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141 lines
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<h1><a href="../webs.html">Sources</a></h1>
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<ul>
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<li><a href="../inweb/index.html">inweb</a></li>
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</ul>
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<h2>Foundation</h2>
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<ul>
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<li><a href="../foundation-module/index.html">foundation-module</a></li>
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<li><a href="../foundation-test/index.html">foundation-test</a></li>
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<!--Weave of 'Image Dimensions' generated by 7-->
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<ul class="crumbs"><li><a href="../webs.html">Source</a></li><li><a href="index.html">foundation</a></li><li><a href="index.html#6">Chapter 6: Media</a></li><li><b>Image Dimensions</b></li></ul><p class="purpose">These utility routines look at the headers of JPEG and PNG files to find the pixel dimensions of any images supplied by the user for cover art and figures.</p>
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<ul class="toc"><li><a href="#SP1">§1. JPEG files</a></li><li><a href="#SP2">§2. PNG files</a></li></ul><hr class="tocbar">
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<p class="inwebparagraph"><a id="SP1"></a><b>§1. JPEG files. </b>The following code, contributed by Toby Nelson, either finds the pixel width
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and height of a given JPEG file and returns <code class="display"><span class="extract">TRUE</span></code> or, if it can't read the
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file or doesn't recognise the header as having JPEG format, returns <code class="display"><span class="extract">FALSE</span></code>.
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</p>
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<p class="inwebparagraph">JPEG is properly speaking not a file format but a compression technique:
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the routine below works with either JIF (JPEG Interchange Format) or its
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simpler cousin JFIF (JPEG File Interchange Format).
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</p>
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<p class="inwebparagraph">We scan the file looking for "markers", each of which begins with an
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<code class="display"><span class="extract">0xFF</span></code> byte and is followed by a marker-type byte which is neither <code class="display"><span class="extract">0x00</span></code>
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nor <code class="display"><span class="extract">0xFF</span></code>. The compulsory marker SOI must appear at the start of the file,
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providing one way to detect probable JPEGs by looking at the first two
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bytes. There must also eventually be a start of frame marker, for the
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actual image: this can have many forms, but in all cases tells us the
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height and width.
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</p>
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<pre class="display">
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<span class="reserved">int</span><span class="plain"> </span><span class="functiontext">ImageFiles::get_JPEG_dimensions</span><span class="plain">(</span><span class="reserved">FILE</span><span class="plain"> *</span><span class="identifier">JPEG_file</span><span class="plain">, </span><span class="reserved">unsigned</span><span class="plain"> </span><span class="reserved">int</span><span class="plain"> *</span><span class="identifier">width</span><span class="plain">, </span><span class="reserved">unsigned</span><span class="plain"> </span><span class="reserved">int</span><span class="plain"> *</span><span class="identifier">height</span><span class="plain">) {</span>
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<span class="reserved">unsigned</span><span class="plain"> </span><span class="reserved">int</span><span class="plain"> </span><span class="identifier">sig</span><span class="plain">, </span><span class="identifier">length</span><span class="plain">;</span>
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<span class="reserved">int</span><span class="plain"> </span><span class="identifier">marker</span><span class="plain">;</span>
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<span class="reserved">if</span><span class="plain"> (!</span><span class="functiontext">BinaryFiles::read_int16</span><span class="plain">(</span><span class="identifier">JPEG_file</span><span class="plain">, &</span><span class="identifier">sig</span><span class="plain">)) </span><span class="reserved">return</span><span class="plain"> </span><span class="constant">FALSE</span><span class="plain">;</span>
