52 lines
7.0 KiB
HTML
52 lines
7.0 KiB
HTML
<html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><title>Chapter 5. Fundamentals of dynamic 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="pt02.html" title="Part II. Basic graph creation"></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">Chapter 5. Fundamentals of dynamic graph generation</th></tr><tr><td width="20%" align="left"> </td><th width="60%" align="center">Part II. Basic graph creation</th><td width="20%" align="right"> </td></tr></table><hr></div><div class="chapter" title="Chapter 5. Fundamentals of dynamic graph generation"><div class="titlepage"><div><div><h2 class="title"><a name="chap.fund-dynamic-graph"></a>Chapter 5. Fundamentals of dynamic graph generation</h2></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="sect1"><a href="ch05.html#sec1.making-sense-of-HTTP-streams">5.1. Making sense of HTTP streams and MIME types</a></span></dt><dt><span class="sect1"><a href="ch05s02.html">5.2. What is an image?</a></span></dt><dt><span class="sect1"><a href="ch05s03.html">5.3. Static vs dynamic images</a></span></dt><dt><span class="sect1"><a href="ch05s04.html">5.4. Dynamic images on the command line</a></span></dt><dt><span class="sect1"><a href="ch05s05.html">5.5. How to generate images with JpGraph library</a></span></dt><dd><dl><dt><span class="sect2"><a href="ch05s05.html#id2494467">5.5.1. The standard steps of setting up a graph</a></span></dt><dt><span class="sect2"><a href="ch05s05.html#id2494641">5.5.2. Choosing the image compression format for JpGraph</a></span></dt><dt><span class="sect2"><a href="ch05s05.html#id2494706">5.5.3. Sending back the image to the browser</a></span></dt><dt><span class="sect2"><a href="ch05s05.html#sec2.writing-miage-to-file">5.5.4. Writing the image directly to a file</a></span></dt><dt><span class="sect2"><a href="ch05s05.html#id2494959">5.5.5. Alternatives to streaming or storing the image</a></span></dt><dt><span class="sect2"><a href="ch05s05.html#sec2.forcing-browser-update">5.5.6. Forcing the browser to update your graph</a></span></dt><dt><span class="sect2"><a href="ch05s05.html#id2495089">5.5.7. Printing the generated image</a></span></dt></dl></dd><dt><span class="sect1"><a href="ch05s06.html">5.6. Efficient graph generation using the built-in cache subsystem</a></span></dt></dl></div>
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<p title="What you will learn in this chapter">
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<b>What you will learn in this chapter. </b>
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The purpose of this chapter is to put dynamic image generation in perspective and
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illustrate how HTML tags is used to call image generating scripts. Even if You are
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familiar with PHP it is strongly recommended to quickly browse through this chapter
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to make sure all concepts are known. If You fully understand and can explain the
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concept of MIME types, HTTP streams and why the "<span class="italic">Headers already sent error</span>" error is a very common
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error when generating dynamic images with PHP it is probably safe to skip this
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chapter. Otherwise it might be wise to read through this chapter at least
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once.
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</p>
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<div class="sect1" title="Making sense of HTTP streams and MIME types"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="sec1.making-sense-of-HTTP-streams"></a>Making sense of HTTP streams and MIME types</h2></div></div></div>
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<p>The following explanation is slightly simplified since a full description of the
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HTTP protocol would bring us a bit too far in this manual </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem">
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<p>A client (e.g. browser) requests data from the server by issuing a GET
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(or possible a POST) command to the server. This is what happens when
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you enter a URI in the address bar in the browser.</p>
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</li><li class="listitem">
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<p>The server replies with a data stream (or an error if the requested
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data wasn't available). This data stream is prepended with header (MIME
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header) that tells the client (e.g. the browser) how to interpret the
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data that follows. The most common type (and the default type if no
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header is sent by a faulty server) is "<span class="bold"><strong>text/html</strong></span>" . This tells the client to
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interpret the data as plain text with embedded HTML encoding. </p>
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<p>When the data is to be interpreted as an image the header will instead
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be one of the image headers, for example "<span class="bold"><strong>image/png</strong></span>" or "<span class="bold"><strong>image/jpeg</strong></span>". When the client receives this
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header it will interpret all the following data as an image encoded in
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the indicated format. </p>
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<p>The important thing to keep in mind here is that each server reply
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(initiated by a call from the client) can only have one and only one
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MIME type. This is the key to further understanding the specific issues
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with dynamic image generation. This explains why if a PHP script running
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on the server sends a header first indicating that the following data it
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sends should be interpreted by the client as an image it cannot also
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include text since only one header can be used for one HTTP stream.
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</p>
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</li></ol></div>
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<p>What happens on a WEB-page with, for example, multiple <span class="markup"><img></span>
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tags is that the browser issues a separate GET command for each of these images. So
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even though it can look like a fetching a single WEB-page can have different content
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each individual request to the server only have one single MIME type.</p>
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</div>
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