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<H1><A NAME="12">12 Anti-Spam Challenges</A></H1>
<P> JpGraph includes a module to easily generate Anti-Spam challenges.
 With this we mean an image consisting of numbers and letters which are
 very hard to read for an OCR program. This is often used to secure sign
 up for bulletin boards, on-line mail system or mailing lists since this
 very effectively prevent automatic sign up by robots (spam-bots).</P>
<P> The module in JpGraph responsible for this is<I>
 jpgraph_antispam.php</I> and behaves as a simplified plot module.</P>
<P> The example below illustrates how such an image can look like<DIV class="example">
<BR> <A href="exframes/frame_antispamex01.html" target="blank"><IMG border="0"
HEIGHT="34"  src="img/antispamex01.png" WIDTH="101"></A>
<BR><B>Figure 191:</B> Simple illustration of spam challenge image <A href="exframes/frame_antispamex01.html"
target="blank">[src]</A>&nbsp;
<P></P>
</DIV></P>
<P>Anti spam images have less functionality then the usual images
 generated with JpGraph in order to keep this utility reasonable small.
 The primary limitation is that there are now additional formatting
 options for the images and the image generated will always use the<I>
 JPEG</I> image format. Hence it is not possible to change this to use,
 for example, PNG.</P>
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