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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<title>convolution: VIPS Reference Manual</title>
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                  <a href="#libvips-convolution.description" class="shortcut">Description</a></span>
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<div class="refentry">
<a name="libvips-convolution"></a><div class="titlepage"></div>
<div class="refnamediv"><table width="100%"><tr>
<td valign="top">
<h2><span class="refentrytitle"><a name="libvips-convolution.top_of_page"></a>convolution</span></h2>
<p>convolution — convolve and correlate images</p>
</td>
<td class="gallery_image" valign="top" align="right"></td>
</tr></table></div>
<div class="refsect1">
<a name="libvips-convolution.stability-level"></a><h2>Stability Level</h2>
<acronym title="The intention of a Stable interface is to enable arbitrary third parties to
develop applications to these interfaces, release them, and have confidence that
they will run on all minor releases of the product (after the one in which the
interface was introduced, and within the same major release). Even at a major
release, incompatible changes are expected to be rare, and to have strong
justifications.
"><span class="acronym">Stable</span></acronym>, unless otherwise indicated
</div>
<div class="refsect1">
<a name="libvips-convolution.functions"></a><h2>Functions</h2>
<div class="informaltable"><table class="informaltable" width="100%" border="0">
<colgroup>
<col width="150px" class="functions_return">
<col class="functions_name">
</colgroup>
<tbody>
<tr>
<td class="function_type">
<span class="returnvalue">int</span>
</td>
<td class="function_name">
<a class="link" href="libvips-convolution.html#vips-conv" title="vips_conv ()">vips_conv</a> <span class="c_punctuation">()</span>
</td>
</tr>
<tr>
<td class="function_type">
<span class="returnvalue">int</span>
</td>
<td class="function_name">
<a class="link" href="libvips-convolution.html#vips-convf" title="vips_convf ()">vips_convf</a> <span class="c_punctuation">()</span>
</td>
</tr>
<tr>
<td class="function_type">
<span class="returnvalue">int</span>
</td>
<td class="function_name">
<a class="link" href="libvips-convolution.html#vips-convi" title="vips_convi ()">vips_convi</a> <span class="c_punctuation">()</span>
</td>
</tr>
<tr>
<td class="function_type">
<span class="returnvalue">int</span>
</td>
<td class="function_name">
<a class="link" href="libvips-convolution.html#vips-conva" title="vips_conva ()">vips_conva</a> <span class="c_punctuation">()</span>
</td>
</tr>
<tr>
<td class="function_type">
<span class="returnvalue">int</span>
</td>
<td class="function_name">
<a class="link" href="libvips-convolution.html#vips-convsep" title="vips_convsep ()">vips_convsep</a> <span class="c_punctuation">()</span>
</td>
</tr>
<tr>
<td class="function_type">
<span class="returnvalue">int</span>
</td>
<td class="function_name">
<a class="link" href="libvips-convolution.html#vips-convasep" title="vips_convasep ()">vips_convasep</a> <span class="c_punctuation">()</span>
</td>
</tr>
<tr>
<td class="function_type">
<span class="returnvalue">int</span>
</td>
<td class="function_name">
<a class="link" href="libvips-convolution.html#vips-compass" title="vips_compass ()">vips_compass</a> <span class="c_punctuation">()</span>
</td>
</tr>
<tr>
<td class="function_type">
<span class="returnvalue">int</span>
</td>
<td class="function_name">
<a class="link" href="libvips-convolution.html#vips-gaussblur" title="vips_gaussblur ()">vips_gaussblur</a> <span class="c_punctuation">()</span>
</td>
</tr>
<tr>
<td class="function_type">
<span class="returnvalue">int</span>
</td>
<td class="function_name">
<a class="link" href="libvips-convolution.html#vips-sharpen" title="vips_sharpen ()">vips_sharpen</a> <span class="c_punctuation">()</span>
</td>
</tr>
<tr>
<td class="function_type">
<span class="returnvalue">int</span>
</td>
<td class="function_name">
<a class="link" href="libvips-convolution.html#vips-spcor" title="vips_spcor ()">vips_spcor</a> <span class="c_punctuation">()</span>
</td>
</tr>
<tr>
<td class="function_type">
<span class="returnvalue">int</span>
</td>
<td class="function_name">
<a class="link" href="libvips-convolution.html#vips-fastcor" title="vips_fastcor ()">vips_fastcor</a> <span class="c_punctuation">()</span>
</td>
</tr>
</tbody>
</table></div>
</div>
<div class="refsect1">
<a name="libvips-convolution.other"></a><h2>Types and Values</h2>
<div class="informaltable"><table class="informaltable" width="100%" border="0">
<colgroup>
<col width="150px" class="name">
<col class="description">
</colgroup>
<tbody><tr>
<td class="datatype_keyword">enum</td>
<td class="function_name"><a class="link" href="libvips-convolution.html#VipsCombine" title="enum VipsCombine">VipsCombine</a></td>
</tr></tbody>
</table></div>
</div>
<div class="refsect1">
<a name="libvips-convolution.includes"></a><h2>Includes</h2>
<pre class="synopsis">#include &lt;vips/vips.h&gt;
</pre>
</div>
<div class="refsect1">
<a name="libvips-convolution.description"></a><h2>Description</h2>
<p>These operations convolve an image in some way, or are operations based on
simple convolution, or are useful with convolution.</p>
</div>
<div class="refsect1">
<a name="libvips-convolution.functions_details"></a><h2>Functions</h2>
<div class="refsect2">
<a name="vips-conv"></a><h3>vips_conv ()</h3>
<pre class="programlisting"><span class="returnvalue">int</span>
vips_conv (<em class="parameter"><code><a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a> *in</code></em>,
           <em class="parameter"><code><a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a> **out</code></em>,
           <em class="parameter"><code><a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a> *mask</code></em>,
           <em class="parameter"><code>...</code></em>);</pre>
<p>Optional arguments:</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p><em class="parameter"><code>precision</code></em>
: <a class="link" href="libvips-basic.html#VipsPrecision" title="enum VipsPrecision"><span class="type">VipsPrecision</span></a>, calculation accuracy</p></li>
<li class="listitem"><p><em class="parameter"><code>layers</code></em>
: <code class="literal">gint</code>, number of layers for approximation</p></li>
<li class="listitem"><p><em class="parameter"><code>cluster</code></em>
: <code class="literal">gint</code>, cluster lines closer than this distance</p></li>
</ul></div>
<p>Convolution. </p>
<p>Perform a convolution of <em class="parameter"><code>in</code></em>
 with <em class="parameter"><code>mask</code></em>
.
