<?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>Experimental API for cropping, composing and scaling</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="common.html" title="Chapter 1. Common API Elements" /><link rel="prev" href="crop.html" title="Image Cropping, Insertion and Scaling" /><link rel="next" href="streaming-par.html" title="Streaming Parameters" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Experimental API for cropping, composing and scaling</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="crop.html">Prev</a> </td><th width="60%" align="center">Chapter 1. Common API Elements</th><td width="20%" align="right"> <a accesskey="n" href="streaming-par.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="selection-api"></a>Experimental API for cropping, composing and scaling</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="section"><a href="selection-api.html#idm140470032568384">Introduction</a></span></dt><dt><span class="section"><a href="selection-api.html#idm140470032520512">Selection targets</a></span></dt><dt><span class="section"><a href="selection-api.html#idm140470032514272">Configuration</a></span></dt><dt><span class="section"><a href="selection-api.html#idm140470032483376">Comparison with old cropping API</a></span></dt><dt><span class="section"><a href="selection-api.html#idm140470032468064">Examples</a></span></dt></dl></div><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Experimental</h3><p>This is an <a class="link" href="hist-v4l2.html#experimental" title="Experimental API Elements">experimental</a> interface and may change in the future.</p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="idm140470032568384"></a>Introduction</h3></div></div></div><p>Some video capture devices can sample a subsection of a picture and shrink or enlarge it to an image of arbitrary size. Next, the devices can insert the image into larger one. Some video output devices can crop part of an input image, scale it up or down and insert it at an arbitrary scan line and horizontal offset into a video signal. We call these abilities cropping, scaling and composing.</p><p>On a video <span class="emphasis"><em>capture</em></span> device the source is a video signal, and the cropping target determine the area actually sampled. The sink is an image stored in a memory buffer. The composing area specifies which part of the buffer is actually written to by the hardware. </p><p>On a video <span class="emphasis"><em>output</em></span> device the source is an image in a memory buffer, and the cropping target is a part of an image to be shown on a display. The sink is the display or the graphics screen. The application may select the part of display where the image should be displayed. The size and position of such a window is controlled by the compose target.</p><p>Rectangles for all cropping and composing targets are defined even if the device does supports neither cropping nor composing. Their size and position will be fixed in such a case. If the device does not support scaling then the cropping and composing rectangles have the same size.</p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="idm140470032520512"></a>Selection targets</h3></div></div></div><p> </p><div class="figure"><a id="sel-targets-capture"></a><p class="title"><strong>Figure 1.2. Cropping and composing targets</strong></p><div class="figure-contents"><div class="mediaobject"><img src="selection.png" alt="Targets used by a cropping, composing and scaling process" /></div></div></div><p><br class="figure-break" /> </p><p>See <a class="xref" href="apb.html#v4l2-selection-targets" title="Selection targets">the section called “Selection targets”</a> for more information.</p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="idm140470032514272"></a>Configuration</h3></div></div></div><div class="toc"><dl class="toc"><dt><span class="section"><a href="selection-api.html#idm140470032510208">Configuration of video capture</a></span></dt><dt><span class="section"><a href="selection-api.html#idm140470032496608">Configuration of video output</a></span></dt><dt><span class="section"><a href="selection-api.html#idm140470032485872">Scaling control</a></span></dt></dl></div><p>Applications can use the <a class="link" href="vidioc-g-selection.html" title="ioctl VIDIOC_G_SELECTION, VIDIOC_S_SELECTION">selection API</a> to select an area in a video signal or a buffer, and to query for default settings and hardware limits.