1<html><head><meta http-equiv="Content-Type" content="text/html; charset=ANSI_X3.4-1968"><title>Video Output Interface</title><meta name="generator" content="DocBook XSL Stylesheets V1.78.1"><link rel="home" href="index.html" title="LINUX MEDIA INFRASTRUCTURE API"><link rel="up" href="devices.html" title="Chapter 4. Interfaces"><link rel="prev" href="overlay.html" title="Video Overlay Interface"><link rel="next" href="osd.html" title="Video Output Overlay Interface"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Video Output Interface</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="overlay.html">Prev</a> </td><th width="60%" align="center">Chapter 4. Interfaces</th><td width="20%" align="right"> <a accesskey="n" href="osd.html">Next</a></td></tr></table><hr></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="output"></a>Video Output Interface</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="section"><a href="output.html#idp1105304604">Querying Capabilities</a></span></dt><dt><span class="section"><a href="output.html#idp1105350052">Supplemental Functions</a></span></dt><dt><span class="section"><a href="output.html#idp1105353692">Image Format Negotiation</a></span></dt><dt><span class="section"><a href="output.html#idp1105366780">Writing Images</a></span></dt></dl></div><p>Video output devices encode stills or image sequences as 2analog video signal. With this interface applications can 3control the encoding process and move images from user space to 4the driver.</p><p>Conventionally V4L2 video output devices are accessed through 5character device special files named <code class="filename">/dev/video</code> 6and <code class="filename">/dev/video0</code> to 7<code class="filename">/dev/video63</code> with major number 81 and minor 8numbers 0 to 63. <code class="filename">/dev/video</code> is typically a 9symbolic link to the preferred video device. Note the same device 10files are used for video capture devices.</p><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="idp1105304604"></a>Querying Capabilities</h3></div></div></div><p>Devices supporting the video output interface set the 11<code class="constant">V4L2_CAP_VIDEO_OUTPUT</code> or 12<code class="constant">V4L2_CAP_VIDEO_OUTPUT_MPLANE</code> flag in the 13<em class="structfield"><code>capabilities</code></em> field of struct <a class="link" href="vidioc-querycap.html#v4l2-capability" title="Table A.93. struct v4l2_capability">v4l2_capability</a> 14returned by the <a class="link" href="vidioc-querycap.html" title="ioctl VIDIOC_QUERYCAP"><code class="constant">VIDIOC_QUERYCAP</code></a> ioctl. As secondary device functions 15they may also support the <a class="link" href="raw-vbi.html" title="Raw VBI Data Interface">raw VBI 16output</a> (<code class="constant">V4L2_CAP_VBI_OUTPUT</code>) interface. At 17least one of the read/write or streaming I/O methods must be 18supported. Modulators and audio outputs are optional.</p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="idp1105350052"></a>Supplemental Functions</h3></div></div></div><p>Video output devices shall support <a class="link" href="audio.html" title="Audio Inputs and Outputs">audio output</a>, <a class="link" href="tuner.html" title="Tuners and Modulators">modulator</a>, <a class="link" href="control.html" title="User Controls">controls</a>, 19<a class="link" href="crop.html" title="Image Cropping, Insertion and Scaling">cropping and scaling</a> and <a class="link" href="streaming-par.html" title="Streaming Parameters">streaming parameter</a> ioctls as needed. 20The <a class="link" href="video.html" title="Video Inputs and Outputs">video output</a> and <a class="link" href="standard.html" title="Video Standards">video standard</a> ioctls must be supported by 21all video output devices.</p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="idp1105353692"></a>Image Format Negotiation</h3></div></div></div><p>The output is determined by cropping and image format 22parameters. The former select an area of the video picture where the 23image will appear, the latter how images are stored in memory, i. e. in 24RGB or YUV format, the number of bits per pixel or width and height. 25Together they also define how images are scaled in the process.</p><p>As usual these parameters are <span class="emphasis"><em>not</em></span> reset 26at <a class="link" href="func-open.html" title="V4L2 open()"><code class="function">open()</code></a> time to permit Unix tool chains, programming a device 27and then writing to it as if it was a plain file. Well written V4L2 28applications ensure they really get what they want, including cropping 29and scaling.