Each output pixel is calculated as:</p>
<div class="informalexample">
  <table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
    <tbody>
      <tr>
        <td class="listing_lines" align="right"><pre>1</pre></td>
        <td class="listing_code"><pre class="programlisting">sigma<span class="gtkdoc opt">[</span>i<span class="gtkdoc opt">]{</span>pixel<span class="gtkdoc opt">[</span>i<span class="gtkdoc opt">] *</span> mask<span class="gtkdoc opt">[</span>i<span class="gtkdoc opt">]} /</span> scale <span class="gtkdoc opt">+</span> offset</pre></td>
      </tr>
    </tbody>
  </table>
</div>

<p></p>
<p>where scale and offset are part of <em class="parameter"><code>mask</code></em>
. </p>
<p>If <em class="parameter"><code>precision</code></em>
 is <a class="link" href="libvips-basic.html#VIPS-PRECISION-INTEGER:CAPS"><span class="type">VIPS_PRECISION_INTEGER</span></a>, then 
elements of <em class="parameter"><code>mask</code></em>
 are converted to
integers before convolution, using <code class="function">rint()</code>,
and the output image 
always has the same <a class="link" href="VipsImage.html#VipsBandFormat" title="enum VipsBandFormat"><span class="type">VipsBandFormat</span></a> as the input image. </p>
<p>For <a class="link" href="VipsImage.html#VIPS-FORMAT-UCHAR:CAPS"><span class="type">VIPS_FORMAT_UCHAR</span></a> images, <a class="link" href="libvips-convolution.html#vips-conv" title="vips_conv ()"><code class="function">vips_conv()</code></a> uses a fast vector path based on
fixed-point arithmetic. This can produce slightly different results. 
Disable the vector path with <code class="literal">--vips-novector</code> or <code class="literal">VIPS_NOVECTOR</code> or
<code class="function">vips_vector_set_enabled()</code>.</p>
<p>If <em class="parameter"><code>precision</code></em>
 is <a class="link" href="libvips-basic.html#VIPS-PRECISION-FLOAT:CAPS"><span class="type">VIPS_PRECISION_FLOAT</span></a> then the convolution is performed
with floating-point arithmetic. The output image 
is always <a class="link" href="VipsImage.html#VIPS-FORMAT-FLOAT:CAPS"><span class="type">VIPS_FORMAT_FLOAT</span></a> unless <em class="parameter"><code>in</code></em>
 is <a class="link" href="VipsImage.html#VIPS-FORMAT-DOUBLE:CAPS"><span class="type">VIPS_FORMAT_DOUBLE</span></a>, in which case
<em class="parameter"><code>out</code></em>
 is also <a class="link" href="VipsImage.html#VIPS-FORMAT-DOUBLE:CAPS"><span class="type">VIPS_FORMAT_DOUBLE</span></a>. </p>
<p>If <em class="parameter"><code>precision</code></em>
 is <a class="link" href="libvips-basic.html#VIPS-PRECISION-APPROXIMATE:CAPS"><span class="type">VIPS_PRECISION_APPROXIMATE</span></a> then, like
<a class="link" href="libvips-basic.html#VIPS-PRECISION-INTEGER:CAPS"><span class="type">VIPS_PRECISION_INTEGER</span></a>, <em class="parameter"><code>mask</code></em>
 is converted to int before convolution, and 
the output image 
always has the same <a class="link" href="VipsImage.html#VipsBandFormat" title="enum VipsBandFormat"><span class="type">VipsBandFormat</span></a> as the input image. </p>
<p>Larger values for <em class="parameter"><code>layers</code></em>
 give more accurate
results, but are slower. As <em class="parameter"><code>layers</code></em>
 approaches the mask radius, the
accuracy will become close to exact convolution and the speed will drop to 
match. For many large masks, such as Gaussian, <em class="parameter"><code>n_layers</code></em>
 need be only 10% of
this value and accuracy will still be good.</p>
<p>Smaller values of <em class="parameter"><code>cluster</code></em>
 will give more accurate results, but be slower
and use more memory. 10% of the mask radius is a good rule of thumb.</p>
<p>See also: <a class="link" href="libvips-convolution.html#vips-convsep" title="vips_convsep ()"><code class="function">vips_convsep()</code></a>.</p>
<div class="refsect3">
<a name="vips-conv.parameters"></a><h4>Parameters</h4>
<div class="informaltable"><table class="informaltable" width="100%" border="0">
<colgroup>
<col width="150px" class="parameters_name">
<col class="parameters_description">
<col width="200px" class="parameters_annotations">
</colgroup>
<tbody>
<tr>
<td class="parameter_name"><p>in</p></td>
<td class="parameter_description"><p>input image</p></td>
<td class="parameter_annotations"> </td>
</tr>
<tr>
<td class="parameter_name"><p>out</p></td>
<td class="parameter_description"><p>output image</p></td>
<td class="parameter_annotations"> </td>
</tr>
<tr>
<td class="parameter_name"><p>mask</p></td>
<td class="parameter_description"><p>convolve with this mask</p></td>
<td class="parameter_annotations"> </td>
</tr>
<tr>
<td class="parameter_name"><p>...</p></td>
<td class="parameter_description"><p><code class="literal">NULL</code>-terminated list of optional named arguments</p></td>
<td class="parameter_annotations"> </td>
</tr>
</tbody>
</table></div>
</div>
<div class="refsect3">
<a name="vips-conv.returns"></a><h4>Returns</h4>
<p> 0 on success, -1 on error</p>
</div>
</div>
<hr>
<div class="refsect2">
<a name="vips-convf"></a><h3>vips_convf ()</h3>
<pre class="programlisting"><span class="returnvalue">int</span>
vips_convf (<em class="parameter"><code><a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a> *in</code></em>,
            <em class="parameter"><code><a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a> **out</code></em>,
            <em class="parameter"><code><a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a> *mask</code></em>,
            <em class="parameter"><code>...</code></em>);</pre>
<p>Convolution. This is a low-level operation, see <a class="link" href="libvips-convolution.html#vips-conv" title="vips_conv ()"><code class="function">vips_conv()</code></a> for something
more convenient. </p>
<p>Perform a convolution of <em class="parameter"><code>in</code></em>
 with <em class="parameter"><code>mask</code></em>
.
