1 .. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
5 ***********************
6 Video Overlay Interface
7 ***********************
9 **Also known as Framebuffer Overlay or Previewing.**
11 Video overlay devices have the ability to genlock (TV-)video into the
12 (VGA-)video signal of a graphics card, or to store captured images
13 directly in video memory of a graphics card, typically with clipping.
14 This can be considerable more efficient than capturing images and
15 displaying them by other means. In the old days when only nuclear power
16 plants needed cooling towers this used to be the only way to put live
19 Video overlay devices are accessed through the same character special
20 files as :ref:`video capture <capture>` devices.
24 The default function of a ``/dev/video`` device is video
25 capturing. The overlay function is only available after calling
26 the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl.
28 The driver may support simultaneous overlay and capturing using the
29 read/write and streaming I/O methods. If so, operation at the nominal
30 frame rate of the video standard is not guaranteed. Frames may be
31 directed away from overlay to capture, or one field may be used for
32 overlay and the other for capture if the capture parameters permit this.
34 Applications should use different file descriptors for capturing and
35 overlay. This must be supported by all drivers capable of simultaneous
36 capturing and overlay. Optionally these drivers may also permit
37 capturing and overlay with a single file descriptor for compatibility
38 with V4L and earlier versions of V4L2. [#f1]_
40 A common application of two file descriptors is the X11
41 :ref:`Xv/V4L <xvideo>` interface driver and a V4L2 application.
42 While the X server controls video overlay, the application can take
43 advantage of memory mapping and DMA.
48 Devices supporting the video overlay interface set the
49 ``V4L2_CAP_VIDEO_OVERLAY`` flag in the ``capabilities`` field of struct
50 :c:type:`v4l2_capability` returned by the
51 :ref:`VIDIOC_QUERYCAP` ioctl. The overlay I/O
52 method specified below must be supported. Tuners and audio inputs are
56 Supplemental Functions
57 ======================
59 Video overlay devices shall support :ref:`audio input <audio>`,
60 :ref:`tuner`, :ref:`controls <control>`,
61 :ref:`cropping and scaling <crop>` and
62 :ref:`streaming parameter <streaming-par>` ioctls as needed. The
63 :ref:`video input <video>` and :ref:`video standard <standard>`
64 ioctls must be supported by all video overlay devices.
70 Before overlay can commence applications must program the driver with
71 frame buffer parameters, namely the address and size of the frame buffer
72 and the image format, for example RGB 5:6:5. The
73 :ref:`VIDIOC_G_FBUF <VIDIOC_G_FBUF>` and
74 :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>` ioctls are available to get and
75 set these parameters, respectively. The :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>` ioctl is
76 privileged because it allows to set up DMA into physical memory,
77 bypassing the memory protection mechanisms of the kernel. Only the
78 superuser can change the frame buffer address and size. Users are not
79 supposed to run TV applications as root or with SUID bit set. A small
80 helper application with suitable privileges should query the graphics
81 system and program the V4L2 driver at the appropriate time.
83 Some devices add the video overlay to the output signal of the graphics
84 card. In this case the frame buffer is not modified by the video device,
85 and the frame buffer address and pixel format are not needed by the
86 driver. The :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>` ioctl is not privileged. An application
87 can check for this type of device by calling the :ref:`VIDIOC_G_FBUF <VIDIOC_G_FBUF>`
90 A driver may support any (or none) of five clipping/blending methods:
92 1. Chroma-keying displays the overlaid image only where pixels in the
93 primary graphics surface assume a certain color.
95 2. A bitmap can be specified where each bit corresponds to a pixel in
96 the overlaid image. When the bit is set, the corresponding video
97 pixel is displayed, otherwise a pixel of the graphics surface.
99 3. A list of clipping rectangles can be specified. In these regions *no*
100 video is displayed, so the graphics surface can be seen here.
102 4. The framebuffer has an alpha channel that can be used to clip or
103 blend the framebuffer with the video.
105 5. A global alpha value can be specified to blend the framebuffer
106 contents with video images.
