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 .. cpp:function:: int ioctl( int fd, int request, struct v4l2_framebuffer *argp )
20 .. cpp:function:: int ioctl( int fd, int request, const struct v4l2_framebuffer *argp )
27 File descriptor returned by :ref:`open() <func-open>`.
30 VIDIOC_G_FBUF, VIDIOC_S_FBUF
38 Applications can use the :ref:`VIDIOC_G_FBUF <VIDIOC_G_FBUF>` and :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>` ioctl
39 to get and set the framebuffer parameters for a
40 :ref:`Video Overlay <overlay>` or :ref:`Video Output Overlay <osd>`
41 (OSD). The type of overlay is implied by the device type (capture or
42 output device) and can be determined with the
43 :ref:`VIDIOC_QUERYCAP` ioctl. One ``/dev/videoN``
44 device must not support both kinds of overlay.
46 The V4L2 API distinguishes destructive and non-destructive overlays. A
47 destructive overlay copies captured video images into the video memory
48 of a graphics card. A non-destructive overlay blends video images into a
49 VGA signal or graphics into a video signal. *Video Output Overlays* are
50 always non-destructive.
52 To get the current parameters applications call the :ref:`VIDIOC_G_FBUF <VIDIOC_G_FBUF>`
53 ioctl with a pointer to a :ref:`struct v4l2_framebuffer <v4l2-framebuffer>`
54 structure. The driver fills all fields of the structure or returns an
55 EINVAL error code when overlays are not supported.
57 To set the parameters for a *Video Output Overlay*, applications must
58 initialize the ``flags`` field of a struct
59 :ref:`struct v4l2_framebuffer <v4l2-framebuffer>`. Since the framebuffer is
60 implemented on the TV card all other parameters are determined by the
61 driver. When an application calls :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>` with a pointer to
62 this structure, the driver prepares for the overlay and returns the
63 framebuffer parameters as :ref:`VIDIOC_G_FBUF <VIDIOC_G_FBUF>` does, or it returns an error
66 To set the parameters for a *non-destructive Video Overlay*,
67 applications must initialize the ``flags`` field, the ``fmt``
68 substructure, and call :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>`. Again the driver prepares for
69 the overlay and returns the framebuffer parameters as :ref:`VIDIOC_G_FBUF <VIDIOC_G_FBUF>`
70 does, or it returns an error code.
72 For a *destructive Video Overlay* applications must additionally provide
73 a ``base`` address. Setting up a DMA to a random memory location can
74 jeopardize the system security, its stability or even damage the
75 hardware, therefore only the superuser can set the parameters for a
76 destructive video overlay.
79 .. tabularcolumns:: |p{3.5cm}|p{3.5cm}|p{3.5cm}|p{7.0cm}|
83 .. cssclass:: longtable
85 .. flat-table:: struct v4l2_framebuffer
98 - Overlay capability flags set by the driver, see
99 :ref:`framebuffer-cap`.
108 - Overlay control flags set by application and driver, see
109 :ref:`framebuffer-flags`
118 - Physical base address of the framebuffer, that is the address of
119 the pixel in the top left corner of the framebuffer. [#f1]_
126 - This field is irrelevant to *non-destructive Video Overlays*. For
127 *destructive Video Overlays* applications must provide a base
128 address. The driver may accept only base addresses which are a
129 multiple of two, four or eight bytes. For *Video Output Overlays*
130 the driver must return a valid base address, so applications can
131 find the corresponding Linux framebuffer device (see
141 - Layout of the frame buffer.
150 - Width of the frame buffer in pixels.
159 - Height of the frame buffer in pixels.
168 - The pixel format of the framebuffer.
175 - For *non-destructive Video Overlays* this field only defines a
176 format for the struct :ref:`v4l2_window <v4l2-window>`
184 - For *destructive Video Overlays* applications must initialize this
185 field. For *Video Output Overlays* the driver must return a valid
193 - Usually this is an RGB format (for example
194 :ref:`V4L2_PIX_FMT_RGB565 <V4L2-PIX-FMT-RGB565>`) but YUV
195 formats (only packed YUV formats when chroma keying is used, not
196 including ``V4L2_PIX_FMT_YUYV`` and ``V4L2_PIX_FMT_UYVY``) and the
197 ``V4L2_PIX_FMT_PAL8`` format are also permitted. The behavior of
198 the driver when an application requests a compressed format is
199 undefined. See :ref:`pixfmt` for information on pixel formats.