Each output pixel is
calculated as sigma[i]{pixel[i] * mask[i]} / scale + offset, where scale
and offset are part of <em class="parameter"><code>mask</code></em>
. </p>
<p>The convolution is performed with floating-point arithmetic. The output image 
is always <a class="link" href="VipsImage.html#VIPS-FORMAT-FLOAT:CAPS"><span class="type">VIPS_FORMAT_FLOAT</span></a> unless <em class="parameter"><code>in</code></em>
 is <a class="link" href="VipsImage.html#VIPS-FORMAT-DOUBLE:CAPS"><span class="type">VIPS_FORMAT_DOUBLE</span></a>, in which case
<em class="parameter"><code>out</code></em>
 is also <a class="link" href="VipsImage.html#VIPS-FORMAT-DOUBLE:CAPS"><span class="type">VIPS_FORMAT_DOUBLE</span></a>. </p>
<p>See also: <a class="link" href="libvips-convolution.html#vips-conv" title="vips_conv ()"><code class="function">vips_conv()</code></a>.</p>
<div class="refsect3">
<a name="vips-convf.parameters"></a><h4>Parameters</h4>
<div class="informaltable"><table class="informaltable" width="100%" border="0">
<colgroup>
<col width="150px" class="parameters_name">
<col class="parameters_description">
<col width="200px" class="parameters_annotations">
</colgroup>
<tbody>
<tr>
<td class="parameter_name"><p>in</p></td>
<td class="parameter_description"><p>input image</p></td>
<td class="parameter_annotations"> </td>
</tr>
<tr>
<td class="parameter_name"><p>out</p></td>
<td class="parameter_description"><p>output image</p></td>
<td class="parameter_annotations"> </td>
</tr>
<tr>
<td class="parameter_name"><p>mask</p></td>
<td class="parameter_description"><p>convolve with this mask</p></td>
<td class="parameter_annotations"> </td>
</tr>
<tr>
<td class="parameter_name"><p>...</p></td>
<td class="parameter_description"><p><code class="literal">NULL</code>-terminated list of optional named arguments</p></td>
<td class="parameter_annotations"> </td>
</tr>
</tbody>
</table></div>
</div>
<div class="refsect3">
<a name="vips-convf.returns"></a><h4>Returns</h4>
<p> 0 on success, -1 on error</p>
</div>
</div>
<hr>
<div class="refsect2">
<a name="vips-convi"></a><h3>vips_convi ()</h3>
<pre class="programlisting"><span class="returnvalue">int</span>
vips_convi (<em class="parameter"><code><a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a> *in</code></em>,
            <em class="parameter"><code><a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a> **out</code></em>,
            <em class="parameter"><code><a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a> *mask</code></em>,
            <em class="parameter"><code>...</code></em>);</pre>
<p>Integer convolution. This is a low-level operation, see <a class="link" href="libvips-convolution.html#vips-conv" title="vips_conv ()"><code class="function">vips_conv()</code></a> for 
something more convenient. </p>
<p><em class="parameter"><code>mask</code></em>
 is converted to an integer mask with <code class="function">rint()</code> of each element, rint of
scale and rint of offset. Each output pixel is then calculated as </p>
<div class="informalexample">
  <table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
    <tbody>
      <tr>
        <td class="listing_lines" align="right"><pre>1</pre></td>
        <td class="listing_code"><pre class="programlisting">sigma<span class="gtkdoc opt">[</span>i<span class="gtkdoc opt">]{</span>pixel<span class="gtkdoc opt">[</span>i<span class="gtkdoc opt">] *</span> mask<span class="gtkdoc opt">[</span>i<span class="gtkdoc opt">]} /</span> scale <span class="gtkdoc opt">+</span> offset</pre></td>
      </tr>
    </tbody>
  </table>
</div>

<p></p>
<p>The output image always has the same <a class="link" href="VipsImage.html#VipsBandFormat" title="enum VipsBandFormat"><span class="type">VipsBandFormat</span></a> as the input image. </p>
<p>For <a class="link" href="VipsImage.html#VIPS-FORMAT-UCHAR:CAPS"><span class="type">VIPS_FORMAT_UCHAR</span></a> images, <a class="link" href="libvips-convolution.html#vips-convi" title="vips_convi ()"><code class="function">vips_convi()</code></a> uses a fast vector path based on
fixed-point arithmetic. This can produce slightly different results. 
