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166 lines
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<html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><title>Using anti-aliasing in the graph generation</title><link rel="stylesheet" type="text/css" href="manual.css"><meta name="generator" content="DocBook XSL Stylesheets V1.76.0"><link rel="home" href="index.html" title="JpGraph Manual"><link rel="up" href="ch14.html" title="Chapter 14. Common features for all Cartesian (x,y) graph types"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Using anti-aliasing in the graph generation</th></tr><tr><td width="20%" align="left"> </td><th width="60%" align="center">Chapter 14. Common features for all Cartesian (x,y) graph types</th><td width="20%" align="right"> </td></tr></table><hr></div><div class="sect1" title="Using anti-aliasing in the graph generation"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2551867"></a>Using anti-aliasing in the graph generation</h2></div></div></div>
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<p>The library have partial support for the use of anti-aliasing when crating graphs.
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The level of support is different depending on the actual graph type and there are
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some caveats to be aware of.</p>
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<p>
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</p><div class="note" title="Note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3>
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<p>One question that is often asked is why does the library not support full
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anti-aliasing in every type of graphs? The answer is that since neither GD
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graphic library does not support real anti-aliasing this is done in the
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library itself most of the time. Doing full image anti-aliasing is very CPU
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intensive and can for large graph easily take four times longer than no
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anti-aliasing. In addition the way full image anti-aliasing is done is by
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creating larger canvas which the graph is drawn on and then do a smoothing
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operation, basically scale the image to the correct size puts some
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limitation on the details resolution. For example text needs special
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handling since the TTF fonts are already anti-aliased and cannot be
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re-anti-aliased a second time.</p>
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</div><p>
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</p>
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<div class="sect2" title="Anti-aliasing for line drawing graphs"><div class="titlepage"><div><div><h3 class="title"><a name="id2551920"></a>Anti-aliasing for line drawing graphs</h3></div></div></div>
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<p>This applies to all graphs that makes use if line drawing. For example line
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plots and radar charts. This is the only "true" anti-aliasing done in the
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library. </p>
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<p>This anti aliasing is enabled by calling the method</p>
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<p>
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</p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
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<p><code class="code">Image::SetAntiAliasing($aFlg=true)</code></p>
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</li></ul></div><p>
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</p>
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<p>As a concrete example the following line can be used to enable anti-aliasing
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for both line and wind rose plots</p>
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<p>
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</p><div class="hl-main"><table class="hl-table" width="100%"><tr><td class="hl-gutter" align="right" valign="top"><pre>1
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</pre></td><td class="hl-main" valign="top"><pre><span class="hl-code">$graph->img->SetAntiAliasing();</span></pre></td></tr></table></div><p>
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</p>
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<p>The algorithm used for line anti-aliasing makes a gradient color transition
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between foreground line color and background while plotting a line using a
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standard mid-point line algorithm. To achieve best visual result always use a
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dark line color on a light colored background.</p>
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<p>The images below shows the difference between a plain graph on the left and
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the same graph drawn with anti-aliasing enabled on the right.</p>
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<p>
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</p><div class="informaltable">
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<table border="0"><colgroup><col class="c1"><col class="c2"></colgroup><tbody><tr><td>
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<div class="figure"><a name="fig.radarex8"></a><p class="title"><b>Figure 14.80. Plain radar plot <code class="uri"><a class="uri" href="example_src/radarex8.html" target="_top">(<code class="filename">radarex8.php</code>)</a></code> </b></p><div class="figure-contents"> <span class="inlinemediaobject"><img src="images/radarex8.png" alt="Plain radar plot (radarex8.php)"></span> </div></div><br class="figure-break">
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</td><td>
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<div class="figure"><a name="fig.radarex8.1"></a><p class="title"><b>Figure 14.81. Anti-aliased radar plot <code class="uri"><a class="uri" href="example_src/radarex8.1.html" target="_top">(<code class="filename">radarex8.1.php</code>)</a></code> </b></p><div class="figure-contents"> <span class="inlinemediaobject"><img src="images/radarex8.1.png" alt="Anti-aliased radar plot (radarex8.1.php)"></span> </div></div><br class="figure-break">
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</td></tr></tbody></table>
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</div><p>
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</p>
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<p>There are a number of limitations with the current implementation that should
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be noted. The reason these limitation exists is that for reasons of CPU load we
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have chosen not to implement full anti-aliasing for arbitrary shapes. </p>
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<p>
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</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem">
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<p>Speed! Doing anti-aliased lines in PHP is not cheap. Anti-aliased
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line drawing is roughly 6-8 times slower then lines without
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anti-aliasing So using anti-aliasing isn't suitable together with
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high-load often updated graphs. Remember to use the caching feature
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(see <a class="xref" href="ch05s06.html" title="Efficient graph generation using the built-in cache subsystem">Efficient graph generation using the built-in cache subsystem</a>.)</p>
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</li><li class="listitem">
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<p>Lines will ignore any width and only have a single line-width of
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approximately=1. It is not possible to set the line width when
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anti-alias is used. (This would require a full implementation of
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anti-aliased polygons which is not implemented.) </p>
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</li><li class="listitem">
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<p>The colors will be a little bit "weaker" since they will now not
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consist of one dominant color but rather being made up by at least
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two different colors. </p>
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</li><li class="listitem">
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<p>Anti-aliasing does not work very well together with background
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images since it assumes a the same solid color on each side of the
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line. Doing a more advanced anti-aliasing algorithm would simple
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take to much processing power.</p>