108 When simultaneous capturing and overlay is supported and the hardware
109 prohibits different image and frame buffer formats, the format requested
110 first takes precedence. The attempt to capture
111 (:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`) or overlay
112 (:ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>`) may fail with an ``EBUSY`` error
113 code or return accordingly modified parameters..
119 The overlaid image is determined by cropping and overlay window
120 parameters. The former select an area of the video picture to capture,
121 the latter how images are overlaid and clipped. Cropping initialization
122 at minimum requires to reset the parameters to defaults. An example is
123 given in :ref:`crop`.
125 The overlay window is described by a struct
126 :c:type:`v4l2_window`. It defines the size of the image,
127 its position over the graphics surface and the clipping to be applied.
128 To get the current parameters applications set the ``type`` field of a
129 struct :c:type:`v4l2_format` to
130 ``V4L2_BUF_TYPE_VIDEO_OVERLAY`` and call the
131 :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` ioctl. The driver fills the
132 struct :c:type:`v4l2_window` substructure named ``win``. It is not
133 possible to retrieve a previously programmed clipping list or bitmap.
135 To program the overlay window applications set the ``type`` field of a
136 struct :c:type:`v4l2_format` to
137 ``V4L2_BUF_TYPE_VIDEO_OVERLAY``, initialize the ``win`` substructure and
138 call the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl. The driver
139 adjusts the parameters against hardware limits and returns the actual
140 parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` does. Like :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`, the
141 :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctl can be used to learn
142 about driver capabilities without actually changing driver state. Unlike
143 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` this also works after the overlay has been enabled.
145 The scaling factor of the overlaid image is implied by the width and
146 height given in struct :c:type:`v4l2_window` and the size
147 of the cropping rectangle. For more information see :ref:`crop`.
149 When simultaneous capturing and overlay is supported and the hardware
150 prohibits different image and window sizes, the size requested first
151 takes precedence. The attempt to capture or overlay as well
152 (:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`) may fail with an ``EBUSY`` error
153 code or return accordingly modified parameters.
156 .. c:type:: v4l2_window
161 ``struct v4l2_rect w``
162 Size and position of the window relative to the top, left corner of
163 the frame buffer defined with
164 :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>`. The window can extend the
165 frame buffer width and height, the ``x`` and ``y`` coordinates can
166 be negative, and it can lie completely outside the frame buffer. The
167 driver clips the window accordingly, or if that is not possible,
168 modifies its size and/or position.
170 ``enum v4l2_field field``
171 Applications set this field to determine which video field shall be
172 overlaid, typically one of ``V4L2_FIELD_ANY`` (0),
173 ``V4L2_FIELD_TOP``, ``V4L2_FIELD_BOTTOM`` or
174 ``V4L2_FIELD_INTERLACED``. Drivers may have to choose a different
175 field order and return the actual setting here.
178 When chroma-keying has been negotiated with
179 :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>` applications set this field
180 to the desired pixel value for the chroma key. The format is the
181 same as the pixel format of the framebuffer (struct
182 :c:type:`v4l2_framebuffer` ``fmt.pixelformat``
183 field), with bytes in host order. E. g. for
184 :ref:`V4L2_PIX_FMT_BGR24 <V4L2-PIX-FMT-BGR32>` the value should
185 be 0xRRGGBB on a little endian, 0xBBGGRR on a big endian host.
187 ``struct v4l2_clip * clips``
188 When chroma-keying has *not* been negotiated and
189 :ref:`VIDIOC_G_FBUF <VIDIOC_G_FBUF>` indicated this capability,
190 applications can set this field to point to an array of clipping
193 Like the window coordinates w, clipping rectangles are defined
194 relative to the top, left corner of the frame buffer. However
195 clipping rectangles must not extend the frame buffer width and
196 height, and they must not overlap. If possible applications
197 should merge adjacent rectangles. Whether this must create
198 x-y or y-x bands, or the order of rectangles, is not defined. When
199 clip lists are not supported the driver ignores this field. Its
200 contents after calling :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`
204 When the application set the ``clips`` field, this field must
205 contain the number of clipping rectangles in the list. When clip
206 lists are not supported the driver ignores this field, its contents
207 after calling :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` are undefined. When clip lists are
208 supported but no clipping is desired this field must be set to zero.