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<span class="reserved">if</span><span class="plain"> (</span><span class="identifier">sig</span><span class="plain"> != </span><span class="constant">0xFFD8</span><span class="plain">) </span><span class="reserved">return</span><span class="plain"> </span><span class="constant">FALSE</span><span class="plain">; </span><span class="comment"> <code class="display"><span class="extract">0xFF</span></code> (marker) then <code class="display"><span class="extract">0xD8</span></code> (SOI)</span>
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<span class="reserved">do</span><span class="plain"> {</span>
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<span class="reserved">do</span><span class="plain"> {</span>
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<span class="identifier">marker</span><span class="plain"> = </span><span class="identifier">getc</span><span class="plain">(</span><span class="identifier">JPEG_file</span><span class="plain">);</span>
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<span class="reserved">if</span><span class="plain"> (</span><span class="identifier">marker</span><span class="plain"> == </span><span class="identifier">EOF</span><span class="plain">) </span><span class="reserved">return</span><span class="plain"> </span><span class="constant">FALSE</span><span class="plain">;</span>
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<span class="plain">} </span><span class="reserved">while</span><span class="plain"> (</span><span class="identifier">marker</span><span class="plain"> != </span><span class="constant">0xff</span><span class="plain">); </span><span class="comment"> skip to next <code class="display"><span class="extract">0xFF</span></code> byte</span>
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<span class="reserved">do</span><span class="plain"> {</span>
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<span class="identifier">marker</span><span class="plain"> = </span><span class="identifier">getc</span><span class="plain">(</span><span class="identifier">JPEG_file</span><span class="plain">);</span>
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<span class="plain">} </span><span class="reserved">while</span><span class="plain"> (</span><span class="identifier">marker</span><span class="plain"> == </span><span class="constant">0xff</span><span class="plain">); </span><span class="comment"> skip to next non <code class="display"><span class="extract">FF</span></code> byte</span>
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<span class="reserved">if</span><span class="plain"> (!</span><span class="functiontext">BinaryFiles::read_int16</span><span class="plain">(</span><span class="identifier">JPEG_file</span><span class="plain">, &</span><span class="identifier">length</span><span class="plain">)) </span><span class="reserved">return</span><span class="plain"> </span><span class="constant">FALSE</span><span class="plain">; </span><span class="comment"> length of marker</span>
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<span class="reserved">switch</span><span class="plain">(</span><span class="identifier">marker</span><span class="plain">) {</span>
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<span class="comment"> all variant forms of "start of frame": e.g., <code class="display"><span class="extract">0xC0</span></code> is a baseline DCT image</span>
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<span class="reserved">case</span><span class="plain"> </span><span class="constant">0xc0</span><span class="identifier">:</span>
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<span class="reserved">case</span><span class="plain"> </span><span class="constant">0xc1</span><span class="identifier">:</span><span class="plain"> </span><span class="reserved">case</span><span class="plain"> </span><span class="constant">0xc2</span><span class="identifier">:</span><span class="plain"> </span><span class="reserved">case</span><span class="plain"> </span><span class="constant">0xc3</span><span class="identifier">:</span>
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<span class="reserved">case</span><span class="plain"> </span><span class="constant">0xc5</span><span class="identifier">:</span><span class="plain"> </span><span class="reserved">case</span><span class="plain"> </span><span class="constant">0xc6</span><span class="identifier">:</span><span class="plain"> </span><span class="reserved">case</span><span class="plain"> </span><span class="constant">0xc7</span><span class="identifier">:</span>
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<span class="reserved">case</span><span class="plain"> </span><span class="constant">0xc9</span><span class="identifier">:</span><span class="plain"> </span><span class="reserved">case</span><span class="plain"> </span><span class="constant">0xca</span><span class="identifier">:</span><span class="plain"> </span><span class="reserved">case</span><span class="plain"> </span><span class="constant">0xcb</span><span class="identifier">:</span>
