1 .. -*- coding: utf-8; mode: rst -*-
5 **********************************
6 ioctl VIDIOC_G_FBUF, VIDIOC_S_FBUF
7 **********************************
12 VIDIOC_G_FBUF - VIDIOC_S_FBUF - Get or set frame buffer overlay parameters
18 .. c:function:: int ioctl( int fd, VIDIOC_G_FBUF, struct v4l2_framebuffer *argp )
21 .. c:function:: int ioctl( int fd, VIDIOC_S_FBUF, const struct v4l2_framebuffer *argp )
29 File descriptor returned by :ref:`open() <func-open>`.
37 Applications can use the :ref:`VIDIOC_G_FBUF <VIDIOC_G_FBUF>` and :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>` ioctl
38 to get and set the framebuffer parameters for a
39 :ref:`Video Overlay <overlay>` or :ref:`Video Output Overlay <osd>`
40 (OSD). The type of overlay is implied by the device type (capture or
41 output device) and can be determined with the
42 :ref:`VIDIOC_QUERYCAP` ioctl. One ``/dev/videoN``
43 device must not support both kinds of overlay.
45 The V4L2 API distinguishes destructive and non-destructive overlays. A
46 destructive overlay copies captured video images into the video memory
47 of a graphics card. A non-destructive overlay blends video images into a
48 VGA signal or graphics into a video signal. *Video Output Overlays* are
49 always non-destructive.
51 To get the current parameters applications call the :ref:`VIDIOC_G_FBUF <VIDIOC_G_FBUF>`
52 ioctl with a pointer to a struct :c:type:`v4l2_framebuffer`
53 structure. The driver fills all fields of the structure or returns an
54 EINVAL error code when overlays are not supported.
56 To set the parameters for a *Video Output Overlay*, applications must
57 initialize the ``flags`` field of a struct
58 struct :c:type:`v4l2_framebuffer`. Since the framebuffer is
59 implemented on the TV card all other parameters are determined by the
60 driver. When an application calls :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>` with a pointer to
61 this structure, the driver prepares for the overlay and returns the
62 framebuffer parameters as :ref:`VIDIOC_G_FBUF <VIDIOC_G_FBUF>` does, or it returns an error
65 To set the parameters for a *non-destructive Video Overlay*,
66 applications must initialize the ``flags`` field, the ``fmt``
67 substructure, and call :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>`. Again the driver prepares for
68 the overlay and returns the framebuffer parameters as :ref:`VIDIOC_G_FBUF <VIDIOC_G_FBUF>`
69 does, or it returns an error code.
71 For a *destructive Video Overlay* applications must additionally provide
72 a ``base`` address. Setting up a DMA to a random memory location can
73 jeopardize the system security, its stability or even damage the
74 hardware, therefore only the superuser can set the parameters for a
75 destructive video overlay.
78 .. tabularcolumns:: |p{3.5cm}|p{3.5cm}|p{3.5cm}|p{7.0cm}|
80 .. c:type:: v4l2_framebuffer
82 .. cssclass:: longtable
84 .. flat-table:: struct v4l2_framebuffer
97 - Overlay capability flags set by the driver, see
98 :ref:`framebuffer-cap`.
107 - Overlay control flags set by application and driver, see
108 :ref:`framebuffer-flags`
117 - Physical base address of the framebuffer, that is the address of
118 the pixel in the top left corner of the framebuffer. [#f1]_
125 - This field is irrelevant to *non-destructive Video Overlays*. For
126 *destructive Video Overlays* applications must provide a base
127 address. The driver may accept only base addresses which are a
128 multiple of two, four or eight bytes. For *Video Output Overlays*
129 the driver must return a valid base address, so applications can
130 find the corresponding Linux framebuffer device (see
140 - Layout of the frame buffer.
149 - Width of the frame buffer in pixels.
158 - Height of the frame buffer in pixels.
167 - The pixel format of the framebuffer.
