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<a href="#libvips-convolution.description" class="shortcut">Description</a>
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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 valign="top" align="right"></td>
</tr></table></div>
<div class="refsect1">
<a name="libvips-convolution.stability-level"></a><h2>Stability Level</h2>
Stable, unless otherwise indicated
</div>
<div class="refsynopsisdiv">
<a name="libvips-convolution.synopsis"></a><h2>Synopsis</h2>
<pre class="synopsis">
#include <vips/vips.h>
enum <a class="link" href="libvips-convolution.html#VipsPrecision" title="enum VipsPrecision">VipsPrecision</a>;
enum <a class="link" href="libvips-convolution.html#VipsCombine" title="enum VipsCombine">VipsCombine</a>;
<span class="returnvalue">int</span> <a class="link" href="libvips-convolution.html#vips-conv" title="vips_conv ()">vips_conv</a> (<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>);
<span class="returnvalue">int</span> <a class="link" href="libvips-convolution.html#vips-compass" title="vips_compass ()">vips_compass</a> (<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>);
<span class="returnvalue">int</span> <a class="link" href="libvips-convolution.html#vips-convsep" title="vips_convsep ()">vips_convsep</a> (<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>);
<span class="returnvalue">int</span> <a class="link" href="libvips-convolution.html#vips-sharpen" title="vips_sharpen ()">vips_sharpen</a> (<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>);
<span class="returnvalue">int</span> <a class="link" href="libvips-convolution.html#vips-gaussblur" title="vips_gaussblur ()">vips_gaussblur</a> (<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">int</span> radius</code></em>,
<em class="parameter"><code>...</code></em>);
</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.details"></a><h2>Details</h2>
<div class="refsect2">
<a name="VipsPrecision"></a><h3>enum VipsPrecision</h3>
<pre class="programlisting">typedef enum {
VIPS_PRECISION_INTEGER,
VIPS_PRECISION_FLOAT,
VIPS_PRECISION_APPROXIMATE,
VIPS_PRECISION_LAST
} VipsPrecision;
</pre>
<p>
How accurate an operation should be.
</p>
<div class="variablelist"><table border="0" class="variablelist">
<colgroup>
<col align="left" valign="top">
<col>
</colgroup>
<tbody>
<tr>
<td><p><a name="VIPS-PRECISION-INTEGER:CAPS"></a><span class="term"><code class="literal">VIPS_PRECISION_INTEGER</code></span></p></td>
<td>int everywhere
</td>
</tr>
<tr>
<td><p><a name="VIPS-PRECISION-FLOAT:CAPS"></a><span class="term"><code class="literal">VIPS_PRECISION_FLOAT</code></span></p></td>
<td>float everywhere
</td>
</tr>
<tr>
<td><p><a name="VIPS-PRECISION-APPROXIMATE:CAPS"></a><span class="term"><code class="literal">VIPS_PRECISION_APPROXIMATE</code></span></p></td>
<td>approximate integer output
</td>
</tr>
<tr>
<td><p><a name="VIPS-PRECISION-LAST:CAPS"></a><span class="term"><code class="literal">VIPS_PRECISION_LAST</code></span></p></td>
<td>
</td>
</tr>
</tbody>
</table></div>
</div>
<hr>
<div class="refsect2">
<a name="VipsCombine"></a><h3>enum VipsCombine</h3>
<pre class="programlisting">typedef enum {
VIPS_COMBINE_MAX,
VIPS_COMBINE_SUM,
VIPS_COMBINE_LAST
} VipsCombine;
</pre>
<p>
</p>
<div class="variablelist"><table border="0" class="variablelist">
<colgroup>
<col align="left" valign="top">
<col>
</colgroup>
<tbody>
<tr>
<td><p><a name="VIPS-COMBINE-MAX:CAPS"></a><span class="term"><code class="literal">VIPS_COMBINE_MAX</code></span></p></td>
<td>
</td>
</tr>
<tr>
<td><p><a name="VIPS-COMBINE-SUM:CAPS"></a><span class="term"><code class="literal">VIPS_COMBINE_SUM</code></span></p></td>
<td>
</td>
</tr>
<tr>
<td><p><a name="VIPS-COMBINE-LAST:CAPS"></a><span class="term"><code class="literal">VIPS_COMBINE_LAST</code></span></p></td>
<td>
</td>
</tr>
</tbody>
</table></div>
</div>
<hr>
<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>
<p>
<em class="parameter"><code>precision</code></em>: calculation accuracy
<em class="parameter"><code>layers</code></em>: number of layers for approximation
<em class="parameter"><code>cluster</code></em>: cluster lines closer than this distance
</p>
<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 sigma[i]{pixel[i] * mask[i]} / scale + offset, 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-convolution.html#VIPS-PRECISION-INTEGER:CAPS"><span class="type">VIPS_PRECISION_INTEGER</span></a> then the convolution is performed
with integer arithmetic and the output image
always has the same <span class="type">VipsBandFmt</span> as the input image.
