<?xml version="1.0" encoding="ANSI_X3.4-1968" standalone="no"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=ANSI_X3.4-1968" /><title>Field Order</title><meta name="generator" content="DocBook XSL Stylesheets V1.75.2" /><link rel="home" href="index.html" title="LINUX MEDIA INFRASTRUCTURE API" /><link rel="up" href="ch03.html" title="Chapter 3. Input/Output" /><link rel="prev" href="ch03s05.html" title="Buffers" /><link rel="next" href="ch04.html" title="Chapter 4. Interfaces" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Field Order</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch03s05.html">Prev</a> </td><th width="60%" align="center">Chapter 3. Input/Output</th><td width="20%" align="right"> <a accesskey="n" href="ch04.html">Next</a></td></tr></table><hr /></div><div class="section" title="Field Order"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="field-order"></a>Field Order</h2></div></div></div><p>We have to distinguish between progressive and interlaced video. Progressive video transmits all lines of a video image sequentially. Interlaced video divides an image into two fields, containing only the odd and even lines of the image, respectively. Alternating the so called odd and even field are transmitted, and due to a small delay between fields a cathode ray TV displays the lines interleaved, yielding the original frame. This curious technique was invented because at refresh rates similar to film the image would fade out too quickly. Transmitting fields reduces the flicker without the necessity of doubling the frame rate and with it the bandwidth required for each channel.</p><p>It is important to understand a video camera does not expose one frame at a time, merely transmitting the frames separated into fields. The fields are in fact captured at two different instances in time. An object on screen may well move between one field and the next. For applications analysing motion it is of paramount importance to recognize which field of a frame is older, the <span class="emphasis"><em>temporal order</em></span>.</p><p>When the driver provides or accepts images field by field rather than interleaved, it is also important applications understand how the fields combine to frames. We distinguish between top and bottom fields, the <span class="emphasis"><em>spatial order</em></span>: The first line of the top field is the first line of an interlaced frame, the first line of the bottom field is the second line of that frame.</p><p>However because fields were captured one after the other, arguing whether a frame commences with the top or bottom field is pointless. Any two successive top and bottom, or bottom and top fields yield a valid frame. Only when the source was progressive to begin with, e. g. when transferring film to video, two fields may come from the same frame, creating a natural order.</p><p>Counter to intuition the top field is not necessarily the older field. Whether the older field contains the top or bottom lines is a convention determined by the video standard. Hence the distinction between temporal and spatial order of fields. The diagrams below should make this clearer.</p><p>All video capture and output devices must report the current field order. Some drivers may permit the selection of a different order, to this end applications initialize the <em class="structfield"><code>field</code></em> field of struct <a class="link" href="ch02.html#v4l2-pix-format" title="Table 2.1. struct v4l2_pix_format">v4l2_pix_format</a> before calling the <a class="link" href="re41.html" title="ioctl VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT"><code class="constant">VIDIOC_S_FMT</code></a> ioctl. If this is not desired it should have the value <code class="constant">V4L2_FIELD_ANY</code> (0).</p><div class="table"><a id="v4l2-field"></a><p class="title"><b>Table 3.8. enum v4l2_field</b></p><div class="table-contents"><table summary="enum v4l2_field" width="100%" border="0"><colgroup><col /><col /><col /></colgroup><tbody valign="top"><tr><td valign="top"><code class="constant">V4L2_FIELD_ANY</code></td><td valign="top">0</td><td valign="top">Applications request this field order when any one of the <code class="constant">V4L2_FIELD_NONE</code>, <code class="constant">V4L2_FIELD_TOP</code>, <code class="constant">V4L2_FIELD_BOTTOM</code>, or <code class="constant">V4L2_FIELD_INTERLACED</code> formats is acceptable. Drivers choose depending on hardware capabilities or e. g. the requested image size, and return the actual field order. struct <a class="link" href="ch03s05.html#v4l2-buffer" title="Table 3.1. struct v4l2_buffer">v4l2_buffer</a> <em class="structfield"><code>field</code></em> can never be <code class="constant">V4L2_FIELD_ANY</code>.