174 - For *non-destructive Video Overlays* this field only defines a
175 format for the struct :c:type:`v4l2_window`
183 - For *destructive Video Overlays* applications must initialize this
184 field. For *Video Output Overlays* the driver must return a valid
192 - Usually this is an RGB format (for example
193 :ref:`V4L2_PIX_FMT_RGB565 <V4L2-PIX-FMT-RGB565>`) but YUV
194 formats (only packed YUV formats when chroma keying is used, not
195 including ``V4L2_PIX_FMT_YUYV`` and ``V4L2_PIX_FMT_UYVY``) and the
196 ``V4L2_PIX_FMT_PAL8`` format are also permitted. The behavior of
197 the driver when an application requests a compressed format is
198 undefined. See :ref:`pixfmt` for information on pixel formats.
203 - enum :c:type:`v4l2_field`
207 - Drivers and applications shall ignore this field. If applicable,
208 the field order is selected with the
209 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, using the ``field``
210 field of struct :c:type:`v4l2_window`.
219 - Distance in bytes between the leftmost pixels in two adjacent
226 This field is irrelevant to *non-destructive Video Overlays*.
228 For *destructive Video Overlays* both applications and drivers can
229 set this field to request padding bytes at the end of each line.
230 Drivers however may ignore the requested value, returning
231 ``width`` times bytes-per-pixel or a larger value required by the
232 hardware. That implies applications can just set this field to
233 zero to get a reasonable default.
235 For *Video Output Overlays* the driver must return a valid value.
237 Video hardware may access padding bytes, therefore they must
238 reside in accessible memory. Consider for example the case where
239 padding bytes after the last line of an image cross a system page
240 boundary. Capture devices may write padding bytes, the value is
241 undefined. Output devices ignore the contents of padding bytes.
243 When the image format is planar the ``bytesperline`` value applies
244 to the first plane and is divided by the same factor as the
245 ``width`` field for the other planes. For example the Cb and Cr
246 planes of a YUV 4:2:0 image have half as many padding bytes
247 following each line as the Y plane. To avoid ambiguities drivers
248 must return a ``bytesperline`` value rounded up to a multiple of
258 - This field is irrelevant to *non-destructive Video Overlays*. For
259 *destructive Video Overlays* applications must initialize this
260 field. For *Video Output Overlays* the driver must return a valid
263 Together with ``base`` it defines the framebuffer memory
264 accessible by the driver.
269 - enum :c:type:`v4l2_colorspace`
273 - This information supplements the ``pixelformat`` and must be set
274 by the driver, see :ref:`colorspaces`.
283 - Reserved. Drivers and applications must set this field to zero.
286 .. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|
290 .. flat-table:: Frame Buffer Capability Flags
298 - ``V4L2_FBUF_CAP_EXTERNOVERLAY``
302 - The device is capable of non-destructive overlays. When the driver
303 clears this flag, only destructive overlays are supported. There
304 are no drivers yet which support both destructive and
305 non-destructive overlays. Video Output Overlays are in practice
306 always non-destructive.
310 - ``V4L2_FBUF_CAP_CHROMAKEY``
314 - The device supports clipping by chroma-keying the images. That is,
315 image pixels replace pixels in the VGA or video signal only where
316 the latter assume a certain color. Chroma-keying makes no sense
317 for destructive overlays.
321 - ``V4L2_FBUF_CAP_LIST_CLIPPING``
325 - The device supports clipping using a list of clip rectangles.
329 - ``V4L2_FBUF_CAP_BITMAP_CLIPPING``
333 - The device supports clipping using a bit mask.
337 - ``V4L2_FBUF_CAP_LOCAL_ALPHA``
341 - The device supports clipping/blending using the alpha channel of
342 the framebuffer or VGA signal. Alpha blending makes no sense for
343 destructive overlays.
347 - ``V4L2_FBUF_CAP_GLOBAL_ALPHA``
351 - The device supports alpha blending using a global alpha value.
352 Alpha blending makes no sense for destructive overlays.
356 - ``V4L2_FBUF_CAP_LOCAL_INV_ALPHA``
360 - The device supports clipping/blending using the inverted alpha
361 channel of the framebuffer or VGA signal. Alpha blending makes no
362 sense for destructive overlays.
