352 lines
13 KiB
ReStructuredText
352 lines
13 KiB
ReStructuredText
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.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
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.. c:namespace:: V4L
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.. _VIDIOC_G_FBUF:
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**********************************
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ioctl VIDIOC_G_FBUF, VIDIOC_S_FBUF
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**********************************
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Name
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====
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VIDIOC_G_FBUF - VIDIOC_S_FBUF - Get or set frame buffer overlay parameters
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Synopsis
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========
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.. c:macro:: VIDIOC_G_FBUF
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``int ioctl(int fd, VIDIOC_G_FBUF, struct v4l2_framebuffer *argp)``
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.. c:macro:: VIDIOC_S_FBUF
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``int ioctl(int fd, VIDIOC_S_FBUF, const struct v4l2_framebuffer *argp)``
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Arguments
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=========
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``fd``
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File descriptor returned by :c:func:`open()`.
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``argp``
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Pointer to struct :c:type:`v4l2_framebuffer`.
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Description
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===========
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Applications can use the :ref:`VIDIOC_G_FBUF <VIDIOC_G_FBUF>` and :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>` ioctl
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to get and set the framebuffer parameters for a
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:ref:`Video Overlay <overlay>` or :ref:`Video Output Overlay <osd>`
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(OSD). The type of overlay is implied by the device type (capture or
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output device) and can be determined with the
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:ref:`VIDIOC_QUERYCAP` ioctl. One ``/dev/videoN``
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device must not support both kinds of overlay.
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The V4L2 API distinguishes destructive and non-destructive overlays. A
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destructive overlay copies captured video images into the video memory
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of a graphics card. A non-destructive overlay blends video images into a
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VGA signal or graphics into a video signal. *Video Output Overlays* are
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always non-destructive.
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To get the current parameters applications call the :ref:`VIDIOC_G_FBUF <VIDIOC_G_FBUF>`
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ioctl with a pointer to a struct :c:type:`v4l2_framebuffer`
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structure. The driver fills all fields of the structure or returns an
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EINVAL error code when overlays are not supported.
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To set the parameters for a *Video Output Overlay*, applications must
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initialize the ``flags`` field of a struct
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:c:type:`v4l2_framebuffer`. Since the framebuffer is
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implemented on the TV card all other parameters are determined by the
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driver. When an application calls :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>` with a pointer to
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this structure, the driver prepares for the overlay and returns the
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framebuffer parameters as :ref:`VIDIOC_G_FBUF <VIDIOC_G_FBUF>` does, or it returns an error
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code.
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To set the parameters for a *non-destructive Video Overlay*,
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applications must initialize the ``flags`` field, the ``fmt``
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substructure, and call :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>`. Again the driver prepares for
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the overlay and returns the framebuffer parameters as :ref:`VIDIOC_G_FBUF <VIDIOC_G_FBUF>`
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does, or it returns an error code.
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For a *destructive Video Overlay* applications must additionally provide
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a ``base`` address. Setting up a DMA to a random memory location can
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jeopardize the system security, its stability or even damage the
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hardware, therefore only the superuser can set the parameters for a
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destructive video overlay.
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.. tabularcolumns:: |p{3.5cm}|p{3.5cm}|p{3.5cm}|p{6.6cm}|
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.. c:type:: v4l2_framebuffer
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.. cssclass:: longtable
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.. flat-table:: struct v4l2_framebuffer
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:header-rows: 0
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:stub-columns: 0
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:widths: 1 1 1 2
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* - __u32
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- ``capability``
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-
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- Overlay capability flags set by the driver, see
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:ref:`framebuffer-cap`.
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* - __u32
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- ``flags``
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-
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- Overlay control flags set by application and driver, see
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:ref:`framebuffer-flags`
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* - void *
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- ``base``
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-
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- Physical base address of the framebuffer, that is the address of
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the pixel in the top left corner of the framebuffer. [#f1]_
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* -
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-
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-
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- This field is irrelevant to *non-destructive Video Overlays*. For
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*destructive Video Overlays* applications must provide a base
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address. The driver may accept only base addresses which are a
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multiple of two, four or eight bytes. For *Video Output Overlays*
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the driver must return a valid base address, so applications can
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find the corresponding Linux framebuffer device (see
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:ref:`osd`).
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* - struct
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- ``fmt``
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-
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- Layout of the frame buffer.
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* -
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- __u32
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- ``width``
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- Width of the frame buffer in pixels.
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* -
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- __u32
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- ``height``
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- Height of the frame buffer in pixels.
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* -
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- __u32
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- ``pixelformat``
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- The pixel format of the framebuffer.
