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 .. flat-table:: struct v4l2_framebuffer
94 - Overlay capability flags set by the driver, see
95 :ref:`framebuffer-cap`.
104 - Overlay control flags set by application and driver, see
105 :ref:`framebuffer-flags`
114 - Physical base address of the framebuffer, that is the address of
115 the pixel in the top left corner of the framebuffer. [#f1]_
122 - This field is irrelevant to *non-destructive Video Overlays*. For
123 *destructive Video Overlays* applications must provide a base
124 address. The driver may accept only base addresses which are a
125 multiple of two, four or eight bytes. For *Video Output Overlays*
126 the driver must return a valid base address, so applications can
127 find the corresponding Linux framebuffer device (see
137 - Layout of the frame buffer.
146 - Width of the frame buffer in pixels.
155 - Height of the frame buffer in pixels.
164 - The pixel format of the framebuffer.
171 - For *non-destructive Video Overlays* this field only defines a
172 format for the struct :ref:`v4l2_window <v4l2-window>`
180 - For *destructive Video Overlays* applications must initialize this
181 field. For *Video Output Overlays* the driver must return a valid
189 - Usually this is an RGB format (for example
190 :ref:`V4L2_PIX_FMT_RGB565 <V4L2-PIX-FMT-RGB565>`) but YUV
191 formats (only packed YUV formats when chroma keying is used, not
192 including ``V4L2_PIX_FMT_YUYV`` and ``V4L2_PIX_FMT_UYVY``) and the
193 ``V4L2_PIX_FMT_PAL8`` format are also permitted. The behavior of
194 the driver when an application requests a compressed format is
195 undefined. See :ref:`pixfmt` for information on pixel formats.
200 - enum :ref:`v4l2_field <v4l2-field>`
204 - Drivers and applications shall ignore this field. If applicable,
205 the field order is selected with the
206 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, using the ``field``
207 field of struct :ref:`v4l2_window <v4l2-window>`.
216 - Distance in bytes between the leftmost pixels in two adjacent
223 This field is irrelevant to *non-destructive Video Overlays*.
225 For *destructive Video Overlays* both applications and drivers can
226 set this field to request padding bytes at the end of each line.
227 Drivers however may ignore the requested value, returning
228 ``width`` times bytes-per-pixel or a larger value required by the
229 hardware. That implies applications can just set this field to
230 zero to get a reasonable default.
232 For *Video Output Overlays* the driver must return a valid value.
234 Video hardware may access padding bytes, therefore they must
235 reside in accessible memory. Consider for example the case where
236 padding bytes after the last line of an image cross a system page
237 boundary. Capture devices may write padding bytes, the value is
238 undefined. Output devices ignore the contents of padding bytes.
240 When the image format is planar the ``bytesperline`` value applies
241 to the first plane and is divided by the same factor as the
242 ``width`` field for the other planes. For example the Cb and Cr
243 planes of a YUV 4:2:0 image have half as many padding bytes
244 following each line as the Y plane. To avoid ambiguities drivers
245 must return a ``bytesperline`` value rounded up to a multiple of
255 - This field is irrelevant to *non-destructive Video Overlays*. For
256 *destructive Video Overlays* applications must initialize this
257 field. For *Video Output Overlays* the driver must return a valid
260 Together with ``base`` it defines the framebuffer memory
261 accessible by the driver.
266 - enum :ref:`v4l2_colorspace <v4l2-colorspace>`
270 - This information supplements the ``pixelformat`` and must be set
271 by the driver, see :ref:`colorspaces`.
280 - Reserved. Drivers and applications must set this field to zero.
286 .. flat-table:: Frame Buffer Capability Flags
294 - ``V4L2_FBUF_CAP_EXTERNOVERLAY``
298 - The device is capable of non-destructive overlays. When the driver
299 clears this flag, only destructive overlays are supported. There
300 are no drivers yet which support both destructive and
301 non-destructive overlays. Video Output Overlays are in practice
302 always non-destructive.
306 - ``V4L2_FBUF_CAP_CHROMAKEY``
310 - The device supports clipping by chroma-keying the images. That is,
311 image pixels replace pixels in the VGA or video signal only where
312 the latter assume a certain color. Chroma-keying makes no sense
313 for destructive overlays.
317 - ``V4L2_FBUF_CAP_LIST_CLIPPING``
321 - The device supports clipping using a list of clip rectangles.
325 - ``V4L2_FBUF_CAP_BITMAP_CLIPPING``
329 - The device supports clipping using a bit mask.
333 - ``V4L2_FBUF_CAP_LOCAL_ALPHA``
337 - The device supports clipping/blending using the alpha channel of
338 the framebuffer or VGA signal. Alpha blending makes no sense for
339 destructive overlays.