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</li><li class="listitem">
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<p>Even when anti-aliasing is enabled fast line-drawing will be used
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for horizontal, vertical and diagonal lines since these get sampled
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at high enough frequency anyway and doesn't benefit from
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anti-aliasing. This optimization has a slight visual impact, as can
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be seen in <a class="xref" href="ch14s13.html#fig.radarex8.1" title="Figure 14.81. Anti-aliased radar plot (radarex8.1.php)">Figure 14.81. Anti-aliased radar plot <code class="uri"><a class="uri" href="example_src/radarex8.1.html" target="_top">(<code class="filename">radarex8.1.php</code>)</a></code> </a> The vertical line
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appear to the human eye to have a slightly different color and looks
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a bit thinner. </p>
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</li></ol></div><p>
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</p>
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<p>
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</p><div class="note" title="Note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3>
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<p>The difference between a standard line and one line with anti-aliasing
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is shown in large magnification in ??. The approach used in the library
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roughly corresponds to an even weighted 3x3 Bartlett-filter. It would be
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possible to apply some more advanced filtering techniques to achieve an
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even better result but doing high-intensive 2D signal processing on a
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HTTP-server is not a brilliant idea. The algorithm used in the library
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is a reasonable trade-off between visual appearance and
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efficiency.</p>
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<p>
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</p><div class="figure"><a name="id2552190"></a><p class="title"><b>Figure 14.82. Anti-aliasing up-close. The figure shows the difference
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between a standard line (on-top) and the corresponding
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anti-aliased line (on-the bottom)</b></p><div class="figure-contents">
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<div class="informaltable">
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<table border="0"><colgroup><col class="c1"></colgroup><tbody><tr><td align="center">
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<p><span class="inlinemediaobject"><img src="images/aalias_line.jpg" alt="Anti-aliasing up-close. The figure shows the difference between a standard line (on-top) and the corresponding anti-aliased line (on-the bottom)"></span></p>
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</td></tr></tbody></table>
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</div>
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</div></div><p><br class="figure-break">
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</p>
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</div><p>
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</p>
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<p></p>
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</div>
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<div class="sect2" title="Anti-aliasing in pie graphs"><div class="titlepage"><div><div><h3 class="title"><a name="id2552235"></a>Anti-aliasing in pie graphs</h3></div></div></div>
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<p>For Pie plots a full image anti-aliasing algorithm is used. The original image
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is scaled to twice it's specified with and then down sampled to the required
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size. This will give a low pass filtering effect which takes the edges away from
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the image and gives a smooth appearance. However the filtering is quite
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processor intensive.</p>
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<p>Anti-aliasing for pie plots are enabled by calling</p>
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<p>
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</p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
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<p><code class="code">PieGraph::SetAntiAlising($aFlg=true)</code></p>
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</li></ul></div><p>
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</p>
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<p>as the following code snippet shows</p>
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<p>
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</p><div class="hl-main"><table class="hl-table" width="100%"><tr><td class="hl-gutter" align="right" valign="top"><pre>1
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</pre></td><td class="hl-main" valign="top"><pre><span class="hl-code">$piegraph->SetAntiAliasing();</span></pre></td></tr></table></div><p>
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</p>
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<p>There is inherent limitation using this method and that is that edges on the
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Pie can not be used. </p>
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<p>The figure below shows the visual impact the anti-aliasing have on both 2D and
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3D Pie plots.</p>
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<p>
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</p><div class="figure"><a name="id2552311"></a><p class="title"><b>Figure 14.83. Affects of using anti-alias for Pie-graphs</b></p><div class="figure-contents">
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<div class="informaltable">
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<table border="0"><colgroup><col class="c1"><col class="c2"></colgroup><tbody><tr><td>
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<p><span class="inlinemediaobject"><img src="images/pie2dnoaa.png" alt="Affects of using anti-alias for Pie-graphs"></span></p>
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</td><td>
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<p><span class="inlinemediaobject"><img src="images/pie2daa.png" alt="Affects of using anti-alias for Pie-graphs"></span></p>
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</td></tr><tr><td>
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<p><span class="inlinemediaobject"><img src="images/pie3dnoaa.png" alt="Affects of using anti-alias for Pie-graphs"></span></p>
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</td><td>
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<p><span class="inlinemediaobject"><img src="images/pie3daa.png" alt="Affects of using anti-alias for Pie-graphs"></span></p>
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</td></tr></tbody></table>
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</div>
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</div></div><p><br class="figure-break">
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</p>
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</div>
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<div class="sect2" title="Anti-aliasing in Windrose plots"><div class="titlepage"><div><div><h3 class="title"><a name="id2552417"></a>Anti-aliasing in Windrose plots</h3></div></div></div>
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<p>This uses the same algorithm as the Pie plots with similar restrictions and
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limitations.</p>
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</div>
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<div class="sect2" title="Anti-aliasing for Contour plots"><div class="titlepage"><div><div><h3 class="title"><a name="id2552434"></a>Anti-aliasing for Contour plots</h3></div></div></div>
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<p>This uses almost the same algorithm as Pie plots. The additional feature here
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is that it is possible to specify the size of the over-sampling. This can be
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value between 2-5. However, using an oversampling larger than 3 has very little
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additional visual improvement but will significantly increase the processing
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time. </p>
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</div>
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</div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"> </td><td width="20%" align="center"><a accesskey="u" href="ch14.html">Up</a></td><td width="40%" align="right"> </td></tr><tr><td width="40%" align="left" valign="top"> </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> </td></tr></table></div></body></html>
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