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<span class="reserved">case</span><span class="plain"> </span><span class="constant">0xcd</span><span class="identifier">:</span><span class="plain"> </span><span class="reserved">case</span><span class="plain"> </span><span class="constant">0xce</span><span class="identifier">:</span><span class="plain"> </span><span class="reserved">case</span><span class="plain"> </span><span class="constant">0xcf</span><span class="identifier">:</span><span class="plain"> {</span>
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<span class="comment"> fortunately these markers all then open with the same format</span>
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<span class="reserved">if</span><span class="plain"> (</span><span class="identifier">getc</span><span class="plain">(</span><span class="identifier">JPEG_file</span><span class="plain">) == </span><span class="identifier">EOF</span><span class="plain">) </span><span class="reserved">return</span><span class="plain"> </span><span class="constant">FALSE</span><span class="plain">; </span><span class="comment"> skip 1 byte of data precision</span>
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<span class="reserved">if</span><span class="plain"> (!</span><span class="functiontext">BinaryFiles::read_int16</span><span class="plain">(</span><span class="identifier">JPEG_file</span><span class="plain">, </span><span class="identifier">height</span><span class="plain">)) </span><span class="reserved">return</span><span class="plain"> </span><span class="constant">FALSE</span><span class="plain">;</span>
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<span class="reserved">if</span><span class="plain"> (!</span><span class="functiontext">BinaryFiles::read_int16</span><span class="plain">(</span><span class="identifier">JPEG_file</span><span class="plain">, </span><span class="identifier">width</span><span class="plain">)) </span><span class="reserved">return</span><span class="plain"> </span><span class="constant">FALSE</span><span class="plain">;</span>
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<span class="reserved">return</span><span class="plain"> </span><span class="constant">TRUE</span><span class="plain">;</span>
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<span class="plain">}</span>
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<span class="identifier">default:</span>
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<span class="reserved">if</span><span class="plain"> (</span><span class="identifier">fseek</span><span class="plain">(</span><span class="identifier">JPEG_file</span><span class="plain">, (</span><span class="reserved">long</span><span class="plain">) (</span><span class="identifier">length</span><span class="plain"> - </span><span class="constant">2</span><span class="plain">), </span><span class="identifier">SEEK_CUR</span><span class="plain">) != </span><span class="constant">0</span><span class="plain">) </span><span class="reserved">return</span><span class="plain"> </span><span class="constant">FALSE</span><span class="plain">; </span><span class="comment"> skip rest of marker</span>
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<span class="plain">}</span>
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<span class="plain">}</span>
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<span class="reserved">while</span><span class="plain"> (</span><span class="identifier">marker</span><span class="plain"> != </span><span class="identifier">EOF</span><span class="plain">);</span>
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<span class="reserved">return</span><span class="plain"> </span><span class="constant">FALSE</span><span class="plain">;</span>
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<span class="plain">}</span>
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</pre>
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<p class="inwebparagraph"></p>
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<p class="endnote">The function ImageFiles::get_JPEG_dimensions appears nowhere else.</p>
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<p class="inwebparagraph"><a id="SP2"></a><b>§2. PNG files. </b>The PNG file must start with a signature which indicates that the
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remainder contains a single PNG image, consisting of a series of chunks
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beginning with an IHDR chunk and ending with an IEND chunk ("Portable
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Network Graphics (PNG) Specification", 2nd edition, section 5.2). We only
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need to scan the IHDR chunk, of which the pixel width and height are the
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first two words (section 11.2.2).