211 When chroma-keying has *not* been negotiated and
212 :ref:`VIDIOC_G_FBUF <VIDIOC_G_FBUF>` indicated this capability,
213 applications can set this field to point to a clipping bit mask.
215 It must be of the same size as the window, ``w.width`` and ``w.height``.
216 Each bit corresponds to a pixel in the overlaid image, which is
217 displayed only when the bit is *set*. Pixel coordinates translate to
223 ((__u8 *) bitmap)[w.width * y + x / 8] & (1 << (x & 7))
225 where ``0`` ≤ x < ``w.width`` and ``0`` ≤ y <``w.height``. [#f2]_
227 When a clipping bit mask is not supported the driver ignores this field,
228 its contents after calling :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` are
229 undefined. When a bit mask is supported but no clipping is desired this
230 field must be set to ``NULL``.
232 Applications need not create a clip list or bit mask. When they pass
233 both, or despite negotiating chroma-keying, the results are undefined.
234 Regardless of the chosen method, the clipping abilities of the hardware
235 may be limited in quantity or quality. The results when these limits are
236 exceeded are undefined. [#f3]_
238 ``__u8 global_alpha``
239 The global alpha value used to blend the framebuffer with video
240 images, if global alpha blending has been negotiated
241 (``V4L2_FBUF_FLAG_GLOBAL_ALPHA``, see
242 :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>`,
243 :ref:`framebuffer-flags`).
247 This field was added in Linux 2.6.23, extending the
248 structure. However the :ref:`VIDIOC_[G|S|TRY]_FMT <VIDIOC_G_FMT>`
249 ioctls, which take a pointer to a :c:type:`v4l2_format`
250 parent structure with padding bytes at the end, are not affected.
253 .. c:type:: v4l2_clip
255 struct v4l2_clip [#f4]_
256 -----------------------
258 ``struct v4l2_rect c``
259 Coordinates of the clipping rectangle, relative to the top, left
260 corner of the frame buffer. Only window pixels *outside* all
261 clipping rectangles are displayed.
263 ``struct v4l2_clip * next``
264 Pointer to the next clipping rectangle, ``NULL`` when this is the last
265 rectangle. Drivers ignore this field, it cannot be used to pass a
266 linked list of clipping rectangles.
269 .. c:type:: v4l2_rect
275 Horizontal offset of the top, left corner of the rectangle, in
279 Vertical offset of the top, left corner of the rectangle, in pixels.
280 Offsets increase to the right and down.
283 Width of the rectangle, in pixels.
286 Height of the rectangle, in pixels.
292 To start or stop the frame buffer overlay applications call the
293 :ref:`VIDIOC_OVERLAY` ioctl.
296 In the opinion of the designers of this API, no driver writer taking
297 the efforts to support simultaneous capturing and overlay will
298 restrict this ability by requiring a single file descriptor, as in
299 V4L and earlier versions of V4L2. Making this optional means
300 applications depending on two file descriptors need backup routines
301 to be compatible with all drivers, which is considerable more work
302 than using two fds in applications which do not. Also two fd's fit
303 the general concept of one file descriptor for each logical stream.
304 Hence as a complexity trade-off drivers *must* support two file
305 descriptors and *may* support single fd operation.
308 Should we require ``w.width`` to be a multiple of eight?
311 When the image is written into frame buffer memory it will be
312 undesirable if the driver clips out less pixels than expected,
313 because the application and graphics system are not aware these
314 regions need to be refreshed. The driver should clip out more pixels
315 or not write the image at all.
318 The X Window system defines "regions" which are vectors of ``struct
319 BoxRec { short x1, y1, x2, y2; }`` with ``width = x2 - x1`` and
320 ``height = y2 - y1``, so one cannot pass X11 clip lists directly.