Disable the vector path with <code class="literal">--vips-novector</code> or <code class="literal">VIPS_NOVECTOR</code> or
<code class="function">vips_vector_set_enabled()</code>.</p>
<p>See also: <a class="link" href="libvips-convolution.html#vips-conv" title="vips_conv ()"><code class="function">vips_conv()</code></a>.</p>
<div class="refsect3">
<a name="vips-convi.parameters"></a><h4>Parameters</h4>
<div class="informaltable"><table class="informaltable" width="100%" border="0">
<colgroup>
<col width="150px" class="parameters_name">
<col class="parameters_description">
<col width="200px" class="parameters_annotations">
</colgroup>
<tbody>
<tr>
<td class="parameter_name"><p>in</p></td>
<td class="parameter_description"><p>input image</p></td>
<td class="parameter_annotations"> </td>
</tr>
<tr>
<td class="parameter_name"><p>out</p></td>
<td class="parameter_description"><p>output image</p></td>
<td class="parameter_annotations"> </td>
</tr>
<tr>
<td class="parameter_name"><p>mask</p></td>
<td class="parameter_description"><p>convolve with this mask</p></td>
<td class="parameter_annotations"> </td>
</tr>
<tr>
<td class="parameter_name"><p>...</p></td>
<td class="parameter_description"><p><code class="literal">NULL</code>-terminated list of optional named arguments</p></td>
<td class="parameter_annotations"> </td>
</tr>
</tbody>
</table></div>
</div>
<div class="refsect3">
<a name="vips-convi.returns"></a><h4>Returns</h4>
<p> 0 on success, -1 on error</p>
</div>
</div>
<hr>
<div class="refsect2">
<a name="vips-conva"></a><h3>vips_conva ()</h3>
<pre class="programlisting"><span class="returnvalue">int</span>
vips_conva (<em class="parameter"><code><a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a> *in</code></em>,
            <em class="parameter"><code><a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a> **out</code></em>,
            <em class="parameter"><code><a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a> *mask</code></em>,
            <em class="parameter"><code>...</code></em>);</pre>
<p>Optional arguments:</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p><em class="parameter"><code>layers</code></em>
: <code class="literal">gint</code>, number of layers for approximation</p></li>
<li class="listitem"><p><em class="parameter"><code>cluster</code></em>
: <code class="literal">gint</code>, cluster lines closer than this distance</p></li>
</ul></div>
<p>Perform an approximate integer convolution of <em class="parameter"><code>in</code></em>
 with <em class="parameter"><code>mask</code></em>
.
This is a low-level operation, see 
<a class="link" href="libvips-convolution.html#vips-conv" title="vips_conv ()"><code class="function">vips_conv()</code></a> for something more convenient. </p>
<p>The output image 
always has the same <a class="link" href="VipsImage.html#VipsBandFormat" title="enum VipsBandFormat"><span class="type">VipsBandFormat</span></a> as the input image. 
Elements of <em class="parameter"><code>mask</code></em>
 are converted to
integers before convolution.</p>
<p>Larger values for <em class="parameter"><code>layers</code></em>
 give more accurate
results, but are slower. As <em class="parameter"><code>layers</code></em>
 approaches the mask radius, the
accuracy will become close to exact convolution and the speed will drop to 
match. For many large masks, such as Gaussian, <em class="parameter"><code>layers</code></em>
 need be only 10% of
this value and accuracy will still be good.</p>
<p>Smaller values of <em class="parameter"><code>cluster</code></em>
 will give more accurate results, but be slower
and use more memory. 10% of the mask radius is a good rule of thumb.</p>
<p>See also: <a class="link" href="libvips-convolution.html#vips-conv" title="vips_conv ()"><code class="function">vips_conv()</code></a>.</p>
<div class="refsect3">
<a name="vips-conva.parameters"></a><h4>Parameters</h4>
<div class="informaltable"><table class="informaltable" width="100%" border="0">
<colgroup>
<col width="150px" class="parameters_name">
<col class="parameters_description">
<col width="200px" class="parameters_annotations">
</colgroup>
<tbody>
<tr>
<td class="parameter_name"><p>in</p></td>
<td class="parameter_description"><p>input image</p></td>
<td class="parameter_annotations"> </td>
</tr>
<tr>
<td class="parameter_name"><p>out</p></td>
<td class="parameter_description"><p>output image</p></td>
<td class="parameter_annotations"> </td>
</tr>
<tr>
<td class="parameter_name"><p>mask</p></td>
<td class="parameter_description"><p>convolution mask</p></td>
<td class="parameter_annotations"> </td>
</tr>
<tr>
<td class="parameter_name"><p>...</p></td>
<td class="parameter_description"><p><code class="literal">NULL</code>-terminated list of optional named arguments</p></td>
<td class="parameter_annotations"> </td>
</tr>
</tbody>
</table></div>
</div>
<div class="refsect3">
<a name="vips-conva.returns"></a><h4>Returns</h4>
<p> 0 on success, -1 on error</p>
</div>
</div>
<hr>
<div class="refsect2">
<a name="vips-convsep"></a><h3>vips_convsep ()</h3>
<pre class="programlisting"><span class="returnvalue">int</span>
vips_convsep (<em class="parameter"><code><a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a> *in</code></em>,
              <em class="parameter"><code><a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a> **out</code></em>,
              <em class="parameter"><code><a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a> *mask</code></em>,
              <em class="parameter"><code>...</code></em>);</pre>
<p>Optional arguments:</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p><em class="parameter"><code>precision</code></em>
: calculation accuracy</p></li>
<li class="listitem"><p><em class="parameter"><code>layers</code></em>
: number of layers for approximation</p></li>
<li class="listitem"><p><em class="parameter"><code>cluster</code></em>
: cluster lines closer than this distance</p></li>
</ul></div>
<p>Perform a separable convolution of <em class="parameter"><code>in</code></em>
 with <em class="parameter"><code>mask</code></em>
.
