Adding upstream version 4.19.249.upstream/4.19.249upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+.. -*- coding: utf-8; mode: rst -*-
+
+.. _overlay:
+
+***********************
+Video Overlay Interface
+***********************
+
+**Also known as Framebuffer Overlay or Previewing.**
+
+Video overlay devices have the ability to genlock (TV-)video into the
+(VGA-)video signal of a graphics card, or to store captured images
+directly in video memory of a graphics card, typically with clipping.
+This can be considerable more efficient than capturing images and
+displaying them by other means. In the old days when only nuclear power
+plants needed cooling towers this used to be the only way to put live
+video into a window.
+
+Video overlay devices are accessed through the same character special
+files as :ref:`video capture <capture>` devices.
+
+.. note::
+
+   The default function of a ``/dev/video`` device is video
+   capturing. The overlay function is only available after calling
+   the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl.
+
+The driver may support simultaneous overlay and capturing using the
+read/write and streaming I/O methods. If so, operation at the nominal
+frame rate of the video standard is not guaranteed. Frames may be
+directed away from overlay to capture, or one field may be used for
+overlay and the other for capture if the capture parameters permit this.
+
+Applications should use different file descriptors for capturing and
+overlay. This must be supported by all drivers capable of simultaneous
+capturing and overlay. Optionally these drivers may also permit
+capturing and overlay with a single file descriptor for compatibility
+with V4L and earlier versions of V4L2. [#f1]_
+
+
+Querying Capabilities
+=====================
+
+Devices supporting the video overlay interface set the
+``V4L2_CAP_VIDEO_OVERLAY`` flag in the ``capabilities`` field of struct
+:c:type:`v4l2_capability` returned by the
+:ref:`VIDIOC_QUERYCAP` ioctl. The overlay I/O
+method specified below must be supported. Tuners and audio inputs are
+optional.
+
+
+Supplemental Functions
+======================
+
+Video overlay devices shall support :ref:`audio input <audio>`,
+:ref:`tuner`, :ref:`controls <control>`,
+:ref:`cropping and scaling <crop>` and
+:ref:`streaming parameter <streaming-par>` ioctls as needed. The
+:ref:`video input <video>` and :ref:`video standard <standard>`
+ioctls must be supported by all video overlay devices.
+
+
+Setup
+=====
+
+Before overlay can commence applications must program the driver with
+frame buffer parameters, namely the address and size of the frame buffer
+and the image format, for example RGB 5:6:5. The
+:ref:`VIDIOC_G_FBUF <VIDIOC_G_FBUF>` and
+:ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>` ioctls are available to get and
+set these parameters, respectively. The :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>` ioctl is
+privileged because it allows to set up DMA into physical memory,
+bypassing the memory protection mechanisms of the kernel. Only the
+superuser can change the frame buffer address and size. Users are not
+supposed to run TV applications as root or with SUID bit set. A small
+helper application with suitable privileges should query the graphics
+system and program the V4L2 driver at the appropriate time.
+
+Some devices add the video overlay to the output signal of the graphics
+card. In this case the frame buffer is not modified by the video device,
+and the frame buffer address and pixel format are not needed by the
+driver. The :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>` ioctl is not privileged. An application
+can check for this type of device by calling the :ref:`VIDIOC_G_FBUF <VIDIOC_G_FBUF>`
+ioctl.
+
+A driver may support any (or none) of five clipping/blending methods:
+
+1. Chroma-keying displays the overlaid image only where pixels in the
+   primary graphics surface assume a certain color.
+
+2. A bitmap can be specified where each bit corresponds to a pixel in
+   the overlaid image. When the bit is set, the corresponding video
+   pixel is displayed, otherwise a pixel of the graphics surface.
+
+3. A list of clipping rectangles can be specified. In these regions *no*
+   video is displayed, so the graphics surface can be seen here.
+
+4. The framebuffer has an alpha channel that can be used to clip or
+   blend the framebuffer with the video.
+
+5. A global alpha value can be specified to blend the framebuffer
+   contents with video images.
+
+When simultaneous capturing and overlay is supported and the hardware
+prohibits different image and frame buffer formats, the format requested
+first takes precedence. The attempt to capture
+(:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`) or overlay
+(:ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>`) may fail with an ``EBUSY`` error
+code or return accordingly modified parameters..