</p><p>Video hardware can have various cropping, composing and scaling limitations. It may only scale up or down, support only discrete scaling factors, or have different scaling abilities in the horizontal and vertical directions. Also it may not support scaling at all. At the same time the cropping/composing rectangles may have to be aligned, and both the source and the sink may have arbitrary upper and lower size limits. Therefore, as usual, drivers are expected to adjust the requested parameters and return the actual values selected. An application can control the rounding behaviour using <a class="link" href="apb.html#v4l2-selection-flags" title="Selection flags"> constraint flags </a>.</p><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="idm140470032510208"></a>Configuration of video capture</h4></div></div></div><p>See figure <a class="xref" href="selection-api.html#sel-targets-capture" title="Figure 1.2. Cropping and composing targets">Figure 1.2, “Cropping and composing targets”</a> for examples of the selection targets available for a video capture device. It is recommended to configure the cropping targets before to the composing targets.</p><p>The range of coordinates of the top left corner, width and height of areas that can be sampled is given by the <code class="constant"> V4L2_SEL_TGT_CROP_BOUNDS </code> target. It is recommended for the driver developers to put the top/left corner at position <code class="constant"> (0,0) </code>. The rectangle's coordinates are expressed in pixels.</p><p>The top left corner, width and height of the source rectangle, that is the area actually sampled, is given by the <code class="constant"> V4L2_SEL_TGT_CROP </code> target. It uses the same coordinate system as <code class="constant"> V4L2_SEL_TGT_CROP_BOUNDS </code>. The active cropping area must lie completely inside the capture boundaries. The driver may further adjust the requested size and/or position according to hardware limitations.</p><p>Each capture device has a default source rectangle, given by the <code class="constant"> V4L2_SEL_TGT_CROP_DEFAULT </code> target. This rectangle shall over what the driver writer considers the complete picture. Drivers shall set the active crop rectangle to the default when the driver is first loaded, but not later.</p><p>The composing targets refer to a memory buffer. The limits of composing coordinates are obtained using <code class="constant"> V4L2_SEL_TGT_COMPOSE_BOUNDS </code>. All coordinates are expressed in pixels. The rectangle's top/left corner must be located at position <code class="constant"> (0,0) </code>. The width and height are equal to the image size set by <code class="constant"> VIDIOC_S_FMT </code>. </p><p>The part of a buffer into which the image is inserted by the hardware is controlled by the <code class="constant"> V4L2_SEL_TGT_COMPOSE </code> target. The rectangle's coordinates are also expressed in the same coordinate system as the bounds rectangle. The composing rectangle must lie completely inside bounds rectangle. The driver must adjust the composing rectangle to fit to the bounding limits. Moreover, the driver can perform other adjustments according to hardware limitations. The application can control rounding behaviour using <a class="link" href="apb.html#v4l2-selection-flags" title="Selection flags"> constraint flags </a>.</p><p>For capture devices the default composing rectangle is queried using <code class="constant"> V4L2_SEL_TGT_COMPOSE_DEFAULT </code>. It is usually equal to the bounding rectangle.</p><p>The part of a buffer that is modified by the hardware is given by <code class="constant"> V4L2_SEL_TGT_COMPOSE_PADDED </code>. It contains all pixels defined using <code class="constant"> V4L2_SEL_TGT_COMPOSE </code> plus all padding data modified by hardware during insertion process. All pixels outside this rectangle <span class="emphasis"><em>must not</em></span> be changed by the hardware. The content of pixels that lie inside the padded area but outside active area is undefined. The application can use the padded and active rectangles to detect where the rubbish pixels are located and remove them if needed.</p></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="idm140470032496608"></a>Configuration of video output</h4></div></div></div><p>For output devices targets and ioctls are used similarly to the video capture case. The <span class="emphasis"><em> composing </em></span> rectangle refers to the insertion of an image into a video signal. The cropping rectangles refer to a memory buffer. It is recommended to configure the composing targets before to the cropping targets.