145 lines
6.9 KiB
ReStructuredText
145 lines
6.9 KiB
ReStructuredText
.. Permission is granted to copy, distribute and/or modify this
|
|
.. document under the terms of the GNU Free Documentation License,
|
|
.. Version 1.1 or any later version published by the Free Software
|
|
.. Foundation, with no Invariant Sections, no Front-Cover Texts
|
|
.. and no Back-Cover Texts. A copy of the license is included at
|
|
.. Documentation/media/uapi/fdl-appendix.rst.
|
|
..
|
|
.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections
|
|
|
|
*************
|
|
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 cover 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.
|