+
+
+Overlay Window
+==============
+
+The overlaid image is determined by cropping and overlay window
+parameters. The former select an area of the video picture to capture,
+the latter how images are overlaid and clipped. Cropping initialization
+at minimum requires to reset the parameters to defaults. An example is
+given in :ref:`crop`.
+
+The overlay window is described by a struct
+:c:type:`v4l2_window`. It defines the size of the image,
+its position over the graphics surface and the clipping to be applied.
+To get the current parameters applications set the ``type`` field of a
+struct :c:type:`v4l2_format` to
+``V4L2_BUF_TYPE_VIDEO_OVERLAY`` and call the
+:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` ioctl. The driver fills the
+struct :c:type:`v4l2_window` substructure named ``win``. It is not
+possible to retrieve a previously programmed clipping list or bitmap.
+
+To program the overlay window applications set the ``type`` field of a
+struct :c:type:`v4l2_format` to
+``V4L2_BUF_TYPE_VIDEO_OVERLAY``, initialize the ``win`` substructure and
+call the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl. The driver
+adjusts the parameters against hardware limits and returns the actual
+parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` does. Like :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`, the
+:ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctl can be used to learn
+about driver capabilities without actually changing driver state. Unlike
+:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` this also works after the overlay has been enabled.
+
+The scaling factor of the overlaid image is implied by the width and
+height given in struct :c:type:`v4l2_window` and the size
+of the cropping rectangle. For more information see :ref:`crop`.
+
+When simultaneous capturing and overlay is supported and the hardware
+prohibits different image and window sizes, the size requested first
+takes precedence. The attempt to capture or overlay as well
+(:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`) may fail with an ``EBUSY`` error
+code or return accordingly modified parameters.
+
+
+.. c:type:: v4l2_window
+
+struct v4l2_window
+------------------
+
+``struct v4l2_rect w``
+    Size and position of the window relative to the top, left corner of
+    the frame buffer defined with
+    :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>`. The window can extend the
+    frame buffer width and height, the ``x`` and ``y`` coordinates can
+    be negative, and it can lie completely outside the frame buffer. The
+    driver clips the window accordingly, or if that is not possible,
+    modifies its size and/or position.
+
+``enum v4l2_field field``
+    Applications set this field to determine which video field shall be
+    overlaid, typically one of ``V4L2_FIELD_ANY`` (0),
+    ``V4L2_FIELD_TOP``, ``V4L2_FIELD_BOTTOM`` or
+    ``V4L2_FIELD_INTERLACED``. Drivers may have to choose a different
+    field order and return the actual setting here.
+
+``__u32 chromakey``
+    When chroma-keying has been negotiated with
+    :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>` applications set this field
+    to the desired pixel value for the chroma key. The format is the
+    same as the pixel format of the framebuffer (struct
+    :c:type:`v4l2_framebuffer` ``fmt.pixelformat``
+    field), with bytes in host order. E. g. for
+    :ref:`V4L2_PIX_FMT_BGR24 <V4L2-PIX-FMT-BGR32>` the value should
+    be 0xRRGGBB on a little endian, 0xBBGGRR on a big endian host.
+
+``struct v4l2_clip * clips``
+    When chroma-keying has *not* been negotiated and
+    :ref:`VIDIOC_G_FBUF <VIDIOC_G_FBUF>` indicated this capability,
+    applications can set this field to point to an array of clipping
+    rectangles.
+
+    Like the window coordinates w, clipping rectangles are defined
+    relative to the top, left corner of the frame buffer. However
+    clipping rectangles must not extend the frame buffer width and
+    height, and they must not overlap. If possible applications
+    should merge adjacent rectangles. Whether this must create
+    x-y or y-x bands, or the order of rectangles, is not defined. When
+    clip lists are not supported the driver ignores this field. Its
+    contents after calling :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`
+    are undefined.
+
+``__u32 clipcount``
+    When the application set the ``clips`` field, this field must
+    contain the number of clipping rectangles in the list. When clip
+    lists are not supported the driver ignores this field, its contents
+    after calling :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` are undefined. When clip lists are
+    supported but no clipping is desired this field must be set to zero.
+
+``void * bitmap``
+    When chroma-keying has *not* been negotiated and
+    :ref:`VIDIOC_G_FBUF <VIDIOC_G_FBUF>` indicated this capability,
+    applications can set this field to point to a clipping bit mask.