</p><p>The cropping targets refer to the memory buffer that contains an image to be inserted into a video signal or graphical screen. The limits of cropping coordinates are obtained using <code class="constant"> V4L2_SEL_TGT_CROP_BOUNDS </code>. All coordinates are expressed in pixels. The top/left corner is always point <code class="constant"> (0,0) </code>. The width and height is equal to the image size specified using <code class="constant"> VIDIOC_S_FMT </code> ioctl.</p><p>The top left corner, width and height of the source rectangle, that is the area from which image date are processed by the hardware, is given by the <code class="constant"> V4L2_SEL_TGT_CROP </code>. Its coordinates are expressed in in the same coordinate system as the bounds rectangle. The active cropping area must lie completely inside the crop boundaries and the driver may further adjust the requested size and/or position according to hardware limitations.</p><p>For output devices the default cropping rectangle is queried using <code class="constant"> V4L2_SEL_TGT_CROP_DEFAULT </code>. It is usually equal to the bounding rectangle.</p><p>The part of a video signal or graphics display where the image is inserted by the hardware is controlled by <code class="constant"> V4L2_SEL_TGT_COMPOSE </code> target. The rectangle's coordinates are expressed in pixels. The composing rectangle must lie completely inside the bounds rectangle. The driver must adjust the area to fit to the bounding limits. Moreover, the driver can perform other adjustments according to hardware limitations. </p><p>The device has a default composing rectangle, given by the <code class="constant"> V4L2_SEL_TGT_COMPOSE_DEFAULT </code> target. This rectangle shall cover what the driver writer considers the complete picture. It is recommended for the driver developers to put the top/left corner at position <code class="constant"> (0,0) </code>. Drivers shall set the active composing rectangle to the default one when the driver is first loaded.</p><p>The devices may introduce additional content to video signal other than an image from memory buffers. It includes borders around an image. However, such a padded area is driver-dependent feature not covered by this document. Driver developers are encouraged to keep padded rectangle equal to active one. The padded target is accessed by the <code class="constant"> V4L2_SEL_TGT_COMPOSE_PADDED </code> identifier. It must contain all pixels from the <code class="constant"> V4L2_SEL_TGT_COMPOSE </code> target.</p></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="idm140470032485872"></a>Scaling control</h4></div></div></div><p>An application can detect if scaling is performed by comparing the width and the height of rectangles obtained using <code class="constant"> V4L2_SEL_TGT_CROP </code> and <code class="constant"> V4L2_SEL_TGT_COMPOSE </code> targets. If these are not equal then the scaling is applied. The application can compute the scaling ratios using these values.</p></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="idm140470032483376"></a>Comparison with old cropping API</h3></div></div></div><p>The selection API was introduced to cope with deficiencies of previous <a class="link" href="crop.html" title="Image Cropping, Insertion and Scaling"> API </a>, that was designed to control simple capture devices. Later the cropping API was adopted by video output drivers. The ioctls are used to select a part of the display were the video signal is inserted. It should be considered as an API abuse because the described operation is actually the composing. The selection API makes a clear distinction between composing and cropping operations by setting the appropriate targets. The V4L2 API lacks any support for composing to and cropping from an image inside a memory buffer. The application could configure a capture device to fill only a part of an image by abusing V4L2 API. Cropping a smaller image from a larger one is achieved by setting the field 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>::bytesperline</code></em>. Introducing an image offsets could be done by modifying field struct <a class="link" href="buffer.html#v4l2-buffer" title="Table 3.1. struct v4l2_buffer">v4l2_buffer</a><em class="structfield"><code>::m_userptr</code></em> before calling <code class="constant"> VIDIOC_QBUF </code>. Those operations should be avoided because they are not portable (endianness), and do not work for macroblock and Bayer formats and mmap buffers. The selection API deals with configuration of buffer cropping/composing in a clear, intuitive and portable way. Next, with the selection API the concepts of the padded target and constraints flags are introduced. Finally, struct <a class="link" href="vidioc-g-crop.html#v4l2-crop" title="Table A.51. struct v4l2_crop">v4l2_crop</a> and struct <a