path: root/Documentation/DocBook/v4l/vidioc-g-fbuf.xml

<refentry id="vidioc-g-fbuf">
  <refmeta>
    <refentrytitle>ioctl VIDIOC_G_FBUF, VIDIOC_S_FBUF</refentrytitle>
    &manvol;
  </refmeta>

  <refnamediv>
    <refname>VIDIOC_G_FBUF</refname>
    <refname>VIDIOC_S_FBUF</refname>
    <refpurpose>Get or set frame buffer overlay parameters</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <funcsynopsis>
      <funcprototype>
	<funcdef>int <function>ioctl</function></funcdef>
	<paramdef>int <parameter>fd</parameter></paramdef>
	<paramdef>int <parameter>request</parameter></paramdef>
	<paramdef>struct v4l2_framebuffer *<parameter>argp</parameter></paramdef>
      </funcprototype>
    </funcsynopsis>
    <funcsynopsis>
      <funcprototype>
	<funcdef>int <function>ioctl</function></funcdef>
	<paramdef>int <parameter>fd</parameter></paramdef>
	<paramdef>int <parameter>request</parameter></paramdef>
	<paramdef>const struct v4l2_framebuffer *<parameter>argp</parameter></paramdef>
      </funcprototype>
    </funcsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Arguments</title>

    <variablelist>
      <varlistentry>
	<term><parameter>fd</parameter></term>
	<listitem>
	  <para>&fd;</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><parameter>request</parameter></term>
	<listitem>
	  <para>VIDIOC_G_FBUF, VIDIOC_S_FBUF</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><parameter>argp</parameter></term>
	<listitem>
	  <para></para>
	</listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>Description</title>

    <para>Applications can use the <constant>VIDIOC_G_FBUF</constant> and
<constant>VIDIOC_S_FBUF</constant> ioctl to get and set the
framebuffer parameters for a <link linkend="overlay">Video
Overlay</link> or <link linkend="osd">Video Output Overlay</link>
(OSD). The type of overlay is implied by the device type (capture or
output device) and can be determined with the &VIDIOC-QUERYCAP; ioctl.
One <filename>/dev/videoN</filename> device must not support both
kinds of overlay.</para>

    <para>The V4L2 API distinguishes destructive and non-destructive
overlays. A destructive overlay copies captured video images into the
video memory of a graphics card. A non-destructive overlay blends
video images into a VGA signal or graphics into a video signal.
<wordasword>Video Output Overlays</wordasword> are always
non-destructive.</para>

    <para>To get the current parameters applications call the
<constant>VIDIOC_G_FBUF</constant> ioctl with a pointer to a
<structname>v4l2_framebuffer</structname> structure. The driver fills
all fields of the structure or returns an &EINVAL; when overlays are
not supported.</para>

    <para>To set the parameters for a <wordasword>Video Output
Overlay</wordasword>, applications must initialize the
<structfield>flags</structfield> field of a struct
<structname>v4l2_framebuffer</structname>. Since the framebuffer is
implemented on the TV card all other parameters are determined by the
driver. When an application calls <constant>VIDIOC_S_FBUF</constant>
with a pointer to this structure, the driver prepares for the overlay
and returns the framebuffer parameters as
<constant>VIDIOC_G_FBUF</constant> does, or it returns an error
code.</para>

    <para>To set the parameters for a <wordasword>non-destructive
Video Overlay</wordasword>, applications must initialize the
<structfield>flags</structfield> field, the
<structfield>fmt</structfield> substructure, and call
<constant>VIDIOC_S_FBUF</constant>. Again the driver prepares for the
overlay and returns the framebuffer parameters as
<constant>VIDIOC_G_FBUF</constant> does, or it returns an error
code.</para>

    <para>For a <wordasword>destructive Video Overlay</wordasword>
applications must additionally provide a
<structfield>base</structfield> address. Setting up a DMA to a
random memory location can jeopardize the system security, its
stability or even damage the hardware, therefore only the superuser
can set the parameters for a destructive video overlay.</para>