204 - enum :ref:`v4l2_field <v4l2-field>`
208 - Drivers and applications shall ignore this field. If applicable,
209 the field order is selected with the
210 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, using the ``field``
211 field of struct :ref:`v4l2_window <v4l2-window>`.
220 - Distance in bytes between the leftmost pixels in two adjacent
227 This field is irrelevant to *non-destructive Video Overlays*.
229 For *destructive Video Overlays* both applications and drivers can
230 set this field to request padding bytes at the end of each line.
231 Drivers however may ignore the requested value, returning
232 ``width`` times bytes-per-pixel or a larger value required by the
233 hardware. That implies applications can just set this field to
234 zero to get a reasonable default.
236 For *Video Output Overlays* the driver must return a valid value.
238 Video hardware may access padding bytes, therefore they must
239 reside in accessible memory. Consider for example the case where
240 padding bytes after the last line of an image cross a system page
241 boundary. Capture devices may write padding bytes, the value is
242 undefined. Output devices ignore the contents of padding bytes.
244 When the image format is planar the ``bytesperline`` value applies
245 to the first plane and is divided by the same factor as the
246 ``width`` field for the other planes. For example the Cb and Cr
247 planes of a YUV 4:2:0 image have half as many padding bytes
248 following each line as the Y plane. To avoid ambiguities drivers
249 must return a ``bytesperline`` value rounded up to a multiple of
259 - This field is irrelevant to *non-destructive Video Overlays*. For
260 *destructive Video Overlays* applications must initialize this
261 field. For *Video Output Overlays* the driver must return a valid
264 Together with ``base`` it defines the framebuffer memory
265 accessible by the driver.
270 - enum :ref:`v4l2_colorspace <v4l2-colorspace>`
274 - This information supplements the ``pixelformat`` and must be set
275 by the driver, see :ref:`colorspaces`.
284 - Reserved. Drivers and applications must set this field to zero.
287 .. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|
291 .. flat-table:: Frame Buffer Capability Flags
299 - ``V4L2_FBUF_CAP_EXTERNOVERLAY``
303 - The device is capable of non-destructive overlays. When the driver
304 clears this flag, only destructive overlays are supported. There
305 are no drivers yet which support both destructive and
306 non-destructive overlays. Video Output Overlays are in practice
307 always non-destructive.
311 - ``V4L2_FBUF_CAP_CHROMAKEY``
315 - The device supports clipping by chroma-keying the images. That is,
316 image pixels replace pixels in the VGA or video signal only where
317 the latter assume a certain color. Chroma-keying makes no sense
318 for destructive overlays.
322 - ``V4L2_FBUF_CAP_LIST_CLIPPING``
326 - The device supports clipping using a list of clip rectangles.
330 - ``V4L2_FBUF_CAP_BITMAP_CLIPPING``
334 - The device supports clipping using a bit mask.
338 - ``V4L2_FBUF_CAP_LOCAL_ALPHA``
342 - The device supports clipping/blending using the alpha channel of
343 the framebuffer or VGA signal. Alpha blending makes no sense for
344 destructive overlays.
348 - ``V4L2_FBUF_CAP_GLOBAL_ALPHA``
352 - The device supports alpha blending using a global alpha value.
353 Alpha blending makes no sense for destructive overlays.
357 - ``V4L2_FBUF_CAP_LOCAL_INV_ALPHA``
361 - The device supports clipping/blending using the inverted alpha
362 channel of the framebuffer or VGA signal. Alpha blending makes no
363 sense for destructive overlays.