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</p>
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<pre class="display">
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<span class="reserved">int</span><span class="plain"> </span><span class="functiontext">ImageFiles::get_PNG_dimensions</span><span class="plain">(</span><span class="reserved">FILE</span><span class="plain"> *</span><span class="identifier">PNG_file</span><span class="plain">, </span><span class="reserved">unsigned</span><span class="plain"> </span><span class="reserved">int</span><span class="plain"> *</span><span class="identifier">width</span><span class="plain">, </span><span class="reserved">unsigned</span><span class="plain"> </span><span class="reserved">int</span><span class="plain"> *</span><span class="identifier">height</span><span class="plain">) {</span>
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<span class="reserved">unsigned</span><span class="plain"> </span><span class="reserved">int</span><span class="plain"> </span><span class="identifier">sig1</span><span class="plain">, </span><span class="identifier">sig2</span><span class="plain">, </span><span class="identifier">length</span><span class="plain">, </span><span class="identifier">type</span><span class="plain">;</span>
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<span class="comment"> Check PNG signature</span>
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<span class="reserved">if</span><span class="plain"> (!</span><span class="functiontext">BinaryFiles::read_int32</span><span class="plain">(</span><span class="identifier">PNG_file</span><span class="plain">, &</span><span class="identifier">sig1</span><span class="plain">)) </span><span class="reserved">return</span><span class="plain"> </span><span class="constant">FALSE</span><span class="plain">;</span>
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<span class="reserved">if</span><span class="plain"> (!</span><span class="functiontext">BinaryFiles::read_int32</span><span class="plain">(</span><span class="identifier">PNG_file</span><span class="plain">, &</span><span class="identifier">sig2</span><span class="plain">)) </span><span class="reserved">return</span><span class="plain"> </span><span class="constant">FALSE</span><span class="plain">;</span>
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<span class="reserved">if</span><span class="plain"> ((</span><span class="identifier">sig1</span><span class="plain"> != </span><span class="constant">0x89504e47</span><span class="plain">) || (</span><span class="identifier">sig2</span><span class="plain"> != </span><span class="constant">0x0d0a1a0a</span><span class="plain">)) </span><span class="reserved">return</span><span class="plain"> </span><span class="constant">FALSE</span><span class="plain">;</span>
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<span class="comment"> Read first chunk</span>
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<span class="reserved">if</span><span class="plain"> (!</span><span class="functiontext">BinaryFiles::read_int32</span><span class="plain">(</span><span class="identifier">PNG_file</span><span class="plain">, &</span><span class="identifier">length</span><span class="plain">)) </span><span class="reserved">return</span><span class="plain"> </span><span class="constant">FALSE</span><span class="plain">;</span>
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<span class="reserved">if</span><span class="plain"> (!</span><span class="functiontext">BinaryFiles::read_int32</span><span class="plain">(</span><span class="identifier">PNG_file</span><span class="plain">, &</span><span class="identifier">type</span><span class="plain">)) </span><span class="reserved">return</span><span class="plain"> </span><span class="constant">FALSE</span><span class="plain">;</span>
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<span class="comment"> First chunk must be IHDR</span>
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<span class="reserved">if</span><span class="plain"> (</span><span class="identifier">type</span><span class="plain"> != </span><span class="constant">0x49484452</span><span class="plain">) </span><span class="reserved">return</span><span class="plain"> </span><span class="constant">FALSE</span><span class="plain">;</span>
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<span class="comment"> Width and height follow</span>
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<span class="reserved">if</span><span class="plain"> (!</span><span class="functiontext">BinaryFiles::read_int32</span><span class="plain">(</span><span class="identifier">PNG_file</span><span class="plain">, </span><span class="identifier">width</span><span class="plain">)) </span><span class="reserved">return</span><span class="plain"> </span><span class="constant">FALSE</span><span class="plain">;</span>
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<span class="reserved">if</span><span class="plain"> (!</span><span class="functiontext">BinaryFiles::read_int32</span><span class="plain">(</span><span class="identifier">PNG_file</span><span class="plain">, </span><span class="identifier">height</span><span class="plain">)) </span><span class="reserved">return</span><span class="plain"> </span><span class="constant">FALSE</span><span class="plain">;</span>
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<span class="reserved">return</span><span class="plain"> </span><span class="constant">TRUE</span><span class="plain">;</span>
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<span class="plain">}</span>
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</pre>
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<p class="inwebparagraph"></p>
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<p class="endnote">The function ImageFiles::get_PNG_dimensions appears nowhere else.</p>
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<hr class="tocbar">
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<ul class="toc"><li><a href="6-bf.html">Back to 'Binary Files'</a></li><li><a href="6-sd.html">Continue with 'Sound Durations'</a></li></ul><hr class="tocbar">
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