class="link" href="vidioc-cropcap.html#v4l2-cropcap" title="Table A.2. struct v4l2_cropcap">v4l2_cropcap</a> have no reserved fields. Therefore there is no way to extend their functionality. The new struct <a class="link" href="vidioc-g-selection.html#v4l2-selection" title="Table A.79. struct v4l2_selection">v4l2_selection</a> provides a lot of place for future extensions. Driver developers are encouraged to implement only selection API. The former cropping API would be simulated using the new one. </p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="idm140470032468064"></a>Examples</h3></div></div></div><div class="example"><a id="idm140470032467424"></a><p class="title"><strong>Example 1.14. Resetting the cropping parameters</strong></p><div class="example-contents"><p>(A video capture device is assumed; change <code class="constant"> V4L2_BUF_TYPE_VIDEO_CAPTURE </code> for other devices; change target to <code class="constant"> V4L2_SEL_TGT_COMPOSE_* </code> family to configure composing area)</p><pre class="programlisting"> struct <a class="link" href="vidioc-g-selection.html#v4l2-selection" title="Table A.79. struct v4l2_selection">v4l2_selection</a> sel = { .type = V4L2_BUF_TYPE_VIDEO_CAPTURE, .target = V4L2_SEL_TGT_CROP_DEFAULT, }; ret = ioctl(fd, <a class="link" href="vidioc-g-selection.html" title="ioctl VIDIOC_G_SELECTION, VIDIOC_S_SELECTION"><code class="constant">VIDIOC_G_SELECTION</code></a>, &sel); if (ret) exit(-1); sel.target = V4L2_SEL_TGT_CROP; ret = ioctl(fd, <a class="link" href="vidioc-g-selection.html" title="ioctl VIDIOC_G_SELECTION, VIDIOC_S_SELECTION"><code class="constant">VIDIOC_S_SELECTION</code></a>, &sel); if (ret) exit(-1); </pre></div></div><br class="example-break" /><div class="example"><a id="idm140470032466016"></a><p class="title"><strong>Example 1.15. Simple downscaling</strong></p><div class="example-contents"><p>Setting a composing area on output of size of <span class="emphasis"><em> at most </em></span> half of limit placed at a center of a display.</p><pre class="programlisting"> struct <a class="link" href="vidioc-g-selection.html#v4l2-selection" title="Table A.79. struct v4l2_selection">v4l2_selection</a> sel = { .type = V4L2_BUF_TYPE_VIDEO_OUTPUT, .target = V4L2_SEL_TGT_COMPOSE_BOUNDS, }; struct v4l2_rect r; ret = ioctl(fd, <a class="link" href="vidioc-g-selection.html" title="ioctl VIDIOC_G_SELECTION, VIDIOC_S_SELECTION"><code class="constant">VIDIOC_G_SELECTION</code></a>, &sel); if (ret) exit(-1); /* setting smaller compose rectangle */ r.width = sel.r.width / 2; r.height = sel.r.height / 2; r.left = sel.r.width / 4; r.top = sel.r.height / 4; sel.r = r; sel.target = V4L2_SEL_TGT_COMPOSE; sel.flags = V4L2_SEL_FLAG_LE; ret = ioctl(fd, <a class="link" href="vidioc-g-selection.html" title="ioctl VIDIOC_G_SELECTION, VIDIOC_S_SELECTION"><code class="constant">VIDIOC_S_SELECTION</code></a>, &sel); if (ret) exit(-1); </pre></div></div><br class="example-break" /><div class="example"><a id="idm140470032459856"></a><p class="title"><strong>Example 1.16. Querying for scaling factors</strong></p><div class="example-contents"><p>A video output device is assumed; change <code class="constant"> V4L2_BUF_TYPE_VIDEO_OUTPUT </code> for other devices</p><pre class="programlisting"> struct <a class="link" href="vidioc-g-selection.html#v4l2-selection" title="Table A.79. struct v4l2_selection">v4l2_selection</a> compose = { .type = V4L2_BUF_TYPE_VIDEO_OUTPUT, .target = V4L2_SEL_TGT_COMPOSE, }; struct <a class="link" href="vidioc-g-selection.html#v4l2-selection" title="Table A.79. struct v4l2_selection">v4l2_selection</a> crop = { .type = V4L2_BUF_TYPE_VIDEO_OUTPUT, .target = V4L2_SEL_TGT_CROP, }; double hscale, vscale; ret = ioctl(fd, <a class="link" href="vidioc-g-selection.html" title="ioctl VIDIOC_G_SELECTION, VIDIOC_S_SELECTION"><code class="constant">VIDIOC_G_SELECTION</code></a>, &compose); if (ret) exit(-1); ret = ioctl(fd, <a class="link" href="vidioc-g-selection.html" title="ioctl VIDIOC_G_SELECTION, VIDIOC_S_SELECTION"><code class="constant">VIDIOC_G_SELECTION</code></a>, &crop); if (ret) exit(-1); /* computing scaling factors */ hscale = (double)compose.r.width / crop.r.width; vscale = (double)compose.r.height / crop.r.height; </pre></div></div><br class="example-break" /></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="crop.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="common.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="streaming-par.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Image Cropping, Insertion and Scaling </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Streaming Parameters</td></tr></table></div></body></html>