    <!-- NB v4l2_pix_format is also specified in pixfmt.sgml.-->

    <table pgwide="1" frame="none" id="v4l2-framebuffer">
      <title>struct <structname>v4l2_framebuffer</structname></title>
      <tgroup cols="4">
	&cs-ustr;
	<tbody valign="top">
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>capability</structfield></entry>
	    <entry></entry>
	    <entry>Overlay capability flags set by the driver, see
<xref linkend="framebuffer-cap" />.</entry>
	  </row>
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>flags</structfield></entry>
	    <entry></entry>
	    <entry>Overlay control flags set by application and
driver, see <xref linkend="framebuffer-flags" /></entry>
	  </row>
	  <row>
	    <entry>void *</entry>
	    <entry><structfield>base</structfield></entry>
	    <entry></entry>
	    <entry>Physical base address of the framebuffer,
that is the address of the pixel in the top left corner of the
framebuffer.<footnote><para>A physical base address may not suit all
platforms. GK notes in theory we should pass something like PCI device
+ memory region + offset instead. If you encounter problems please
discuss on the linux-media mailing list: &v4l-ml;.</para></footnote></entry>
	  </row>
	  <row>
	    <entry></entry>
	    <entry></entry>
	    <entry></entry>
	    <entry>This field is irrelevant to
<wordasword>non-destructive Video Overlays</wordasword>. For
<wordasword>destructive Video Overlays</wordasword> applications must
provide a base address. The driver may accept only base addresses
which are a multiple of two, four or eight bytes. For
<wordasword>Video Output Overlays</wordasword> the driver must return
a valid base address, so applications can find the corresponding Linux
framebuffer device (see <xref linkend="osd" />).</entry>
	  </row>
	  <row>
	    <entry>&v4l2-pix-format;</entry>
	    <entry><structfield>fmt</structfield></entry>
	    <entry></entry>
	    <entry>Layout of the frame buffer. The
<structname>v4l2_pix_format</structname> structure is defined in <xref
linkend="pixfmt" />, for clarification the fields and acceptable values
	    are listed below:</entry>
	  </row>
	  <row>
	    <entry></entry>
	    <entry>__u32</entry>
	    <entry><structfield>width</structfield></entry>
	    <entry>Width of the frame buffer in pixels.</entry>
	  </row>
	  <row>
	    <entry></entry>
	    <entry>__u32</entry>
	    <entry><structfield>height</structfield></entry>
	    <entry>Height of the frame buffer in pixels.</entry>
	  </row>
	  <row>
	    <entry></entry>
	    <entry>__u32</entry>
	    <entry><structfield>pixelformat</structfield></entry>
	    <entry>The pixel format of the
framebuffer.</entry>
	  </row>
	  <row>
	    <entry></entry>
	    <entry></entry>
	    <entry></entry>
	    <entry>For <wordasword>non-destructive Video
Overlays</wordasword> this field only defines a format for the
&v4l2-window; <structfield>chromakey</structfield> field.</entry>
	  </row>
	  <row>
	    <entry></entry>
	    <entry></entry>
	    <entry></entry>
	    <entry>For <wordasword>destructive Video
Overlays</wordasword> applications must initialize this field. For
<wordasword>Video Output Overlays</wordasword> the driver must return
a valid format.</entry>
	  </row>
	  <row>
	    <entry></entry>
	    <entry></entry>
	    <entry></entry>
	    <entry>Usually this is an RGB format (for example
<link linkend="V4L2-PIX-FMT-RGB565"><constant>V4L2_PIX_FMT_RGB565</constant></link>)
but YUV formats (only packed YUV formats when chroma keying is used,
not including <constant>V4L2_PIX_FMT_YUYV</constant> and
<constant>V4L2_PIX_FMT_UYVY</constant>) and the
<constant>V4L2_PIX_FMT_PAL8</constant> format are also permitted. The
behavior of the driver when an application requests a compressed
format is undefined. See <xref linkend="pixfmt" /> for information on
pixel formats.</entry>
	  </row>
	  <row>
	    <entry></entry>
	    <entry>&v4l2-field;</entry>
	    <entry><structfield>field</structfield></entry>
	    <entry>Drivers and applications shall ignore this field.
If applicable, the field order is selected with the &VIDIOC-S-FMT;
ioctl, using the <structfield>field</structfield> field of
&v4l2-window;.</entry>
	  </row>
	  <row>
	    <entry></entry>
	    <entry>__u32</entry>
	    <entry><structfield>bytesperline</structfield></entry>
	    <entry>Distance in bytes between the leftmost pixels in
two adjacent lines.</entry>
	  </row>
	  <row>
	    <entry spanname="hspan"><para>This field is irrelevant to
<wordasword>non-destructive Video
Overlays</wordasword>.</para><para>For <wordasword>destructive Video
Overlays</wordasword> both applications and drivers can set this field
to request padding bytes at the end of each line. Drivers however may
ignore the requested value, returning <structfield>width</structfield>
times bytes-per-pixel or a larger value required by the hardware. That
implies applications can just set this field to zero to get a
reasonable default.</para><para>For <wordasword>Video Output
Overlays</wordasword> the driver must return a valid
value.</para><para>Video hardware may access padding bytes, therefore
they must reside in accessible memory. Consider for example the case
where padding bytes after the last line of an image cross a system
page boundary. Capture devices may write padding bytes, the value is
undefined. Output devices ignore the contents of padding
bytes.</para><para>When the image format is planar the
<structfield>bytesperline</structfield> value applies to the largest
plane and is divided by the same factor as the
<structfield>width</structfield> field for any smaller planes. For
example the Cb and Cr planes of a YUV 4:2:0 image have half as many
padding bytes following each line as the Y plane. To avoid ambiguities
drivers must return a <structfield>bytesperline</structfield> value
rounded up to a multiple of the scale factor.</para></entry>
	  </row>
	  <row>
	    <entry></entry>
	    <entry>__u32</entry>
	    <entry><structfield>sizeimage</structfield></entry>
	    <entry><para>This field is irrelevant to
<wordasword>non-destructive Video Overlays</wordasword>. For
<wordasword>destructive Video Overlays</wordasword> applications must
initialize this field. For <wordasword>Video Output
Overlays</wordasword> the driver must return a valid
format.</para><para>Together with <structfield>base</structfield> it
defines the framebuffer memory accessible by the
driver.</para></entry>
	  </row>
	  <row>
	    <entry></entry>
	    <entry>&v4l2-colorspace;</entry>
	    <entry><structfield>colorspace</structfield></entry>
	    <entry>This information supplements the
<structfield>pixelformat</structfield> and must be set by the driver,
see <xref linkend="colorspaces" />.</entry>
	  </row>
	  <row>
	    <entry></entry>
	    <entry>__u32</entry>
	    <entry><structfield>priv</structfield></entry>
	    <entry>Reserved for additional information about custom
(driver defined) formats. When not used drivers and applications must
set this field to zero.</entry>
	  </row>
	</tbody>
      </tgroup>
    </table>

