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 .. flat-table:: struct v4l2_framebuffer
96 - Overlay capability flags set by the driver, see
97 :ref:`framebuffer-cap`.
106 - Overlay control flags set by application and driver, see
107 :ref:`framebuffer-flags`
116 - Physical base address of the framebuffer, that is the address of
117 the pixel in the top left corner of the framebuffer. [#f1]_
124 - This field is irrelevant to *non-destructive Video Overlays*. For
125 *destructive Video Overlays* applications must provide a base
126 address. The driver may accept only base addresses which are a
127 multiple of two, four or eight bytes. For *Video Output Overlays*
128 the driver must return a valid base address, so applications can
129 find the corresponding Linux framebuffer device (see
139 - Layout of the frame buffer.
148 - Width of the frame buffer in pixels.
157 - Height of the frame buffer in pixels.
166 - The pixel format of the framebuffer.
173 - For *non-destructive Video Overlays* this field only defines a
174 format for the struct :ref:`v4l2_window <v4l2-window>`
182 - For *destructive Video Overlays* applications must initialize this
183 field. For *Video Output Overlays* the driver must return a valid
191 - Usually this is an RGB format (for example
192 :ref:`V4L2_PIX_FMT_RGB565 <V4L2-PIX-FMT-RGB565>`) but YUV
193 formats (only packed YUV formats when chroma keying is used, not
194 including ``V4L2_PIX_FMT_YUYV`` and ``V4L2_PIX_FMT_UYVY``) and the
195 ``V4L2_PIX_FMT_PAL8`` format are also permitted. The behavior of
196 the driver when an application requests a compressed format is
197 undefined. See :ref:`pixfmt` for information on pixel formats.
202 - enum :ref:`v4l2_field <v4l2-field>`
206 - Drivers and applications shall ignore this field. If applicable,
207 the field order is selected with the
208 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, using the ``field``
209 field of struct :ref:`v4l2_window <v4l2-window>`.
218 - Distance in bytes between the leftmost pixels in two adjacent
225 This field is irrelevant to *non-destructive Video Overlays*.
227 For *destructive Video Overlays* both applications and drivers can
228 set this field to request padding bytes at the end of each line.
229 Drivers however may ignore the requested value, returning
230 ``width`` times bytes-per-pixel or a larger value required by the
231 hardware. That implies applications can just set this field to
232 zero to get a reasonable default.
234 For *Video Output Overlays* the driver must return a valid value.
236 Video hardware may access padding bytes, therefore they must
237 reside in accessible memory. Consider for example the case where
238 padding bytes after the last line of an image cross a system page
239 boundary. Capture devices may write padding bytes, the value is
240 undefined. Output devices ignore the contents of padding bytes.
242 When the image format is planar the ``bytesperline`` value applies
243 to the first plane and is divided by the same factor as the
244 ``width`` field for the other planes. For example the Cb and Cr
245 planes of a YUV 4:2:0 image have half as many padding bytes
246 following each line as the Y plane. To avoid ambiguities drivers
247 must return a ``bytesperline`` value rounded up to a multiple of
257 - This field is irrelevant to *non-destructive Video Overlays*. For
258 *destructive Video Overlays* applications must initialize this
259 field. For *Video Output Overlays* the driver must return a valid
262 Together with ``base`` it defines the framebuffer memory
263 accessible by the driver.
268 - enum :ref:`v4l2_colorspace <v4l2-colorspace>`
272 - This information supplements the ``pixelformat`` and must be set
273 by the driver, see :ref:`colorspaces`.
282 - Reserved. Drivers and applications must set this field to zero.
288 .. 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``
376 .. _framebuffer-flags:
378 .. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|
380 .. flat-table:: Frame Buffer Flags
388 - ``V4L2_FBUF_FLAG_PRIMARY``
392 - The framebuffer is the primary graphics surface. In other words,
393 the overlay is destructive. This flag is typically set by any
394 driver that doesn't have the ``V4L2_FBUF_CAP_EXTERNOVERLAY``
395 capability and it is cleared otherwise.
399 - ``V4L2_FBUF_FLAG_OVERLAY``
403 - If this flag is set for a video capture device, then the driver
404 will set the initial overlay size to cover the full framebuffer
405 size, otherwise the existing overlay size (as set by
406 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`) will be used. Only one
407 video capture driver (bttv) supports this flag. The use of this
408 flag for capture devices is deprecated. There is no way to detect
409 which drivers support this flag, so the only reliable method of
410 setting the overlay size is through
411 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`. If this flag is set for a
412 video output device, then the video output overlay window is
413 relative to the top-left corner of the framebuffer and restricted
414 to the size of the framebuffer. If it is cleared, then the video
415 output overlay window is relative to the video output display.
419 - ``V4L2_FBUF_FLAG_CHROMAKEY``
423 - Use chroma-keying. The chroma-key color is determined by the
424 ``chromakey`` field of struct :ref:`v4l2_window <v4l2-window>`
425 and negotiated with the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`
426 ioctl, see :ref:`overlay` and :ref:`osd`.
430 - :cspan:`2` There are no flags to enable clipping using a list of
431 clip rectangles or a bitmap. These methods are negotiated with the
432 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, see :ref:`overlay`
437 - ``V4L2_FBUF_FLAG_LOCAL_ALPHA``
441 - Use the alpha channel of the framebuffer to clip or blend
442 framebuffer pixels with video images. The blend function is:
443 output = framebuffer pixel * alpha + video pixel * (1 - alpha).
444 The actual alpha depth depends on the framebuffer pixel format.
448 - ``V4L2_FBUF_FLAG_GLOBAL_ALPHA``
452 - Use a global alpha value to blend the framebuffer with video
453 images. The blend function is: output = (framebuffer pixel * alpha
454 + video pixel * (255 - alpha)) / 255. The alpha value is
455 determined by the ``global_alpha`` field of struct
456 :ref:`v4l2_window <v4l2-window>` and negotiated with the
457 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, see :ref:`overlay`
462 - ``V4L2_FBUF_FLAG_LOCAL_INV_ALPHA``
466 - Like ``V4L2_FBUF_FLAG_LOCAL_ALPHA``, use the alpha channel of the
467 framebuffer to clip or blend framebuffer pixels with video images,
468 but with an inverted alpha value. The blend function is: output =
469 framebuffer pixel * (1 - alpha) + video pixel * alpha. The actual
470 alpha depth depends on the framebuffer pixel format.
474 - ``V4L2_FBUF_FLAG_SRC_CHROMAKEY``
478 - Use source chroma-keying. The source chroma-key color is
479 determined by the ``chromakey`` field of struct
480 :ref:`v4l2_window <v4l2-window>` and negotiated with the
481 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl, see :ref:`overlay`
482 and :ref:`osd`. Both chroma-keying are mutual exclusive to each
483 other, so same ``chromakey`` field of struct
484 :ref:`v4l2_window <v4l2-window>` is being used.
490 On success 0 is returned, on error -1 and the ``errno`` variable is set
491 appropriately. The generic error codes are described at the
492 :ref:`Generic Error Codes <gen-errors>` chapter.
495 :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>` can only be called by a privileged user to
496 negotiate the parameters for a destructive overlay.
499 The :ref:`VIDIOC_S_FBUF <VIDIOC_G_FBUF>` parameters are unsuitable.
502 A physical base address may not suit all platforms. GK notes in
503 theory we should pass something like PCI device + memory region +
504 offset instead. If you encounter problems please discuss on the
505 linux-media mailing list:
506 `https://linuxtv.org/lists.php <https://linuxtv.org/lists.php>`__.