</p>
<p>
Convolutions on unsigned 8-bit images are calculated with the
processor's vector unit, if possible. Disable this with --vips-novector or
IM_NOVECTOR.
</p>
<p>
If <em class="parameter"><code>precision</code></em> is <a class="link" href="libvips-convolution.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"><code class="literal">VIPS_FORMAT_FLOAT</code></a> unless <em class="parameter"><code>in</code></em> is <a class="link" href="VipsImage.html#VIPS-FORMAT-DOUBLE:CAPS"><code class="literal">VIPS_FORMAT_DOUBLE</code></a>, in which case
<em class="parameter"><code>out</code></em> is also <a class="link" href="VipsImage.html#VIPS-FORMAT-DOUBLE:CAPS"><code class="literal">VIPS_FORMAT_DOUBLE</code></a>.
</p>
<p>
If <em class="parameter"><code>precision</code></em> is <a class="link" href="libvips-convolution.html#VIPS-PRECISION-APPROXIMATE:CAPS"><span class="type">VIPS_PRECISION_APPROXIMATE</span></a> then the output image
always has the same <span class="type">VipsBandFmt</span> 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>
<div class="variablelist"><table border="0" class="variablelist">
<colgroup>
<col align="left" valign="top">
<col>
</colgroup>
<tbody>
<tr>
<td><p><span class="term"><em class="parameter"><code>in</code></em> :</span></p></td>
<td>input image</td>
</tr>
<tr>
<td><p><span class="term"><em class="parameter"><code>out</code></em> :</span></p></td>
<td>output image</td>
</tr>
<tr>
<td><p><span class="term"><em class="parameter"><code>mask</code></em> :</span></p></td>
<td>convolve with this mask</td>
</tr>
<tr>
<td><p><span class="term"><em class="parameter"><code>...</code></em> :</span></p></td>
<td>
<code class="literal">NULL</code>-terminated list of optional named arguments</td>
</tr>
<tr>
<td><p><span class="term"><span class="emphasis"><em>Returns</em></span> :</span></p></td>
<td>0 on success, -1 on error</td>
</tr>
</tbody>
</table></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>
<p>
</p>
</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>
<p>
<em class="parameter"><code>precision</code></em>: calculation accuracy
<em class="parameter"><code>layers</code></em>: number of layers for approximation
<em class="parameter"><code>cluster</code></em>: cluster lines closer than this distance
</p>
<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="variablelist"><table border="0" class="variablelist">
<colgroup>
<col align="left" valign="top">
<col>
</colgroup>
<tbody>
<tr>
<td><p><span class="term"><em class="parameter"><code>in</code></em> :</span></p></td>
<td>input image</td>
</tr>
<tr>
<td><p><span class="term"><em class="parameter"><code>out</code></em> :</span></p></td>
<td>output image</td>
</tr>
<tr>
<td><p><span class="term"><em class="parameter"><code>mask</code></em> :</span></p></td>
<td>convolution mask</td>
</tr>
<tr>
<td><p><span class="term"><span class="emphasis"><em>Returns</em></span> :</span></p></td>
<td>0 on success, -1 on error</td>
</tr>
</tbody>
</table></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>
<p>
<em class="parameter"><code>radius</code></em>: how large a mask to use
<em class="parameter"><code>x1</code></em>: flat/jaggy threshold
<em class="parameter"><code>y2</code></em>: maximum amount of brightening
<em class="parameter"><code>y3</code></em>: maximum amount of darkening
<em class="parameter"><code>m1</code></em>: slope for flat areas
<em class="parameter"><code>m2</code></em>: slope for jaggy areas
</p>
<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>
<p>
</p>
<div class="informalexample"><pre class="programlisting">
^
y2 |- - - - - -----------
| /
| / slope m2
| .../
-x1 | ... |
-------------------...---------------------->
| ... | x1
|... slope m1
/ |
/ m2 |
/ |
/ |
/ |
/ |
______/ _ _ _ _ _ _ | -y3
|
</pre></div>
<p>
</p>
<p>
For printing, we recommend the following settings (the defaults):
</p>
<p>
</p>
<div class="informalexample"><pre class="programlisting">
radius == 3
x1 == 1.5
y2 == 20 (don't brighten by more than 20 L*)
y3 == 50 (can darken by up to 50 L*)
m1 == 1 (some sharpening in flat areas)
m2 == 2 (more sharpening in jaggy areas)
</pre></div>
<p>
</p>
<p>
If you want more or less sharpening, we suggest you just change the m1
and m2 parameters.