366 - ``V4L2_FBUF_CAP_SRC_CHROMAKEY``
370 - The device supports Source Chroma-keying. Video pixels with the
371 chroma-key colors are replaced by framebuffer pixels, which is
372 exactly opposite of ``V4L2_FBUF_CAP_CHROMAKEY``
375 .. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|
377 .. _framebuffer-flags:
379 .. cssclass:: longtable
381 .. flat-table:: Frame Buffer Flags
389 - ``V4L2_FBUF_FLAG_PRIMARY``
393 - The framebuffer is the primary graphics surface. In other words,
394 the overlay is destructive. This flag is typically set by any
395 driver that doesn't have the ``V4L2_FBUF_CAP_EXTERNOVERLAY``
396 capability and it is cleared otherwise.
400 - ``V4L2_FBUF_FLAG_OVERLAY``
404 - If this flag is set for a video capture device, then the driver
405 will set the initial overlay size to cover the full framebuffer
406 size, otherwise the existing overlay size (as set by
407 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`) will be used. Only one
408 video capture driver (bttv) supports this flag. The use of this
409 flag for capture devices is deprecated. There is no way to detect
410 which drivers support this flag, so the only reliable method of
411 setting the overlay size is through
412 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`. If this flag is set for a
413 video output device, then the video output overlay window is
414 relative to the top-left corner of the framebuffer and restricted
415 to the size of the framebuffer. If it is cleared, then the video
416 output overlay window is relative to the video output display.
420 - ``V4L2_FBUF_FLAG_CHROMAKEY``
424 - Use chroma-keying. The chroma-key color is determined by the
425 ``chromakey`` field of struct :c:type:`v4l2_window`
426 and negotiated with the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`
427 ioctl, see :ref:`overlay` and :ref:`osd`.
431 - :cspan:`2` There are no flags to enable clipping using a list of
432 clip rectangles or a bitmap. These methods are negotiated with the
433 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, see :ref:`overlay`
438 - ``V4L2_FBUF_FLAG_LOCAL_ALPHA``
442 - Use the alpha channel of the framebuffer to clip or blend
443 framebuffer pixels with video images. The blend function is:
444 output = framebuffer pixel * alpha + video pixel * (1 - alpha).
445 The actual alpha depth depends on the framebuffer pixel format.
449 - ``V4L2_FBUF_FLAG_GLOBAL_ALPHA``
453 - Use a global alpha value to blend the framebuffer with video
454 images. The blend function is: output = (framebuffer pixel * alpha
455 + video pixel * (255 - alpha)) / 255. The alpha value is
456 determined by the ``global_alpha`` field of struct
457 :c:type:`v4l2_window` and negotiated with the
458 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, see :ref:`overlay`
463 - ``V4L2_FBUF_FLAG_LOCAL_INV_ALPHA``
467 - Like ``V4L2_FBUF_FLAG_LOCAL_ALPHA``, use the alpha channel of the
468 framebuffer to clip or blend framebuffer pixels with video images,
469 but with an inverted alpha value. The blend function is: output =
470 framebuffer pixel * (1 - alpha) + video pixel * alpha. The actual
471 alpha depth depends on the framebuffer pixel format.
475 - ``V4L2_FBUF_FLAG_SRC_CHROMAKEY``
479 - Use source chroma-keying. The source chroma-key color is
480 determined by the ``chromakey`` field of struct
481 :c:type:`v4l2_window` and negotiated with the
482 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, see :ref:`overlay`
483 and :ref:`osd`. Both chroma-keying are mutual exclusive to each
484 other, so same ``chromakey`` field of struct
485 :c:type:`v4l2_window` is being used.
491 On success 0 is returned, on error -1 and the ``errno`` variable is set
492 appropriately. The generic error codes are described at the
493 :ref:`Generic Error Codes <gen-errors>` chapter.
496 :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>` can only be called by a privileged user to
497 negotiate the parameters for a destructive overlay.
500 The :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>` parameters are unsuitable.
503 A physical base address may not suit all platforms. GK notes in
504 theory we should pass something like PCI device + memory region +
505 offset instead. If you encounter problems please discuss on the
506 linux-media mailing list:
507 `https://linuxtv.org/lists.php <https://linuxtv.org/lists.php>`__.