</td></tr><tr><td valign="top"><code class="constant">V4L2_FIELD_NONE</code></td><td valign="top">1</td><td valign="top">Images are in progressive format, not interlaced. The driver may also indicate this order when it cannot distinguish between <code class="constant">V4L2_FIELD_TOP</code> and <code class="constant">V4L2_FIELD_BOTTOM</code>.</td></tr><tr><td valign="top"><code class="constant">V4L2_FIELD_TOP</code></td><td valign="top">2</td><td valign="top">Images consist of the top field only.</td></tr><tr><td valign="top"><code class="constant">V4L2_FIELD_BOTTOM</code></td><td valign="top">3</td><td valign="top">Images consist of the bottom field only. Applications may wish to prevent a device from capturing interlaced images because they will have "comb" or "feathering" artefacts around moving objects.</td></tr><tr><td valign="top"><code class="constant">V4L2_FIELD_INTERLACED</code></td><td valign="top">4</td><td valign="top">Images contain both fields, interleaved line by line. The temporal order of the fields (whether the top or bottom field is first transmitted) depends on the current video standard. M/NTSC transmits the bottom field first, all other standards the top field first.</td></tr><tr><td valign="top"><code class="constant">V4L2_FIELD_SEQ_TB</code></td><td valign="top">5</td><td valign="top">Images contain both fields, the top field lines are stored first in memory, immediately followed by the bottom field lines. Fields are always stored in temporal order, the older one first in memory. Image sizes refer to the frame, not fields.</td></tr><tr><td valign="top"><code class="constant">V4L2_FIELD_SEQ_BT</code></td><td valign="top">6</td><td valign="top">Images contain both fields, the bottom field lines are stored first in memory, immediately followed by the top field lines. Fields are always stored in temporal order, the older one first in memory. Image sizes refer to the frame, not fields.</td></tr><tr><td valign="top"><code class="constant">V4L2_FIELD_ALTERNATE</code></td><td valign="top">7</td><td valign="top">The two fields of a frame are passed in separate buffers, in temporal order, i. e. the older one first. To indicate the field parity (whether the current field is a top or bottom field) the driver or application, depending on data direction, must set struct <a class="link" href="ch03s05.html#v4l2-buffer" title="Table 3.1. struct v4l2_buffer">v4l2_buffer</a> <em class="structfield"><code>field</code></em> to <code class="constant">V4L2_FIELD_TOP</code> or <code class="constant">V4L2_FIELD_BOTTOM</code>. Any two successive fields pair to build a frame. If fields are successive, without any dropped fields between them (fields can drop individually), can be determined from the struct <a class="link" href="ch03s05.html#v4l2-buffer" title="Table 3.1. struct v4l2_buffer">v4l2_buffer</a> <em class="structfield"><code>sequence</code></em> field. Image sizes refer to the frame, not fields. This format cannot be selected when using the read/write I/O method.</td></tr><tr><td valign="top"><code class="constant">V4L2_FIELD_INTERLACED_TB</code></td><td valign="top">8</td><td valign="top">Images contain both fields, interleaved line by line, top field first. The top field is transmitted first.</td></tr><tr><td valign="top"><code class="constant">V4L2_FIELD_INTERLACED_BT</code></td><td valign="top">9</td><td valign="top">Images contain both fields, interleaved line by line, top field first. The bottom field is transmitted first.</td></tr></tbody></table></div></div><br class="table-break" /><div class="figure"><a id="fieldseq-tb"></a><p class="title"><b>Figure 3.1. Field Order, Top Field First Transmitted</b></p><div class="figure-contents"><div class="mediaobject"><img src="fieldseq_tb.gif" alt="Field Order, Top Field First Transmitted" /></div></div></div><br class="figure-break" /><div class="figure"><a id="fieldseq-bt"></a><p class="title"><b>Figure 3.2. Field Order, Bottom Field First Transmitted</b></p><div class="figure-contents"><div class="mediaobject"><img src="fieldseq_bt.gif" alt="Field Order, Bottom Field First Transmitted" /></div></div></div><br class="figure-break" /></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ch03s05.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ch03.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ch04.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Buffers </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 4. Interfaces</td></tr></table></div></body></html>
