diff options
Diffstat (limited to 'Documentation/userspace-api/media/v4l/dev-overlay.rst')
| -rw-r--r-- | Documentation/userspace-api/media/v4l/dev-overlay.rst | 326 |
1 files changed, 326 insertions, 0 deletions
diff --git a/Documentation/userspace-api/media/v4l/dev-overlay.rst b/Documentation/userspace-api/media/v4l/dev-overlay.rst new file mode 100644 index 0000000000..d52977120b --- /dev/null +++ b/Documentation/userspace-api/media/v4l/dev-overlay.rst @@ -0,0 +1,326 @@ +.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later + +.. _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]_ + +A common application of two file descriptors is the X11 +: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. + +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 +===== + +*Note: support for this has been removed.* +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. *Note: support for this has been removed.* + 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. *Note: support for this has been removed.* + 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`` + *Note: support for this has been removed.* + 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`` + *Note: support for this has been removed.* + 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`` + *Note: support for this has been removed.* + 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] + 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. |