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<title>Magick++ API: Working with Pixels </title>
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<h1 align="center">Magick::Pixels</h1>
<p>The <i>Pixels</i> class provides efficient access to raw image
pixels. Image pixels (of type <a href="Quantum.html"><i>Quantum</i></a>)
may be accessed directly via the <i>Image Pixel Cache</i>. The image
pixel cache is a rectangular window (a view) into the actual image
pixels (which may be in memory, memory-mapped from a disk file, or
entirely on disk). Obtain existing image pixels via <i>get()</i>.
Create a new pixel region using <i>set().</i> </p>
<p>Depending on the capabilities of the operating system, and the
relationship of the window to the image, the pixel cache may be a copy
of the pixels in the selected window, or it may be the actual image
pixels. In any case calling <i>sync()</i> insures that the base image
is updated with the contents of the modified pixel cache. The method <i>decode()</i>supports
copying foreign pixel data formats into the pixel cache according to
the <i>QuantumTypes</i>. The method <i>encode()</i> supports copying
the pixels in the cache to a foreign pixel representation according to
the format specified by <i>QuantumTypes</i>. </p>
<p>Setting a view using the Pixels class does not cause the number of
references to the underlying image to be reduced to one. Therefore, in
order to ensure that only the current generation of the image is
modified, the Image's <a href="https://imagemagick.org/Magick++/Image++.html#modifyImage">modifyImage()</a>
method should be invoked to reduce the reference count on the underlying
image to one. If this is not done, then it is possible for a previous
generation of the image to be modified due to the use of reference
counting when copying or constructing an Image. </p>
<p>The <i>Quantum</i>* returned by the <i>set</i> and <i>get</i>
methods, and the <i>void</i>* returned by the <i>indexes</i>
method point to pixel data managed by the <i>Pixels</i> class. The <i>Pixels</i>
class is responsible for releasing resources associated with the pixel
view. This means that the pointer should never be passed to delete() or
free(). </p>
<p style="margin-bottom: 0cm;">The pixel view is a small image in which
the pixels may be accessed, addressed, and updated, as shown in the
following example, which produces an image similar to the one on the
right (minus lines and text): </p>
<p class="image"><img class="icon" src="Cache.png" name="Graphic1" align="bottom" width="254" border="0" /></p>
<div class="viewport">
#include <Magick++.h>
#include <iostream>
using namespace std;
using namespace Magick;
int main(int argc,char **argv)
{
InitializeMagick(*argv);
// Create base image
Image image(Geometry(254,218), "white");
// Set the image type to TrueColor DirectClass representation.
image.type(TrueColorType);
// Ensure that there is only one reference to underlying image
// If this is not done, then image pixels will not be modified.
image.modifyImage();
// Allocate pixel view
Pixels view(image);
// Set all pixels in region anchored at 38x36, with size 160x230 to green.
size_t columns = 196; size_t rows = 162;
Color green("green");
Quantum *pixels = view.get(38,36,columns,rows);
for ( ssize_t row = 0; row < rows ; ++row )
for ( ssize_t column = 0; column < columns ; ++column )
{
*pixels++=QuantumRange*green.quantumRed();
*pixels++=QuantumRange*green.quantumGreen();
*pixels++=QuantumRange*green.quantumBlue();
}
// Save changes to image.
view.sync();
// Set all pixels in region anchored at 86x72, with size 108x67 to yellow.
columns = 108; rows = 67;
Color yellow("yellow");
pixels = view.get(86,72,columns,rows);
for ( ssize_t row = 0; row < rows ; ++row )
for ( ssize_t column = 0; column < columns ; ++column )
{
*pixels++=QuantumRange*yellow.quantumRed();
*pixels++=QuantumRange*yellow.quantumGreen();
*pixels++=QuantumRange*yellow.quantumBlue();
}
view.sync();
// Set pixel at position 108,94 to red
Color red("red");
pixels = view.get(108,94,1,1);
*pixels++=QuantumRange*red.quantumRed();
*pixels++=QuantumRange*red.quantumGreen();
*pixels++=QuantumRange*red.quantumBlue();
// Save changes to image.
view.sync();
image.write( "logo.png" );
}
</div>
<p style="margin-bottom: 0cm;"><i>Pixels</i> supports the following
methods: </p>
<p align="center" style="margin-bottom: 0cm;"><b>Pixel Cache Methods</b></p>
<table width="100%" border="1" cellpadding="2" cellspacing="2">
<tbody>
<tr>
<td>
<p align="center"><b>Method</b></p>
</td>
<td>
<p align="center"><b>Returns</b></p>
</td>
<td>
<p align="center"><b>Signature</b></p>
</td>
<td>
<p align="center"><b>Description</b></p>
</td>
</tr>
<tr>
<td>
<p align="center"><a name="get"></a><font size="2">get</font></p>
</td>
<td>
<p><font size="2"><a href="Quantum.html">Quantum</a>*</font></p>
</td>
<td>
<p><font size="2">const ssize_t x_, const ssize_t y_, const size_t
columns_, const size_t rows_</font></p>
</td>
<td>
<p><font size="2">Transfers read-write pixels from the image to
the pixel cache as defined by the specified rectangular region.
