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