    <table pgwide="1" frame="none" id="framebuffer-cap">
      <title>Frame Buffer Capability Flags</title>
      <tgroup cols="3">
	&cs-def;
	<tbody valign="top">
	  <row>
	    <entry><constant>V4L2_FBUF_CAP_EXTERNOVERLAY</constant></entry>
	    <entry>0x0001</entry>
	    <entry>The device is capable of non-destructive overlays.
When the driver clears this flag, only destructive overlays are
supported. There are no drivers yet which support both destructive and
non-destructive overlays.</entry>
	  </row>
	  <row>
	    <entry><constant>V4L2_FBUF_CAP_CHROMAKEY</constant></entry>
	    <entry>0x0002</entry>
	    <entry>The device supports clipping by chroma-keying the
images. That is, image pixels replace pixels in the VGA or video
signal only where the latter assume a certain color. Chroma-keying
makes no sense for destructive overlays.</entry>
	  </row>
	  <row>
	    <entry><constant>V4L2_FBUF_CAP_LIST_CLIPPING</constant></entry>
	    <entry>0x0004</entry>
	    <entry>The device supports clipping using a list of clip
rectangles.</entry>
	  </row>
	  <row>
	    <entry><constant>V4L2_FBUF_CAP_BITMAP_CLIPPING</constant></entry>
	    <entry>0x0008</entry>
	    <entry>The device supports clipping using a bit mask.</entry>
	  </row>
	  <row>
	    <entry><constant>V4L2_FBUF_CAP_LOCAL_ALPHA</constant></entry>
	    <entry>0x0010</entry>
	    <entry>The device supports clipping/blending using the
alpha channel of the framebuffer or VGA signal. Alpha blending makes
no sense for destructive overlays.</entry>
	  </row>
	  <row>
	    <entry><constant>V4L2_FBUF_CAP_GLOBAL_ALPHA</constant></entry>
	    <entry>0x0020</entry>
	    <entry>The device supports alpha blending using a global
alpha value. Alpha blending makes no sense for destructive overlays.</entry>
	  </row>
	  <row>
	    <entry><constant>V4L2_FBUF_CAP_LOCAL_INV_ALPHA</constant></entry>
	    <entry>0x0040</entry>
	    <entry>The device supports clipping/blending using the
inverted alpha channel of the framebuffer or VGA signal. Alpha
blending makes no sense for destructive overlays.</entry>
	  </row>
	</tbody>
      </tgroup>
    </table>