</p>
<p>
The <em class="parameter"><code>radius</code></em> parameter changes the width of the fringe and can be
adjusted according to the output printing resolution. As an approximate
guideline, use 1 for 4 pixels/mm (CRT display resolution), 2 for 8
pixels/mm, 3 for 12 pixels/mm and 4 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 href="http://http://www.vips.ecs.soton.ac.uk/libvips-convolution.html#im-conv"><code class="function">im_conv()</code></a>.
</p>
<div class="variablelist"><table border="0" class="variablelist">
<colgroup>
<col align="left" valign="top">
<col>
</colgroup>
<tbody>
<tr>
<td><p><span class="term"><em class="parameter"><code>in</code></em> :</span></p></td>
<td>input image</td>
</tr>
<tr>
<td><p><span class="term"><em class="parameter"><code>out</code></em> :</span></p></td>
<td>output image</td>
</tr>
<tr>
<td><p><span class="term"><em class="parameter"><code>...</code></em> :</span></p></td>
<td>
<code class="literal">NULL</code>-terminated list of optional named arguments</td>
</tr>
<tr>
<td><p><span class="term"><span class="emphasis"><em>Returns</em></span> :</span></p></td>
<td>0 on success, -1 on error.</td>
</tr>
</tbody>
</table></div>
</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">int</span> radius</code></em>,
<em class="parameter"><code>...</code></em>);</pre>
<p>
Optional arguments:
</p>
<p>
<em class="parameter"><code>precision</code></em>: <a class="link" href="libvips-convolution.html#VipsPrecision" title="enum VipsPrecision"><span class="type">VipsPrecision</span></a> for blur
</p>
<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.
</p>
<p>
<em class="parameter"><code>radius</code></em> is not used directly. Instead the standard deviation of
<a class="link" href="libvips-create.html#vips-gaussmat" title="vips_gaussmat ()"><code class="function">vips_gaussmat()</code></a> is set to <em class="parameter"><code>radius</code></em> / 2 and the minimum amplitude set to 20%.
This gives a mask radius of approximately <em class="parameter"><code>radius</code></em> pixels.
</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-conv" title="vips_conv ()"><code class="function">vips_conv()</code></a>.
</p>
<div class="variablelist"><table border="0" class="variablelist">
<colgroup>
<col align="left" valign="top">
<col>
</colgroup>
<tbody>
<tr>
<td><p><span class="term"><em class="parameter"><code>in</code></em> :</span></p></td>
<td>input image</td>
</tr>
<tr>
<td><p><span class="term"><em class="parameter"><code>out</code></em> :</span></p></td>
<td>output image</td>
</tr>
<tr>
<td><p><span class="term"><em class="parameter"><code>radius</code></em> :</span></p></td>
<td>how large a mask to use</td>
</tr>
<tr>
<td><p><span class="term"><em class="parameter"><code>...</code></em> :</span></p></td>
<td>
<code class="literal">NULL</code>-terminated list of optional named arguments</td>
</tr>
<tr>
<td><p><span class="term"><span class="emphasis"><em>Returns</em></span> :</span></p></td>
<td>0 on success, -1 on error.</td>
</tr>
</tbody>
</table></div>
</div>
</div>
</div>
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