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* -
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-
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-
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- For *non-destructive Video Overlays* this field only defines a
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format for the struct :c:type:`v4l2_window`
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``chromakey`` field.
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* -
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-
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-
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- For *destructive Video Overlays* applications must initialize this
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field. For *Video Output Overlays* the driver must return a valid
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format.
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* -
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-
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-
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- Usually this is an RGB format (for example
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:ref:`V4L2_PIX_FMT_RGB565 <V4L2-PIX-FMT-RGB565>`) but YUV
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formats (only packed YUV formats when chroma keying is used, not
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including ``V4L2_PIX_FMT_YUYV`` and ``V4L2_PIX_FMT_UYVY``) and the
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``V4L2_PIX_FMT_PAL8`` format are also permitted. The behavior of
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the driver when an application requests a compressed format is
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undefined. See :ref:`pixfmt` for information on pixel formats.
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* -
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- enum :c:type:`v4l2_field`
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- ``field``
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- Drivers and applications shall ignore this field. If applicable,
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the field order is selected with the
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:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, using the ``field``
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field of struct :c:type:`v4l2_window`.
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* -
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- __u32
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- ``bytesperline``
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- Distance in bytes between the leftmost pixels in two adjacent
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lines.
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* - :cspan:`3`
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This field is irrelevant to *non-destructive Video Overlays*.
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For *destructive Video Overlays* both applications and drivers can
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set this field to request padding bytes at the end of each line.
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Drivers however may ignore the requested value, returning
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``width`` times bytes-per-pixel or a larger value required by the
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hardware. That implies applications can just set this field to
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zero to get a reasonable default.
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For *Video Output Overlays* the driver must return a valid value.
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Video hardware may access padding bytes, therefore they must
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reside in accessible memory. Consider for example the case where
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padding bytes after the last line of an image cross a system page
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boundary. Capture devices may write padding bytes, the value is
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undefined. Output devices ignore the contents of padding bytes.
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When the image format is planar the ``bytesperline`` value applies
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to the first plane and is divided by the same factor as the
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``width`` field for the other planes. For example the Cb and Cr
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planes of a YUV 4:2:0 image have half as many padding bytes
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following each line as the Y plane. To avoid ambiguities drivers
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must return a ``bytesperline`` value rounded up to a multiple of
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the scale factor.
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* -
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- __u32
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- ``sizeimage``
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- This field is irrelevant to *non-destructive Video Overlays*. For
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*destructive Video Overlays* applications must initialize this
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field. For *Video Output Overlays* the driver must return a valid
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format.
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Together with ``base`` it defines the framebuffer memory
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accessible by the driver.
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* -
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- enum :c:type:`v4l2_colorspace`
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- ``colorspace``
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- This information supplements the ``pixelformat`` and must be set
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by the driver, see :ref:`colorspaces`.
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* -
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- __u32
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- ``priv``
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- Reserved. Drivers and applications must set this field to zero.
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.. tabularcolumns:: |p{7.4cm}|p{1.6cm}|p{8.3cm}|
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.. _framebuffer-cap:
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.. flat-table:: Frame Buffer Capability Flags
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:header-rows: 0
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:stub-columns: 0
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:widths: 3 1 4
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* - ``V4L2_FBUF_CAP_EXTERNOVERLAY``
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- 0x0001
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- The device is capable of non-destructive overlays. When the driver
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clears this flag, only destructive overlays are supported. There
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are no drivers yet which support both destructive and
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non-destructive overlays. Video Output Overlays are in practice
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always non-destructive.
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* - ``V4L2_FBUF_CAP_CHROMAKEY``
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- 0x0002
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- The device supports clipping by chroma-keying the images. That is,
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image pixels replace pixels in the VGA or video signal only where
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the latter assume a certain color. Chroma-keying makes no sense
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for destructive overlays.
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* - ``V4L2_FBUF_CAP_LIST_CLIPPING``
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- 0x0004
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- The device supports clipping using a list of clip rectangles.
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* - ``V4L2_FBUF_CAP_BITMAP_CLIPPING``
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- 0x0008
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- The device supports clipping using a bit mask.
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* - ``V4L2_FBUF_CAP_LOCAL_ALPHA``
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- 0x0010
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- The device supports clipping/blending using the alpha channel of
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the framebuffer or VGA signal. Alpha blending makes no sense for
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destructive overlays.
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* - ``V4L2_FBUF_CAP_GLOBAL_ALPHA``
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- 0x0020
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- The device supports alpha blending using a global alpha value.
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Alpha blending makes no sense for destructive overlays.
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* - ``V4L2_FBUF_CAP_LOCAL_INV_ALPHA``
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- 0x0040
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- The device supports clipping/blending using the inverted alpha
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channel of the framebuffer or VGA signal. Alpha blending makes no
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sense for destructive overlays.
