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