24 lines
3.4 KiB
HTML
24 lines
3.4 KiB
HTML
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<html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><title>Static vs dynamic images</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="ch05.html" title="Chapter 5. Fundamentals of dynamic graph generation"></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">Static vs dynamic images</th></tr><tr><td width="20%" align="left"> </td><th width="60%" align="center">Chapter 5. Fundamentals of dynamic graph generation</th><td width="20%" align="right"> </td></tr></table><hr></div><div class="sect1" title="Static vs dynamic images"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2494178"></a>Static vs dynamic images</h2></div></div></div>
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<p>Having understood how we can generate images using a PHP script a new idea might
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be forming. There is nothing that says that we have to create the same image using
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the same data every time the graph script is called. We could for example randomly
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select a pre-stored image in a directory and every time we call (the same) graph
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script and new image is returned. Hence, with PHP scripts we can create dynamic
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images that generate different images depending on some yet to be specified
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parameters even though the script name stays the same. This is a new behaviour
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compared with our normal images. </p>
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<p>This could also lead to a new, and very real, problem which has to do with the
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local caching most browsers do in order to avoid re-fetching already fetched objects
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such as images. Normally a browser will check the time stamp on the file on the
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server and compare with the latest fetched version it has stored locally. If the
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browser sees that the file on the server is older than what it already has it will
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not re-fetch the file. </p>
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<p>Since a dynamic image is produced by a script and the script will stay the same
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even if the produced image is changing (due to new data) the browser might still
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think that the image has not changed since the script is the same and will not
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bother re-fetching it again. The exact behavior of the browser cache is usually
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adjustable by the end user and hence out of control for us who writes the script. A
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simple way to always force the client to re-fetch the dynamic image is described in <a class="xref" href="ch05s05.html#sec2.forcing-browser-update" title="Forcing the browser to update your graph">Forcing the browser to update your graph</a></p>
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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="ch05.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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