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.
81 .. 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.
290 .. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|
292 .. flat-table:: Frame Buffer Capability Flags
300 - ``V4L2_FBUF_CAP_EXTERNOVERLAY``
304 - The device is capable of non-destructive overlays. When the driver
305 clears this flag, only destructive overlays are supported. There
306 are no drivers yet which support both destructive and
307 non-destructive overlays. Video Output Overlays are in practice
308 always non-destructive.
312 - ``V4L2_FBUF_CAP_CHROMAKEY``
316 - The device supports clipping by chroma-keying the images. That is,
317 image pixels replace pixels in the VGA or video signal only where
318 the latter assume a certain color. Chroma-keying makes no sense
319 for destructive overlays.
323 - ``V4L2_FBUF_CAP_LIST_CLIPPING``
327 - The device supports clipping using a list of clip rectangles.
331 - ``V4L2_FBUF_CAP_BITMAP_CLIPPING``
335 - The device supports clipping using a bit mask.
339 - ``V4L2_FBUF_CAP_LOCAL_ALPHA``
343 - The device supports clipping/blending using the alpha channel of
344 the framebuffer or VGA signal. Alpha blending makes no sense for
345 destructive overlays.
349 - ``V4L2_FBUF_CAP_GLOBAL_ALPHA``
353 - The device supports alpha blending using a global alpha value.
354 Alpha blending makes no sense for destructive overlays.
358 - ``V4L2_FBUF_CAP_LOCAL_INV_ALPHA``
362 - The device supports clipping/blending using the inverted alpha
363 channel of the framebuffer or VGA signal. Alpha blending makes no
364 sense for destructive overlays.
368 - ``V4L2_FBUF_CAP_SRC_CHROMAKEY``
372 - The device supports Source Chroma-keying. Video pixels with the
373 chroma-key colors are replaced by framebuffer pixels, which is
374 exactly opposite of ``V4L2_FBUF_CAP_CHROMAKEY``
378 .. _framebuffer-flags:
380 .. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|
382 .. cssclass:: longtable
384 .. flat-table:: Frame Buffer Flags
392 - ``V4L2_FBUF_FLAG_PRIMARY``
396 - The framebuffer is the primary graphics surface. In other words,
397 the overlay is destructive. This flag is typically set by any
398 driver that doesn't have the ``V4L2_FBUF_CAP_EXTERNOVERLAY``
399 capability and it is cleared otherwise.
403 - ``V4L2_FBUF_FLAG_OVERLAY``
407 - If this flag is set for a video capture device, then the driver
408 will set the initial overlay size to cover the full framebuffer
409 size, otherwise the existing overlay size (as set by
410 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`) will be used. Only one
411 video capture driver (bttv) supports this flag. The use of this
412 flag for capture devices is deprecated. There is no way to detect
413 which drivers support this flag, so the only reliable method of
414 setting the overlay size is through
415 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`. If this flag is set for a
416 video output device, then the video output overlay window is
417 relative to the top-left corner of the framebuffer and restricted
418 to the size of the framebuffer. If it is cleared, then the video
419 output overlay window is relative to the video output display.
423 - ``V4L2_FBUF_FLAG_CHROMAKEY``
427 - Use chroma-keying. The chroma-key color is determined by the
428 ``chromakey`` field of struct :ref:`v4l2_window <v4l2-window>`
429 and negotiated with the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`
430 ioctl, see :ref:`overlay` and :ref:`osd`.
434 - :cspan:`2` There are no flags to enable clipping using a list of
435 clip rectangles or a bitmap. These methods are negotiated with the
436 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, see :ref:`overlay`
441 - ``V4L2_FBUF_FLAG_LOCAL_ALPHA``
445 - Use the alpha channel of the framebuffer to clip or blend
446 framebuffer pixels with video images. The blend function is:
447 output = framebuffer pixel * alpha + video pixel * (1 - alpha).
448 The actual alpha depth depends on the framebuffer pixel format.
452 - ``V4L2_FBUF_FLAG_GLOBAL_ALPHA``
456 - Use a global alpha value to blend the framebuffer with video
457 images. The blend function is: output = (framebuffer pixel * alpha
458 + video pixel * (255 - alpha)) / 255. The alpha value is
459 determined by the ``global_alpha`` field of struct
460 :ref:`v4l2_window <v4l2-window>` and negotiated with the
461 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, see :ref:`overlay`
466 - ``V4L2_FBUF_FLAG_LOCAL_INV_ALPHA``
470 - Like ``V4L2_FBUF_FLAG_LOCAL_ALPHA``, use the alpha channel of the
471 framebuffer to clip or blend framebuffer pixels with video images,
472 but with an inverted alpha value. The blend function is: output =
473 framebuffer pixel * (1 - alpha) + video pixel * alpha. The actual
474 alpha depth depends on the framebuffer pixel format.
478 - ``V4L2_FBUF_FLAG_SRC_CHROMAKEY``
482 - Use source chroma-keying. The source chroma-key color is
483 determined by the ``chromakey`` field of struct
484 :ref:`v4l2_window <v4l2-window>` and negotiated with the
485 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, see :ref:`overlay`
486 and :ref:`osd`. Both chroma-keying are mutual exclusive to each
487 other, so same ``chromakey`` field of struct
488 :ref:`v4l2_window <v4l2-window>` is being used.
494 On success 0 is returned, on error -1 and the ``errno`` variable is set
495 appropriately. The generic error codes are described at the
496 :ref:`Generic Error Codes <gen-errors>` chapter.
499 :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>` can only be called by a privileged user to
500 negotiate the parameters for a destructive overlay.
503 The :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>` parameters are unsuitable.
506 A physical base address may not suit all platforms. GK notes in
507 theory we should pass something like PCI device + memory region +
508 offset instead. If you encounter problems please discuss on the
509 linux-media mailing list:
510 `https://linuxtv.org/lists.php <https://linuxtv.org/lists.php>`__.