path: root/Documentation/DocBook/media/v4l/pixfmt.xml

  <title>Image Formats</title>

  <para>The V4L2 API was primarily designed for devices exchanging
image data with applications. The
<structname>v4l2_pix_format</structname> and <structname>v4l2_pix_format_mplane
</structname> structures define the format and layout of an image in memory.
The former is used with the single-planar API, while the latter is used with the
multi-planar version (see <xref linkend="planar-apis"/>). Image formats are
negotiated with the &VIDIOC-S-FMT; ioctl. (The explanations here focus on video
capturing and output, for overlay frame buffer formats see also
&VIDIOC-G-FBUF;.)</para>

<section>
  <title>Single-planar format structure</title>
  <table pgwide="1" frame="none" id="v4l2-pix-format">
    <title>struct <structname>v4l2_pix_format</structname></title>
    <tgroup cols="3">
      &cs-str;
      <tbody valign="top">
	<row>
	  <entry>__u32</entry>
	  <entry><structfield>width</structfield></entry>
	  <entry>Image width in pixels.</entry>
	</row>
	<row>
	  <entry>__u32</entry>
	  <entry><structfield>height</structfield></entry>
	  <entry>Image height in pixels.</entry>
	</row>
	<row>
	  <entry spanname="hspan">Applications set these fields to
request an image size, drivers return the closest possible values. In
case of planar formats the <structfield>width</structfield> and
<structfield>height</structfield> applies to the largest plane. To
avoid ambiguities drivers must return values rounded up to a multiple
of the scale factor of any smaller planes. For example when the image
format is YUV 4:2:0, <structfield>width</structfield> and
<structfield>height</structfield> must be multiples of two.</entry>
	</row>
	<row>
	  <entry>__u32</entry>
	  <entry><structfield>pixelformat</structfield></entry>
	  <entry>The pixel format or type of compression, set by the
application. This is a little endian <link
linkend="v4l2-fourcc">four character code</link>. V4L2 defines
standard RGB formats in <xref linkend="rgb-formats" />, YUV formats in <xref
linkend="yuv-formats" />, and reserved codes in <xref
linkend="reserved-formats" /></entry>
	</row>
	<row>
	  <entry>&v4l2-field;</entry>
	  <entry><structfield>field</structfield></entry>
	  <entry>Video images are typically interlaced. Applications
can request to capture or output only the top or bottom field, or both
fields interlaced or sequentially stored in one buffer or alternating
in separate buffers. Drivers return the actual field order selected.
For details see <xref linkend="field-order" />.</entry>
	</row>
	<row>
	  <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>Both applications and drivers
can set this field to request padding bytes at the end of each line.
Drivers however may ignore the value requested by the application,
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>Video hardware may access padding bytes,
therefore they must reside in accessible memory. Consider cases where
padding bytes after the last line of an image cross a system page
boundary. Input 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>__u32</entry>
	  <entry><structfield>sizeimage</structfield></entry>
	  <entry>Size in bytes of the buffer to hold a complete image,
set by the driver. Usually this is
<structfield>bytesperline</structfield> times
<structfield>height</structfield>. When the image consists of variable
length compressed data this is the maximum number of bytes required to
hold an image.</entry>
	</row>
	<row>
	  <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>__u32</entry>
	  <entry><structfield>priv</structfield></entry>
	  <entry>Reserved for custom (driver defined) additional
information about formats. When not used drivers and applications must
set this field to zero.</entry>
	</row>
      </tbody>
    </tgroup>
  </table>
</section>

<section>
  <title>Multi-planar format structures</title>
  <para>The <