See <a class="link" href="libvips-convolution.html#vips-conv" title="vips_conv ()"><code class="function">vips_conv()</code></a> for a detailed description.</p>
<p>The mask must be 1xn or nx1 elements. </p>
<p>The image is convolved twice: once with <em class="parameter"><code>mask</code></em>
 and then again with <em class="parameter"><code>mask</code></em>
 
rotated by 90 degrees. This is much faster for certain types of mask
(gaussian blur, for example) than doing a full 2D convolution.</p>
<p>See also: <a class="link" href="libvips-convolution.html#vips-conv" title="vips_conv ()"><code class="function">vips_conv()</code></a>, <a class="link" href="libvips-create.html#vips-gaussmat" title="vips_gaussmat ()"><code class="function">vips_gaussmat()</code></a>.</p>
<div class="refsect3">
<a name="vips-convsep.parameters"></a><h4>Parameters</h4>
<div class="informaltable"><table class="informaltable" width="100%" border="0">
<colgroup>
<col width="150px" class="parameters_name">
<col class="parameters_description">
<col width="200px" class="parameters_annotations">
</colgroup>
<tbody>
<tr>
<td class="parameter_name"><p>in</p></td>
<td class="parameter_description"><p>input image</p></td>
<td class="parameter_annotations"> </td>
</tr>
<tr>
<td class="parameter_name"><p>out</p></td>
<td class="parameter_description"><p>output image</p></td>
<td class="parameter_annotations"> </td>
</tr>
<tr>
<td class="parameter_name"><p>mask</p></td>
<td class="parameter_description"><p>convolution mask</p></td>
<td class="parameter_annotations"> </td>
</tr>
<tr>
<td class="parameter_name"><p>...</p></td>
<td class="parameter_description"><p><code class="literal">NULL</code>-terminated list of optional named arguments</p></td>
<td class="parameter_annotations"> </td>
</tr>
</tbody>
</table></div>
</div>
<div class="refsect3">
<a name="vips-convsep.returns"></a><h4>Returns</h4>
<p> 0 on success, -1 on error</p>
</div>
</div>
<hr>
<div class="refsect2">
<a name="vips-convasep"></a><h3>vips_convasep ()</h3>
<pre class="programlisting"><span class="returnvalue">int</span>
vips_convasep (<em class="parameter"><code><a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a> *in</code></em>,
               <em class="parameter"><code><a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a> **out</code></em>,
               <em class="parameter"><code><a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a> *mask</code></em>,
               <em class="parameter"><code>...</code></em>);</pre>
<p>Optional arguments:</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p><em class="parameter"><code>layers</code></em>
: <code class="literal">gint</code>, number of layers for approximation</p></li></ul></div>
<p>Approximate separable integer convolution. This is a low-level operation, see 
<a class="link" href="libvips-convolution.html#vips-convsep" title="vips_convsep ()"><code class="function">vips_convsep()</code></a> for something more convenient. </p>
<p>The image is convolved twice: once with <em class="parameter"><code>mask</code></em>
 and then again with <em class="parameter"><code>mask</code></em>
 
rotated by 90 degrees. 
<em class="parameter"><code>mask</code></em>
 must be 1xn or nx1 elements. 
Elements of <em class="parameter"><code>mask</code></em>
 are converted to
integers before convolution.</p>
<p>Larger values for <em class="parameter"><code>layers</code></em>
 give more accurate
results, but are slower. As <em class="parameter"><code>layers</code></em>
 approaches the mask radius, the
accuracy will become close to exact convolution and the speed will drop to 
match. For many large masks, such as Gaussian, <em class="parameter"><code>layers</code></em>
 need be only 10% of
this value and accuracy will still be good.</p>
<p>The output image 
always has the same <a class="link" href="VipsImage.html#VipsBandFormat" title="enum VipsBandFormat"><span class="type">VipsBandFormat</span></a> as the input image. </p>
<p>See also: <a class="link" href="libvips-convolution.html#vips-convsep" title="vips_convsep ()"><code class="function">vips_convsep()</code></a>.</p>
<div class="refsect3">
<a name="vips-convasep.parameters"></a><h4>Parameters</h4>
<div class="informaltable"><table class="informaltable" width="100%" border="0">
<colgroup>
<col width="150px" class="parameters_name">
<col class="parameters_description">
<col width="200px" class="parameters_annotations">
</colgroup>
<tbody>
<tr>
<td class="parameter_name"><p>in</p></td>
<td class="parameter_description"><p>input image</p></td>
<td class="parameter_annotations"> </td>
</tr>
<tr>
<td class="parameter_name"><p>out</p></td>
<td class="parameter_description"><p>output image</p></td>
<td class="parameter_annotations"> </td>
</tr>
<tr>
<td class="parameter_name"><p>mask</p></td>
<td class="parameter_description"><p>convolve with this mask</p></td>
<td class="parameter_annotations"> </td>
</tr>
<tr>
<td class="parameter_name"><p>...</p></td>
<td class="parameter_description"><p><code class="literal">NULL</code>-terminated list of optional named arguments</p></td>
<td class="parameter_annotations"> </td>
</tr>
</tbody>
</table></div>
</div>
<div class="refsect3">
<a name="vips-convasep.returns"></a><h4>Returns</h4>
<p> 0 on success, -1 on error</p>
</div>
</div>
<hr>
<div class="refsect2">
<a name="vips-compass"></a><h3>vips_compass ()</h3>
<pre class="programlisting"><span class="returnvalue">int</span>
vips_compass (<em class="parameter"><code><a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a> *in</code></em>,
              <em class="parameter"><code><a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a> **out</code></em>,
              <em class="parameter"><code><a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a> *mask</code></em>,
              <em class="parameter"><code>...</code></em>);</pre>
</div>
<hr>
<div class="refsect2">
<a name="vips-gaussblur"></a><h3>vips_gaussblur ()</h3>
<pre class="programlisting"><span class="returnvalue">int</span>
vips_gaussblur (<em class="parameter"><code><a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a> *in</code></em>,
                <em class="parameter"><code><a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a> **out</code></em>,
                <em class="parameter"><code><span class="type">double</span> sigma</code></em>,
                <em class="parameter"><code>...</code></em>);</pre>
<p>Optional arguments:</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p><em class="parameter"><code>precision</code></em>
: <a class="link" href="libvips-basic.html#VipsPrecision" title="enum VipsPrecision"><span class="type">VipsPrecision</span></a> for blur, default VIPS_PRECISION_INTEGER</p></li>
<li class="listitem"><p><em class="parameter"><code>min_ampl</code></em>
: minimum amplitude, default 0.2</p></li>
</ul></div>
<p>This operator runs <a class="link" href="libvips-create.html#vips-gaussmat" title="vips_gaussmat ()"><code class="function">vips_gaussmat()</code></a> and <a class="link" href="libvips-convolution.html#vips-convsep" title="vips_convsep ()"><code class="function">vips_convsep()</code></a> for you on an image.
