. At the same
+time the cropping/composing rectangles may have to be aligned, and both
+the source and the sink may have arbitrary upper and lower size limits.
+Therefore, as usual, drivers are expected to adjust the requested
+parameters and return the actual values selected. An application can
+control the rounding behaviour using
+:ref:`constraint flags <v4l2-selection-flags>`.
+
+
+Configuration of video capture
+==============================
+
+See figure :ref:`sel-targets-capture` for examples of the selection
+targets available for a video capture device. It is recommended to
+configure the cropping targets before to the composing targets.
+
+The range of coordinates of the top left corner, width and height of
+areas that can be sampled is given by the ``V4L2_SEL_TGT_CROP_BOUNDS``
+target. It is recommended for the driver developers to put the top/left
+corner at position ``(0,0)``. The rectangle's coordinates are expressed
+in pixels.
+
+The top left corner, width and height of the source rectangle, that is
+the area actually sampled, is given by the ``V4L2_SEL_TGT_CROP`` target.
+It uses the same coordinate system as ``V4L2_SEL_TGT_CROP_BOUNDS``. The
+active cropping area must lie completely inside the capture boundaries.
+The driver may further adjust the requested size and/or position
+according to hardware limitations.
+
+Each capture device has a default source rectangle, given by the
+``V4L2_SEL_TGT_CROP_DEFAULT`` target. This rectangle shall over what the
+driver writer considers the complete picture. Drivers shall set the
+active crop rectangle to the default when the driver is first loaded,
+but not later.
+
+The composing targets refer to a memory buffer. The limits of composing
+coordinates are obtained using ``V4L2_SEL_TGT_COMPOSE_BOUNDS``. All
+coordinates are expressed in pixels. The rectangle's top/left corner
+must be located at position ``(0,0)``. The width and height are equal to
+the image size set by :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`.
+
+The part of a buffer into which the image is inserted by the hardware is
+controlled by the ``V4L2_SEL_TGT_COMPOSE`` target. The rectangle's
+coordinates are also expressed in the same coordinate system as the
+bounds rectangle. The composing rectangle must lie completely inside
+bounds rectangle. The driver must adjust the composing rectangle to fit
+to the bounding limits. Moreover, the driver can perform other
+adjustments according to hardware limitations. The application can
+control rounding behaviour using
+:ref:`constraint flags <v4l2-selection-flags>`.
+
+For capture devices the default composing rectangle is queried using
+``V4L2_SEL_TGT_COMPOSE_DEFAULT``. It is usually equal to the bounding
+rectangle.
+
+The part of a buffer that is modified by the hardware is given by
+``V4L2_SEL_TGT_COMPOSE_PADDED``. It contains all pixels defined using
+``V4L2_SEL_TGT_COMPOSE`` plus all padding data modified by hardware
+during insertion process. All pixels outside this rectangle *must not*
+be changed by the hardware. The content of pixels that lie inside the
+padded area but outside active area is undefined. The application can
+use the padded and active rectangles to detect where the rubbish pixels
+are located and remove them if needed.
+
+
+Configuration of video output
+=============================
+
+For output devices targets and ioctls are used similarly to the video
+capture case. The *composing* rectangle refers to the insertion of an
+image into a video signal. The cropping rectangles refer to a memory
+buffer. It is recommended to configure the composing targets before to
+the cropping targets.
+
+The cropping targets refer to the memory buffer that contains an image
+to be inserted into a video signal or graphical screen. The limits of
+cropping coordinates are obtained using ``V4L2_SEL_TGT_CROP_BOUNDS``.
+All coordinates are expressed in pixels. The top/left corner is always
+point ``(0,0)``. The width and height is equal to the image size
+specified using :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl.
+
+The top left corner, width and height of the source rectangle, that is
+the area from which image date are processed by the hardware, is given
+by the ``V4L2_SEL_TGT_CROP``. Its coordinates are expressed in in the
+same coordinate system as the bounds rectangle. The active cropping area
+must lie completely inside the crop boundaries and the driver may
+further adjust the requested size and/or position according to hardware
+limitations.
+
+For output devices the default cropping rectangle is queried using
+``V4L2_SEL_TGT_CROP_DEFAULT``. It is usually equal to the bounding
+rectangle.
+
+The part of a video signal or graphics display where the image is
+inserted by the hardware is controlled by ``V4L2_SEL_TGT_COMPOSE``
+target. The rectangle's coordinates are expressed in pixels. The
+composing rectangle must lie completely inside the bounds rectangle. The
+driver must adjust the area to fit to the bounding limits. Moreover, the
+driver can perform other adjustments according to hardware limitations.
+
+The device has a default composing rectangle, given by the
+``V4L2_SEL_TGT_COMPOSE_DEFAULT`` target. This rectangle shall cover what
+the driver writer considers the complete picture. It is recommended for
+the driver developers to put the top/left corner at position ``(0,0)``.
+Drivers shall set the active composing rectangle to the default one when
+the driver is first loaded.
+
+The devices may introduce additional content to video signal other than
+an image from memory buffers. It includes borders around an image.
+However, such a padded area is driver-dependent feature not covered by
+this document. Driver developers are encouraged to keep padded rectangle
+equal to active one. The padded target is accessed by the
+``V4L2_SEL_TGT_COMPOSE_PADDED`` identifier. It must contain all pixels
+from the ``V4L2_SEL_TGT_COMPOSE`` target.
+
+
+Scaling control
+===============
+
+An application can detect if scaling is performed by comparing the width
+and the height of rectangles obtained using ``V4L2_SEL_TGT_CROP`` and
+``V4L2_SEL_TGT_COMPOSE`` targets. If these are not equal then the
+scaling is applied. The application can compute the scaling ratios using
+these values.