343 - ``V4L2_FBUF_CAP_GLOBAL_ALPHA``
347 - The device supports alpha blending using a global alpha value.
348 Alpha blending makes no sense for destructive overlays.
352 - ``V4L2_FBUF_CAP_LOCAL_INV_ALPHA``
356 - The device supports clipping/blending using the inverted alpha
357 channel of the framebuffer or VGA signal. Alpha blending makes no
358 sense for destructive overlays.
362 - ``V4L2_FBUF_CAP_SRC_CHROMAKEY``
366 - The device supports Source Chroma-keying. Video pixels with the
367 chroma-key colors are replaced by framebuffer pixels, which is
368 exactly opposite of ``V4L2_FBUF_CAP_CHROMAKEY``
372 .. _framebuffer-flags:
374 .. flat-table:: Frame Buffer Flags
382 - ``V4L2_FBUF_FLAG_PRIMARY``
386 - The framebuffer is the primary graphics surface. In other words,
387 the overlay is destructive. This flag is typically set by any
388 driver that doesn't have the ``V4L2_FBUF_CAP_EXTERNOVERLAY``
389 capability and it is cleared otherwise.
393 - ``V4L2_FBUF_FLAG_OVERLAY``
397 - If this flag is set for a video capture device, then the driver
398 will set the initial overlay size to cover the full framebuffer
399 size, otherwise the existing overlay size (as set by
400 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`) will be used. Only one
401 video capture driver (bttv) supports this flag. The use of this
402 flag for capture devices is deprecated. There is no way to detect
403 which drivers support this flag, so the only reliable method of
404 setting the overlay size is through
405 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`. If this flag is set for a
406 video output device, then the video output overlay window is
407 relative to the top-left corner of the framebuffer and restricted
408 to the size of the framebuffer. If it is cleared, then the video
409 output overlay window is relative to the video output display.
413 - ``V4L2_FBUF_FLAG_CHROMAKEY``
417 - Use chroma-keying. The chroma-key color is determined by the
418 ``chromakey`` field of struct :ref:`v4l2_window <v4l2-window>`
419 and negotiated with the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`
420 ioctl, see :ref:`overlay` and :ref:`osd`.
424 - :cspan:`2` There are no flags to enable clipping using a list of
425 clip rectangles or a bitmap. These methods are negotiated with the
426 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, see :ref:`overlay`
431 - ``V4L2_FBUF_FLAG_LOCAL_ALPHA``
435 - Use the alpha channel of the framebuffer to clip or blend
436 framebuffer pixels with video images. The blend function is:
437 output = framebuffer pixel * alpha + video pixel * (1 - alpha).
438 The actual alpha depth depends on the framebuffer pixel format.
442 - ``V4L2_FBUF_FLAG_GLOBAL_ALPHA``
446 - Use a global alpha value to blend the framebuffer with video
447 images. The blend function is: output = (framebuffer pixel * alpha
448 + video pixel * (255 - alpha)) / 255. The alpha value is
449 determined by the ``global_alpha`` field of struct
450 :ref:`v4l2_window <v4l2-window>` and negotiated with the
451 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, see :ref:`overlay`
456 - ``V4L2_FBUF_FLAG_LOCAL_INV_ALPHA``
460 - Like ``V4L2_FBUF_FLAG_LOCAL_ALPHA``, use the alpha channel of the
461 framebuffer to clip or blend framebuffer pixels with video images,
462 but with an inverted alpha value. The blend function is: output =
463 framebuffer pixel * (1 - alpha) + video pixel * alpha. The actual
464 alpha depth depends on the framebuffer pixel format.
468 - ``V4L2_FBUF_FLAG_SRC_CHROMAKEY``
472 - Use source chroma-keying. The source chroma-key color is
473 determined by the ``chromakey`` field of struct
474 :ref:`v4l2_window <v4l2-window>` and negotiated with the
475 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, see :ref:`overlay`
476 and :ref:`osd`. Both chroma-keying are mutual exclusive to each
477 other, so same ``chromakey`` field of struct
478 :ref:`v4l2_window <v4l2-window>` is being used.
484 On success 0 is returned, on error -1 and the ``errno`` variable is set
485 appropriately. The generic error codes are described at the
486 :ref:`Generic Error Codes <gen-errors>` chapter.
489 :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>` can only be called by a privileged user to
490 negotiate the parameters for a destructive overlay.
493 The :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>` parameters are unsuitable.
496 A physical base address may not suit all platforms. GK notes in
497 theory we should pass something like PCI device + memory region +
498 offset instead. If you encounter problems please discuss on the
499 linux-media mailing list:
500 `https://linuxtv.org/lists.php <https://linuxtv.org/lists.php>`__.