Set <em class="parameter"><code>min_ampl</code></em>
 smaller to generate a larger, more accurate mask. Set <em class="parameter"><code>sigma</code></em>

larger to make the blur more blurry. </p>
<p>See also: <a class="link" href="libvips-create.html#vips-gaussmat" title="vips_gaussmat ()"><code class="function">vips_gaussmat()</code></a>, <a class="link" href="libvips-convolution.html#vips-convsep" title="vips_convsep ()"><code class="function">vips_convsep()</code></a>.</p>
<div class="refsect3">
<a name="vips-gaussblur.parameters"></a><h4>Parameters</h4>
<div class="informaltable"><table class="informaltable" width="100%" border="0">
<colgroup>
<col width="150px" class="parameters_name">
<col class="parameters_description">
<col width="200px" class="parameters_annotations">
</colgroup>
<tbody>
<tr>
<td class="parameter_name"><p>in</p></td>
<td class="parameter_description"><p>input image</p></td>
<td class="parameter_annotations"> </td>
</tr>
<tr>
<td class="parameter_name"><p>out</p></td>
<td class="parameter_description"><p>output image</p></td>
<td class="parameter_annotations"> </td>
</tr>
<tr>
<td class="parameter_name"><p>sigma</p></td>
<td class="parameter_description"><p>how large a mask to use</p></td>
<td class="parameter_annotations"> </td>
</tr>
<tr>
<td class="parameter_name"><p>...</p></td>
<td class="parameter_description"><p><code class="literal">NULL</code>-terminated list of optional named arguments</p></td>
<td class="parameter_annotations"> </td>
</tr>
</tbody>
</table></div>
</div>
<div class="refsect3">
<a name="vips-gaussblur.returns"></a><h4>Returns</h4>
<p> 0 on success, -1 on error.</p>
</div>
</div>
<hr>
<div class="refsect2">
<a name="vips-sharpen"></a><h3>vips_sharpen ()</h3>
<pre class="programlisting"><span class="returnvalue">int</span>
vips_sharpen (<em class="parameter"><code><a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a> *in</code></em>,
              <em class="parameter"><code><a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a> **out</code></em>,
              <em class="parameter"><code>...</code></em>);</pre>
<p>Optional arguments:</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p><em class="parameter"><code>sigma</code></em>
: sigma of gaussian</p></li>
<li class="listitem"><p><em class="parameter"><code>x1</code></em>
: flat/jaggy threshold</p></li>
<li class="listitem"><p><em class="parameter"><code>y2</code></em>
: maximum amount of brightening</p></li>
<li class="listitem"><p><em class="parameter"><code>y3</code></em>
: maximum amount of darkening</p></li>
<li class="listitem"><p><em class="parameter"><code>m1</code></em>
: slope for flat areas</p></li>
<li class="listitem"><p><em class="parameter"><code>m2</code></em>
: slope for jaggy areas</p></li>
</ul></div>
<p>Selectively sharpen the L channel of a LAB image. The input image is
transformed to <a class="link" href="VipsImage.html#VIPS-INTERPRETATION-LABS:CAPS"><span class="type">VIPS_INTERPRETATION_LABS</span></a>. </p>
<p>The operation performs a gaussian blur of radius <em class="parameter"><code>radius</code></em>
 and subtracts 
from <em class="parameter"><code>in</code></em>
 to generate a high-frequency signal. This signal is passed 
through a lookup table formed from the five parameters and added back to 
<em class="parameter"><code>in</code></em>
.</p>
<p>The lookup table is formed like this:</p>
<div class="informalexample">
  <table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
    <tbody>
      <tr>
        <td class="listing_lines" align="right"><pre>1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17</pre></td>
        <td class="listing_code"><pre class="programlisting"><span class="gtkdoc opt">^</span>
                  y2 <span class="gtkdoc opt">|- - - - - -----------</span>
<span class="gtkdoc opt">|         /</span> 
<span class="gtkdoc opt">|        /</span> slope m2
<span class="gtkdoc opt">|    .../</span>    
             <span class="gtkdoc opt">-</span>x1     <span class="gtkdoc opt">| ...   |</span>    
 <span class="gtkdoc opt">-------------------...----------------------&gt;</span>
             <span class="gtkdoc opt">|   ... |</span>      x1           
             <span class="gtkdoc opt">|...</span> slope m1
             <span class="gtkdoc opt">/       |</span>
            <span class="gtkdoc opt">/</span> m2     <span class="gtkdoc opt">|</span>
           <span class="gtkdoc opt">/         |</span>
          <span class="gtkdoc opt">/          |</span>
         <span class="gtkdoc opt">/           |</span>
        <span class="gtkdoc opt">/            |</span>
 ______<span class="gtkdoc opt">/</span> _ _ _ _ _ _ <span class="gtkdoc opt">| -</span>y3
<span class="gtkdoc opt">|</span></pre></td>
      </tr>
    </tbody>
  </table>
</div>

<p></p>
<p>For screen output, we suggest the following settings (the defaults):</p>
<div class="informalexample">
  <table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
    <tbody>
      <tr>
        <td class="listing_lines" align="right"><pre>1
2
3
4
5
6</pre></td>
        <td class="listing_code"><pre class="programlisting">sigma <span class="gtkdoc opt">==</span> <span class="number">0.5</span>
x1 <span class="gtkdoc opt">==</span> <span class="number">2</span>
y2 <span class="gtkdoc opt">==</span> <span class="function">10</span>         <span class="gtkdoc opt">(</span>don<span class="string">'t brighten by more than 10 L*)</span>