367 - ``V4L2_FBUF_CAP_SRC_CHROMAKEY``
371 - The device supports Source Chroma-keying. Video pixels with the
372 chroma-key colors are replaced by framebuffer pixels, which is
373 exactly opposite of ``V4L2_FBUF_CAP_CHROMAKEY``
376 .. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|
378 .. _framebuffer-flags:
380 .. cssclass:: longtable
382 .. flat-table:: Frame Buffer Flags
390 - ``V4L2_FBUF_FLAG_PRIMARY``
394 - The framebuffer is the primary graphics surface. In other words,
395 the overlay is destructive. This flag is typically set by any
396 driver that doesn't have the ``V4L2_FBUF_CAP_EXTERNOVERLAY``
397 capability and it is cleared otherwise.
401 - ``V4L2_FBUF_FLAG_OVERLAY``
405 - If this flag is set for a video capture device, then the driver
406 will set the initial overlay size to cover the full framebuffer
407 size, otherwise the existing overlay size (as set by
408 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`) will be used. Only one
409 video capture driver (bttv) supports this flag. The use of this
410 flag for capture devices is deprecated. There is no way to detect
411 which drivers support this flag, so the only reliable method of
412 setting the overlay size is through
413 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`. If this flag is set for a
414 video output device, then the video output overlay window is
415 relative to the top-left corner of the framebuffer and restricted
416 to the size of the framebuffer. If it is cleared, then the video
417 output overlay window is relative to the video output display.
421 - ``V4L2_FBUF_FLAG_CHROMAKEY``
425 - Use chroma-keying. The chroma-key color is determined by the
426 ``chromakey`` field of struct :ref:`v4l2_window <v4l2-window>`
427 and negotiated with the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`
428 ioctl, see :ref:`overlay` and :ref:`osd`.
432 - :cspan:`2` There are no flags to enable clipping using a list of
433 clip rectangles or a bitmap. These methods are negotiated with the
434 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, see :ref:`overlay`
439 - ``V4L2_FBUF_FLAG_LOCAL_ALPHA``
443 - Use the alpha channel of the framebuffer to clip or blend
444 framebuffer pixels with video images. The blend function is:
445 output = framebuffer pixel * alpha + video pixel * (1 - alpha).
446 The actual alpha depth depends on the framebuffer pixel format.
450 - ``V4L2_FBUF_FLAG_GLOBAL_ALPHA``
454 - Use a global alpha value to blend the framebuffer with video
455 images. The blend function is: output = (framebuffer pixel * alpha
456 + video pixel * (255 - alpha)) / 255. The alpha value is
457 determined by the ``global_alpha`` field of struct
458 :ref:`v4l2_window <v4l2-window>` and negotiated with the
459 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, see :ref:`overlay`
464 - ``V4L2_FBUF_FLAG_LOCAL_INV_ALPHA``
468 - Like ``V4L2_FBUF_FLAG_LOCAL_ALPHA``, use the alpha channel of the
469 framebuffer to clip or blend framebuffer pixels with video images,
470 but with an inverted alpha value. The blend function is: output =
471 framebuffer pixel * (1 - alpha) + video pixel * alpha. The actual
472 alpha depth depends on the framebuffer pixel format.
476 - ``V4L2_FBUF_FLAG_SRC_CHROMAKEY``
480 - Use source chroma-keying. The source chroma-key color is
481 determined by the ``chromakey`` field of struct
482 :ref:`v4l2_window <v4l2-window>` and negotiated with the
483 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, see :ref:`overlay`
484 and :ref:`osd`. Both chroma-keying are mutual exclusive to each
485 other, so same ``chromakey`` field of struct
486 :ref:`v4l2_window <v4l2-window>` is being used.
492 On success 0 is returned, on error -1 and the ``errno`` variable is set
493 appropriately. The generic error codes are described at the
494 :ref:`Generic Error Codes <gen-errors>` chapter.
497 :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>` can only be called by a privileged user to
498 negotiate the parameters for a destructive overlay.
501 The :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>` parameters are unsuitable.
504 A physical base address may not suit all platforms. GK notes in
505 theory we should pass something like PCI device + memory region +
506 offset instead. If you encounter problems please discuss on the
507 linux-media mailing list:
508 `https://linuxtv.org/lists.php <https://linuxtv.org/lists.php>`__.