Modified pixels may be subsequently transferred back to the image
via <i>sync</i>. The value returned is intended for pixel access
only. It should never be deallocated.</font></p>
</td>
</tr>
<tr>
<td>
<p align="center"><a name="getConst"></a><font size="2">getConst</font></p>
</td>
<td>
<p><font size="2">const <a href="Quantum.html">Quantum</a>*</font></p>
</td>
<td>
<p><font size="2">const ssize_t x_, const ssize_t y_, const size_t
columns_, const size_t rows_</font></p>
</td>
<td>
<p><font size="2">Transfers read-only pixels from the image to
the pixel cache as defined by the specified rectangular region.</font></p>
</td>
</tr>
<tr>
<td>
<p align="center"><a name="set"></a><font size="2">set</font></p>
</td>
<td>
<p><font size="2"><a href="Quantum.html">Quantum</a>*</font></p>
</td>
<td>
<p><font size="2">const ssize_t x_, const ssize_t y_, const size_t
columns_, const size_t rows_</font></p>
</td>
<td>
<p><font size="2">Allocates a pixel cache region to store image
pixels as defined by the region rectangle. This area is
subsequently transferred from the pixel cache to the image via <i>sync</i>.
The value returned is intended for pixel access only. It should
never be deallocated.</font></p>
</td>
</tr>
<tr>
<td>
<p align="center"><a name="sync"></a><font size="2">sync</font></p>
</td>
<td>
<p><font size="2">void</font></p>
</td>
<td>
<p><font size="2">void</font></p>
</td>
<td>
<p><font size="2">Transfers the image cache pixels to the image.</font></p>
</td>
</tr>
<tr>
<td>
<p align="center"><a name="indexes"></a><font size="2">indexes</font></p>
</td>
<td>
<p><font size="2">void*</font></p>
</td>
<td>
<p><font size="2">void</font></p>
</td>
<td>
<p><font size="2">Returns the PsuedoColor pixel indexes
corresponding to the pixel region defined by the last <a href="Pixels.html#get">get</a>
, <a href="Pixels.html#getConst">getConst</a>, or <a href="Pixels.html#set">set</a>
call. Only valid for PseudoColor and CMYKA images. The pixel
indexes (an array of type <i>void</i>, which is typedef <i>Quantum</i>,
which is itself typedef <i>unsigned char</i>, or <i>unsigned short</i>,
depending on the value of the <i>QuantumDepth </i>define) provide
the <span lang="en-US">colormap</span> index (see <a
href="https://imagemagick.org/Magick++/Image++.html#colorMap">colorMap</a>) for each pixel in the
image. For CMYKA images, the indexes represent the black
channel. The value returned is intended for pixel access only. It
should never be deallocated.</font></p>
</td>
</tr>
<tr>
<td>
<p align="center"><a name="x"></a><font size="2">x</font></p>
</td>
<td>
<p><font size="2">int</font></p>
</td>
<td>
<p><font size="2">void</font></p>
</td>
<td>
<p><font size="2">Left ordinate of view</font></p>
</td>
</tr>
<tr>
<td>
<p align="center"><a name="y"></a><font size="2">y</font></p>
</td>
<td>
<p><font size="2">int</font></p>
</td>
<td>
<p><font size="2">void</font></p>
</td>
<td>
<p><font size="2">Top ordinate of view</font></p>
</td>
</tr>
<tr>
<td>
<p align="center"><a name="columns"></a><font size="2">columns</font></p>
</td>
<td>
<p><font size="2">size_t</font></p>
</td>
<td>
<p><font size="2">void</font></p>
</td>
<td>
<p><font size="2">Width of view</font></p>
</td>
</tr>
<tr>
<td>
<p align="center"><a name="rows"></a><font size="2">rows</font></p>
</td>
<td>
<p><font size="2">size_t</font></p>
</td>
<td>
<p><font size="2">void</font></p>
</td>
<td>
<p><font size="2">Height of view</font></p>
</td>
</tr>
</tbody>
</table>
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