Figure 195: Simple illustration of spam challenge image [src]
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).
The module in JpGraph responsible for this is jpgraph_antispam.php and behaves as a simplified plot module.
The example below illustrates how such an image can look like
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 JPEG image format. Hence it is not possible to change this to use, for example, PNG.