    <table pgwide="1" frame="none" id="framebuffer-flags">
      <title>Frame Buffer Flags</title>
      <tgroup cols="3">
	&cs-def;
	<tbody valign="top">
	  <row>
	    <entry><constant>V4L2_FBUF_FLAG_PRIMARY</constant></entry>
	    <entry>0x0001</entry>
	    <entry>The framebuffer is the primary graphics surface.
In other words, the overlay is destructive. [?]</entry>
	  </row>
	  <row>
	    <entry><constant>V4L2_FBUF_FLAG_OVERLAY</constant></entry>
	    <entry>0x0002</entry>
	    <entry>The frame buffer is an overlay surface the same
size as the capture. [?]</entry>
	  </row>
	  <row>
	    <entry spanname="hspan">The purpose of
<constant>V4L2_FBUF_FLAG_PRIMARY</constant> and
<constant>V4L2_FBUF_FLAG_OVERLAY</constant> was never quite clear.
Most drivers seem to ignore these flags. For compatibility with the
<wordasword>bttv</wordasword> driver applications should set the
<constant>V4L2_FBUF_FLAG_OVERLAY</constant> flag.</entry>
	  </row>
	  <row>
	    <entry><constant>V4L2_FBUF_FLAG_CHROMAKEY</constant></entry>
	    <entry>0x0004</entry>
	    <entry>Use chroma-keying. The chroma-key color is
determined by the <structfield>chromakey</structfield> field of
&v4l2-window; and negotiated with the &VIDIOC-S-FMT; ioctl, see <xref
		linkend="overlay" />
and
	    <xref linkend="osd" />.</entry>
	  </row>
	  <row>
	    <entry spanname="hspan">There are no flags to enable
clipping using a list of clip rectangles or a bitmap. These methods
are negotiated with the &VIDIOC-S-FMT; ioctl, see <xref
		linkend="overlay" /> and <xref linkend="osd" />.</entry>
	  </row>
	  <row>
	    <entry><constant>V4L2_FBUF_FLAG_LOCAL_ALPHA</constant></entry>
	    <entry>0x0008</entry>
	    <entry>Use the alpha channel of the framebuffer to clip or
blend framebuffer pixels with video images. The blend
function is: output = framebuffer pixel * alpha + video pixel * (1 -
alpha). The actual alpha depth depends on the framebuffer pixel
format.</entry>
	  </row>
	  <row>
	    <entry><constant>V4L2_FBUF_FLAG_GLOBAL_ALPHA</constant></entry>
	    <entry>0x0010</entry>
	    <entry>Use a global alpha value to blend the framebuffer
with video images. The blend function is: output = (framebuffer pixel
* alpha + video pixel * (255 - alpha)) / 255. The alpha value is
determined by the <structfield>global_alpha</structfield> field of
&v4l2-window; and negotiated with the &VIDIOC-S-FMT; ioctl, see <xref
		linkend="overlay" />
and <xref linkend="osd" />.</entry>
	  </row>
	  <row>
	    <entry><constant>V4L2_FBUF_FLAG_LOCAL_INV_ALPHA</constant></entry>
	    <entry>0x0020</entry>
	    <entry>Like
<constant>V4L2_FBUF_FLAG_LOCAL_ALPHA</constant>, use the alpha channel
of the framebuffer to clip or blend framebuffer pixels with video
images, but with an inverted alpha value. The blend function is:
output = framebuffer pixel * (1 - alpha) + video pixel * alpha. The
actual alpha depth depends on the framebuffer pixel format.</entry>
	  </row>
	</tbody>
      </tgroup>
    </table>
  </refsect1>

  <refsect1>
    &return-value;

    <variablelist>
      <varlistentry>
	<term><errorcode>EPERM</errorcode></term>
	<listitem>
	  <para><constant>VIDIOC_S_FBUF</constant> can only be called
by a privileged user to negotiate the parameters for a destructive
overlay.</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><errorcode>EBUSY</errorcode></term>
	<listitem>
	  <para>The framebuffer parameters cannot be changed at this
time because overlay is already enabled, or capturing is enabled
and the hardware cannot capture and overlay simultaneously.</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><errorcode>EINVAL</errorcode></term>
	<listitem>
	  <para>The ioctl is not supported or the
<constant>VIDIOC_S_FBUF</constant> parameters are unsuitable.</para>
	</listitem>
      </varlistentry>
    </variablelist>
  </refsect1>
</refentry>

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