<span class="string">y3 == 20         (can darken by up to 20 L*)</span>
<span class="string">m1 == 0          (no sharpening in flat areas)</span>
<span class="string">m2 == 3          (some sharpening in jaggy areas)</span></pre></td>
      </tr>
    </tbody>
  </table>
</div>

<p></p>
<p>If you want more or less sharpening, we suggest you just change the 
m2 parameter. </p>
<p>The <em class="parameter"><code>sigma</code></em>
 parameter changes the width of the fringe and can be 
adjusted according to the output printing resolution. As an approximate 
guideline, use 0.5 for 4 pixels/mm (display resolution), 
1.0 for 12 pixels/mm and 1.5 for 16 pixels/mm (300 dpi == 12 
pixels/mm). These figures refer to the image raster, not the half-tone 
resolution.</p>
<p>See also: <a class="link" href="libvips-convolution.html#vips-conv" title="vips_conv ()"><code class="function">vips_conv()</code></a>.</p>
<div class="refsect3">
<a name="vips-sharpen.parameters"></a><h4>Parameters</h4>
<div class="informaltable"><table class="informaltable" width="100%" border="0">
<colgroup>
<col width="150px" class="parameters_name">
<col class="parameters_description">
<col width="200px" class="parameters_annotations">
</colgroup>
<tbody>
<tr>
<td class="parameter_name"><p>in</p></td>
<td class="parameter_description"><p>input image</p></td>
<td class="parameter_annotations"> </td>
</tr>
<tr>
<td class="parameter_name"><p>out</p></td>
<td class="parameter_description"><p>output image</p></td>
<td class="parameter_annotations"> </td>
</tr>
<tr>
<td class="parameter_name"><p>...</p></td>
<td class="parameter_description"><p><code class="literal">NULL</code>-terminated list of optional named arguments</p></td>
<td class="parameter_annotations"> </td>
</tr>
</tbody>
</table></div>
</div>
<div class="refsect3">
<a name="vips-sharpen.returns"></a><h4>Returns</h4>
<p> 0 on success, -1 on error.</p>
</div>
</div>
<hr>
<div class="refsect2">
<a name="vips-spcor"></a><h3>vips_spcor ()</h3>
<pre class="programlisting"><span class="returnvalue">int</span>
vips_spcor (<em class="parameter"><code><a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a> *in</code></em>,
            <em class="parameter"><code><a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a> *ref</code></em>,
            <em class="parameter"><code><a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a> **out</code></em>,
            <em class="parameter"><code>...</code></em>);</pre>
<p>Calculate a correlation surface.</p>
<p><em class="parameter"><code>ref</code></em>
 is placed at every position in <em class="parameter"><code>in</code></em>
 and the correlation coefficient
calculated. The output
image is always float.</p>
<p>The output
image is the same size as the input. Extra input edge pixels are made by 
copying the existing edges outwards. </p>
<p>The correlation coefficient is calculated as:</p>
<div class="informalexample">
  <table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
    <tbody>
      <tr>
        <td class="listing_lines" align="right"><pre>1
2
3
4</pre></td>
        <td class="listing_code"><pre class="programlisting"><span class="function">sumij</span> <span class="gtkdoc opt">(</span><span class="function">ref</span><span class="gtkdoc opt">(</span>i<span class="gtkdoc opt">,</span>j<span class="gtkdoc opt">)-</span><span class="function">mean</span><span class="gtkdoc opt">(</span>ref<span class="gtkdoc opt">))(</span><span class="function">inkl</span><span class="gtkdoc opt">(</span>i<span class="gtkdoc opt">,</span>j<span class="gtkdoc opt">)-</span><span class="function">mean</span><span class="gtkdoc opt">(</span>inkl<span class="gtkdoc opt">))</span>
<span class="function">c</span><span class="gtkdoc opt">(</span>k<span class="gtkdoc opt">,</span>l<span class="gtkdoc opt">) = ------------------------------------------------</span>
<span class="function">sqrt</span><span class="gtkdoc opt">(</span><span class="function">sumij</span> <span class="gtkdoc opt">(</span><span class="function">ref</span><span class="gtkdoc opt">(</span>i<span class="gtkdoc opt">,</span>j<span class="gtkdoc opt">)-</span><span class="function">mean</span><span class="gtkdoc opt">(</span>ref<span class="gtkdoc opt">))^</span><span class="number">2</span><span class="gtkdoc opt">) *</span>
            <span class="function">sqrt</span><span class="gtkdoc opt">(</span><span class="function">sumij</span> <span class="gtkdoc opt">(</span><span class="function">inkl</span><span class="gtkdoc opt">(</span>i<span class="gtkdoc opt">,</span>j<span class="gtkdoc opt">)-</span><span class="function">mean</span><span class="gtkdoc opt">(</span>inkl<span class="gtkdoc opt">))^</span><span class="number">2</span><span class="gtkdoc opt">)</span></pre></td>
      </tr>
    </tbody>
  </table>
</div>

<p></p>
<p>where inkl is the area of <em class="parameter"><code>in</code></em>
 centred at position (k,l).</p>
<p>from Niblack "An Introduction to Digital Image Processing", 
Prentice/Hall, pp 138.</p>
<p>If the number of bands differs, one of the images 
must have one band. In this case, an n-band image is formed from the 
one-band image by joining n copies of the one-band image together, and then
the two n-band images are operated upon.</p>