</p><p>Cropping initialization at minimum requires to reset the 30parameters to defaults. An example is given in <a class="xref" href="crop.html" title="Image Cropping, Insertion and Scaling">the section called “Image Cropping, Insertion and Scaling”</a>.</p><p>To query the current image format applications set the 31<em class="structfield"><code>type</code></em> field of a struct <a class="link" href="vidioc-g-fmt.html#v4l2-format" title="Table A.72. struct v4l2_format">v4l2_format</a> to 32<code class="constant">V4L2_BUF_TYPE_VIDEO_OUTPUT</code> or 33<code class="constant">V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE</code> and call the 34<a class="link" href="vidioc-g-fmt.html" title="ioctl VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT"><code class="constant">VIDIOC_G_FMT</code></a> ioctl with a pointer to this structure. Drivers fill 35the struct <a class="link" href="pixfmt.html#v4l2-pix-format" title="Table 2.1. struct v4l2_pix_format">v4l2_pix_format</a> <em class="structfield"><code>pix</code></em> or the 36struct <a class="link" href="ch02s02.html#v4l2-pix-format-mplane" title="Table 2.3. struct v4l2_pix_format_mplane">v4l2_pix_format_mplane</a> <em class="structfield"><code>pix_mp</code></em> member of the 37<em class="structfield"><code>fmt</code></em> union.</p><p>To request different parameters applications set the 38<em class="structfield"><code>type</code></em> field of a struct <a class="link" href="vidioc-g-fmt.html#v4l2-format" title="Table A.72. struct v4l2_format">v4l2_format</a> as above and 39initialize all fields of the struct <a class="link" href="pixfmt.html#v4l2-pix-format" title="Table 2.1. struct v4l2_pix_format">v4l2_pix_format</a> 40<em class="structfield"><code>vbi</code></em> member of the 41<em class="structfield"><code>fmt</code></em> union, or better just modify the 42results of <code class="constant">VIDIOC_G_FMT</code>, and call the 43<a class="link" href="vidioc-g-fmt.html" title="ioctl VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT"><code class="constant">VIDIOC_S_FMT</code></a> ioctl with a pointer to this structure. Drivers may 44adjust the parameters and finally return the actual parameters as 45<code class="constant">VIDIOC_G_FMT</code> does.</p><p>Like <code class="constant">VIDIOC_S_FMT</code> the 46<a class="link" href="vidioc-g-fmt.html" title="ioctl VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT"><code class="constant">VIDIOC_TRY_FMT</code></a> ioctl can be used to learn about hardware limitations 47without disabling I/O or possibly time consuming hardware 48preparations.</p><p>The contents of struct <a class="link" href="pixfmt.html#v4l2-pix-format" title="Table 2.1. struct v4l2_pix_format">v4l2_pix_format</a> and struct <a class="link" href="ch02s02.html#v4l2-pix-format-mplane" title="Table 2.3. struct v4l2_pix_format_mplane">v4l2_pix_format_mplane</a> 49are discussed in <a class="xref" href="pixfmt.html" title="Chapter 2. Image Formats">Chapter 2, <i>Image Formats</i></a>. See also the specification of the 50<code class="constant">VIDIOC_G_FMT</code>, <code class="constant">VIDIOC_S_FMT</code> 51and <code class="constant">VIDIOC_TRY_FMT</code> ioctls for details. Video 52output devices must implement both the 53<code class="constant">VIDIOC_G_FMT</code> and 54<code class="constant">VIDIOC_S_FMT</code> ioctl, even if 55<code class="constant">VIDIOC_S_FMT</code> ignores all requests and always 56returns default parameters as <code class="constant">VIDIOC_G_FMT</code> does. 57<code class="constant">VIDIOC_TRY_FMT</code> is optional.</p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="idp1105366780"></a>Writing Images</h3></div></div></div><p>A video output device may support the <a class="link" href="io.html#rw" title="Read/Write">write() function</a> and/or streaming (<a class="link" href="mmap.html" title="Streaming I/O (Memory Mapping)">memory mapping</a> or <a class="link" href="userp.html" title="Streaming I/O (User Pointers)">user pointer</a>) I/O. See <a class="xref" href="io.html" title="Chapter 3. Input/Output">Chapter 3, <i>Input/Output</i></a> for details.</p></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="overlay.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="devices.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="osd.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Video Overlay Interface </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Video Output Overlay Interface</td></tr></table></div></body></html> 58