+
+It must be of the same size as the window, ``w.width`` and ``w.height``.
+Each bit corresponds to a pixel in the overlaid image, which is
+displayed only when the bit is *set*. Pixel coordinates translate to
+bits like:
+
+
+.. code-block:: c
+
+    ((__u8 *) bitmap)[w.width * y + x / 8] & (1 << (x & 7))
+
+where ``0`` ≤ x < ``w.width`` and ``0`` ≤ y <``w.height``. [#f2]_
+
+When a clipping bit mask is not supported the driver ignores this field,
+its contents after calling :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` are
+undefined. When a bit mask is supported but no clipping is desired this
+field must be set to ``NULL``.
+
+Applications need not create a clip list or bit mask. When they pass
+both, or despite negotiating chroma-keying, the results are undefined.
+Regardless of the chosen method, the clipping abilities of the hardware
+may be limited in quantity or quality. The results when these limits are
+exceeded are undefined. [#f3]_
+
+``__u8 global_alpha``
+    The global alpha value used to blend the framebuffer with video
+    images, if global alpha blending has been negotiated
+    (``V4L2_FBUF_FLAG_GLOBAL_ALPHA``, see
+    :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>`,
+    :ref:`framebuffer-flags`).
+
+.. note::
+
+   This field was added in Linux 2.6.23, extending the
+   structure. However the :ref:`VIDIOC_[G|S|TRY]_FMT <VIDIOC_G_FMT>`
+   ioctls, which take a pointer to a :c:type:`v4l2_format`
+   parent structure with padding bytes at the end, are not affected.
+
+
+.. c:type:: v4l2_clip
+
+struct v4l2_clip [#f4]_
+-----------------------
+
+``struct v4l2_rect c``
+    Coordinates of the clipping rectangle, relative to the top, left
+    corner of the frame buffer. Only window pixels *outside* all
+    clipping rectangles are displayed.
+
+``struct v4l2_clip * next``
+    Pointer to the next clipping rectangle, ``NULL`` when this is the last
+    rectangle. Drivers ignore this field, it cannot be used to pass a
+    linked list of clipping rectangles.
+
+
+.. c:type:: v4l2_rect
+
+struct v4l2_rect
+----------------
+
+``__s32 left``
+    Horizontal offset of the top, left corner of the rectangle, in
+    pixels.
+
+``__s32 top``
+    Vertical offset of the top, left corner of the rectangle, in pixels.
+    Offsets increase to the right and down.
+
+``__u32 width``
+    Width of the rectangle, in pixels.
+
+``__u32 height``
+    Height of the rectangle, in pixels.
+
+
+Enabling Overlay
+================
+
+To start or stop the frame buffer overlay applications call the
+:ref:`VIDIOC_OVERLAY` ioctl.
+
+.. [#f1]
+   A common application of two file descriptors is the XFree86
+   :ref:`Xv/V4L <xvideo>` interface driver and a V4L2 application.
+   While the X server controls video overlay, the application can take
+   advantage of memory mapping and DMA.
+
+   In the opinion of the designers of this API, no driver writer taking
+   the efforts to support simultaneous capturing and overlay will
+   restrict this ability by requiring a single file descriptor, as in
+   V4L and earlier versions of V4L2. Making this optional means
+   applications depending on two file descriptors need backup routines
+   to be compatible with all drivers, which is considerable more work
+   than using two fds in applications which do not. Also two fd's fit
+   the general concept of one file descriptor for each logical stream.
+   Hence as a complexity trade-off drivers *must* support two file
+   descriptors and *may* support single fd operation.
+
+.. [#f2]
+   Should we require ``w.width`` to be a multiple of eight?
+
+.. [#f3]
+   When the image is written into frame buffer memory it will be
+   undesirable if the driver clips out less pixels than expected,
+   because the application and graphics system are not aware these
+   regions need to be refreshed. The driver should clip out more pixels
+   or not write the image at all.
+
+.. [#f4]
+   The X Window system defines "regions" which are vectors of ``struct
+   BoxRec { short x1, y1, x2, y2; }`` with ``width = x2 - x1`` and
+   ``height = y2 - y1``, so one cannot pass X11 clip lists directly.