50 lines
5.7 KiB
HTML
50 lines
5.7 KiB
HTML
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<html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><title>What is an image?</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">What is an image?</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="What is an image?"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2494175"></a>What is an image?</h2></div></div></div>
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<p>This might be strange question and you might already now the answer but having a
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true understanding will help. An image is simply a data stream of octets (bytes)
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specifying the exact color of a number of pixels. A typical image uses 32 bits to
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specify the color which means it can encode 2^32 different colors (Homework: How
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many gray levels is possible with 32 bits?). It would be perfectly possible to send
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this data stream as it is and indeed this is how the bitmap (BMP) format specifies
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the data. However, image data usually have a lot of redundancies that can be used to
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reduce the size of the data that needs to be transmitted to specify any given image.
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These compression formats are what is more commonly known as for example PNG, JPEG
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or GIF type of images. So it is actually more correct, in our view, to look upon
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these formats as different compression techniques rather than as true image formats. </p>
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<p>Contrary to popular belief it is not the suffix on a file per se that identifies a
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particular file as an image on a HTTP server but rather the mapping on the server
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that instructs the server to send back a particular header for a particular file
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type (as identified by the file suffix), the MIME type. This might not seem like an
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important distinction to make but keeping this in mind will help understanding how
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images are created with a PHP script.</p>
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<p>A normal PHP scripts sends back character data (or more likely HTML encoded
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character data) that is displayed by the browser. However it is nothing that stops
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the PHP script from sending back data that instead should be interpreted as an image
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assuming of course we know how to create such data.. </p>
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<p>How the client (i.e. the browser) should interpret the data sent is all controlled
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by the header that is sent before the data. So this now leads us to the idea that if
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our PHP script by some means constructs a valid image, compressed in a known format
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(say PNG), we can tell the client to interpret the data we send back as an image by
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just making sure we first send a header indicating that the data is an image.</p>
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<p>This could now lead us to the perhaps surprising but crucial insight that if we
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call our script that sends back valid image data "<code class="filename">myimage.php</code>"
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and open it in a browser, for example by opening the local URI
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"<code class="filename">http://localhost/myimage.php</code>" for the browser this is no
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different then opening an image directly (for example
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"<code class="filename">http://localhost/myimage.png</code>") </p>
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<p>The only difference is that in the first case (our PHP script) we create the
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header and data our self and in the second case the HTTP server looks up the file
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suffix (*.png) to find out what kind of header to send before reading the file and
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just passing on the data to the client.</p>
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<p>Armed with this new knowledge we realize that our PHP script <span class="italic">is</span> an image for all practical purposes and intent from
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a clients point of view. So we could safely name our PHP script as a target for an
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<span class="markup"><img></span> tag. This gives us the final and most common way to
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call a graph script in a HTML page. </p>
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<p><code class="code"><img src="myimage.php"></code></p>
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<p>in exactly the same way as we would do with the old type of images. The "*.php"
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suffix is no problem since the client (e.g. the browser) don't care what the file
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ending is. It only cares about what header the server returns. If that header tells
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the browser that the data received should be interpreted as a PNG image it will do
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so regardless of the name of the source file on the server.</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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