<p>The output image is always float, unless either of the two inputs is
double, in which case the output is also double.</p>
<p>See also: <a class="link" href="libvips-convolution.html#vips-fastcor" title="vips_fastcor ()"><code class="function">vips_fastcor()</code></a>.</p>
<div class="refsect3">
<a name="vips-spcor.parameters"></a><h4>Parameters</h4>
<div class="informaltable"><table class="informaltable" width="100%" border="0">
<colgroup>
<col width="150px" class="parameters_name">
<col class="parameters_description">
<col width="200px" class="parameters_annotations">
</colgroup>
<tbody>
<tr>
<td class="parameter_name"><p>in</p></td>
<td class="parameter_description"><p>input image</p></td>
<td class="parameter_annotations"> </td>
</tr>
<tr>
<td class="parameter_name"><p>ref</p></td>
<td class="parameter_description"><p>reference image</p></td>
<td class="parameter_annotations"> </td>
</tr>
<tr>
<td class="parameter_name"><p>out</p></td>
<td class="parameter_description"><p>output image</p></td>
<td class="parameter_annotations"> </td>
</tr>
<tr>
<td class="parameter_name"><p>...</p></td>
<td class="parameter_description"><p><code class="literal">NULL</code>-terminated list of optional named arguments</p></td>
<td class="parameter_annotations"> </td>
</tr>
</tbody>
</table></div>
</div>
<div class="refsect3">
<a name="vips-spcor.returns"></a><h4>Returns</h4>
<p> 0 on success, -1 on error</p>
</div>
</div>
<hr>
<div class="refsect2">
<a name="vips-fastcor"></a><h3>vips_fastcor ()</h3>
<pre class="programlisting"><span class="returnvalue">int</span>
vips_fastcor (<em class="parameter"><code><a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a> *in</code></em>,
              <em class="parameter"><code><a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a> *ref</code></em>,
              <em class="parameter"><code><a class="link" href="VipsImage.html" title="VipsImage"><span class="type">VipsImage</span></a> **out</code></em>,
              <em class="parameter"><code>...</code></em>);</pre>
<p>Calculate a fast correlation surface.</p>
<p><em class="parameter"><code>ref</code></em>
 is placed at every position in <em class="parameter"><code>in</code></em>
 and the sum of squares of
differences calculated. </p>
<p>The output
image is the same size as the input. Extra input edge pixels are made by 
copying the existing edges outwards. </p>
<p>If the number of bands differs, one of the images 
must have one band. In this case, an n-band image is formed from the 
one-band image by joining n copies of the one-band image together, and then
the two n-band images are operated upon.</p>
<p>The output type is uint if both inputs are integer, float if both are float
or complex, and double if either is double or double complex. 
In other words, the output type is just large enough to hold the whole
range of possible values.</p>
<p>See also: <a class="link" href="libvips-convolution.html#vips-spcor" title="vips_spcor ()"><code class="function">vips_spcor()</code></a>.</p>
<div class="refsect3">
<a name="vips-fastcor.parameters"></a><h4>Parameters</h4>
<div class="informaltable"><table class="informaltable" width="100%" border="0">
<colgroup>
<col width="150px" class="parameters_name">
<col class="parameters_description">
<col width="200px" class="parameters_annotations">
</colgroup>
<tbody>
<tr>
<td class="parameter_name"><p>in</p></td>
<td class="parameter_description"><p>input image</p></td>
<td class="parameter_annotations"> </td>
</tr>
<tr>
<td class="parameter_name"><p>ref</p></td>
<td class="parameter_description"><p>reference image</p></td>
<td class="parameter_annotations"> </td>
</tr>
<tr>
<td class="parameter_name"><p>out</p></td>
<td class="parameter_description"><p>output image</p></td>
<td class="parameter_annotations"> </td>
</tr>
<tr>
<td class="parameter_name"><p>...</p></td>
<td class="parameter_description"><p><code class="literal">NULL</code>-terminated list of optional named arguments</p></td>
<td class="parameter_annotations"> </td>
</tr>
</tbody>
</table></div>
</div>
<div class="refsect3">
<a name="vips-fastcor.returns"></a><h4>Returns</h4>
<p> 0 on success, -1 on error</p>
</div>
</div>
</div>
<div class="refsect1">
<a name="libvips-convolution.other_details"></a><h2>Types and Values</h2>
<div class="refsect2">
<a name="VipsCombine"></a><h3>enum VipsCombine</h3>
<p>How to combine values.</p>
<div class="refsect3">
<a name="VipsCombine.members"></a><h4>Members</h4>
<div class="informaltable"><table class="informaltable" width="100%" border="0">
<colgroup>
<col width="300px" class="enum_members_name">
<col class="enum_members_description">
<col width="200px" class="enum_members_annotations">
</colgroup>
<tbody>
<tr>
<td class="enum_member_name"><p><a name="VIPS-COMBINE-MAX:CAPS"></a>VIPS_COMBINE_MAX</p></td>
<td class="enum_member_description">
<p>take the maximum of the possible values</p>
</td>
<td class="enum_member_annotations"> </td>
</tr>
<tr>
<td class="enum_member_name"><p><a name="VIPS-COMBINE-SUM:CAPS"></a>VIPS_COMBINE_SUM</p></td>
<td class="enum_member_description">
<p>sum all the values</p>
</td>
<td class="enum_member_annotations"> </td>
</tr>
<tr>
<td class="enum_member_name"><p><a name="VIPS-COMBINE-LAST:CAPS"></a>VIPS_COMBINE_LAST</p></td>
<td> </td>
<td> </td>
</tr>
</tbody>
</table></div>
</div>
</div>
</div>
</div>
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