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* - ``V4L2_FBUF_CAP_SRC_CHROMAKEY``
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- 0x0080
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- The device supports Source Chroma-keying. Video pixels with the
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chroma-key colors are replaced by framebuffer pixels, which is
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exactly opposite of ``V4L2_FBUF_CAP_CHROMAKEY``
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.. tabularcolumns:: |p{7.4cm}|p{1.6cm}|p{8.3cm}|
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.. _framebuffer-flags:
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.. cssclass:: longtable
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.. flat-table:: Frame Buffer Flags
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:header-rows: 0
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:stub-columns: 0
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:widths: 3 1 4
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* - ``V4L2_FBUF_FLAG_PRIMARY``
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- 0x0001
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- The framebuffer is the primary graphics surface. In other words,
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the overlay is destructive. This flag is typically set by any
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driver that doesn't have the ``V4L2_FBUF_CAP_EXTERNOVERLAY``
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capability and it is cleared otherwise.
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* - ``V4L2_FBUF_FLAG_OVERLAY``
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- 0x0002
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- If this flag is set for a video capture device, then the driver
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will set the initial overlay size to cover the full framebuffer
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size, otherwise the existing overlay size (as set by
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:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`) will be used. Only one
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video capture driver (bttv) supports this flag. The use of this
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flag for capture devices is deprecated. There is no way to detect
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which drivers support this flag, so the only reliable method of
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setting the overlay size is through
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:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`. If this flag is set for a
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video output device, then the video output overlay window is
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relative to the top-left corner of the framebuffer and restricted
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to the size of the framebuffer. If it is cleared, then the video
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output overlay window is relative to the video output display.
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* - ``V4L2_FBUF_FLAG_CHROMAKEY``
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- 0x0004
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- Use chroma-keying. The chroma-key color is determined by the
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``chromakey`` field of struct :c:type:`v4l2_window`
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and negotiated with the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`
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ioctl, see :ref:`overlay` and :ref:`osd`.
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* - :cspan:`2` There are no flags to enable clipping using a list of
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clip rectangles or a bitmap. These methods are negotiated with the
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:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, see :ref:`overlay`
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and :ref:`osd`.
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* - ``V4L2_FBUF_FLAG_LOCAL_ALPHA``
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- 0x0008
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- Use the alpha channel of the framebuffer to clip or blend
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framebuffer pixels with video images. The blend function is:
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output = framebuffer pixel * alpha + video pixel * (1 - alpha).
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The actual alpha depth depends on the framebuffer pixel format.
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* - ``V4L2_FBUF_FLAG_GLOBAL_ALPHA``
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- 0x0010
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- Use a global alpha value to blend the framebuffer with video
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images. The blend function is: output = (framebuffer pixel * alpha
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+ video pixel * (255 - alpha)) / 255. The alpha value is
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determined by the ``global_alpha`` field of struct
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:c:type:`v4l2_window` and negotiated with the
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:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, see :ref:`overlay`
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and :ref:`osd`.
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* - ``V4L2_FBUF_FLAG_LOCAL_INV_ALPHA``
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- 0x0020
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- Like ``V4L2_FBUF_FLAG_LOCAL_ALPHA``, use the alpha channel of the
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framebuffer to clip or blend framebuffer pixels with video images,
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but with an inverted alpha value. The blend function is: output =
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framebuffer pixel * (1 - alpha) + video pixel * alpha. The actual
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alpha depth depends on the framebuffer pixel format.
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* - ``V4L2_FBUF_FLAG_SRC_CHROMAKEY``
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- 0x0040
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- Use source chroma-keying. The source chroma-key color is
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determined by the ``chromakey`` field of struct
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:c:type:`v4l2_window` and negotiated with the
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:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, see :ref:`overlay`
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and :ref:`osd`. Both chroma-keying are mutual exclusive to each
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other, so same ``chromakey`` field of struct
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:c:type:`v4l2_window` is being used.
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Return Value
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============
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On success 0 is returned, on error -1 and the ``errno`` variable is set
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appropriately. The generic error codes are described at the
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:ref:`Generic Error Codes <gen-errors>` chapter.
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EPERM
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:ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>` can only be called by a privileged user to
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negotiate the parameters for a destructive overlay.
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EINVAL
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The :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>` parameters are unsuitable.
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.. [#f1]
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A physical base address may not suit all platforms. GK notes in
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theory we should pass something like PCI device + memory region +
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offset instead. If you encounter problems please discuss on the
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linux-media mailing list:
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`https://linuxtv.org/lists.php <https://linuxtv.org/lists.php>`__.
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