148 lines
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148 lines
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<ul><li><a href="../inweb/index.html">inweb</a></li>
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<!--Weave of 'Image Dimensions' generated by Inweb-->
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<ul class="crumbs"><li><a href="../index.html">Home</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="6-id.html#SP1">§1. JPEG files</a></li><li><a href="6-id.html#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-syntax">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-syntax">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-syntax">0xFF</span></code> byte and is followed by a marker-type byte which is neither <code class="display"><span class="extract-syntax">0x00</span></code>
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nor <code class="display"><span class="extract-syntax">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="displayed-code all-displayed-code">
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<span class="reserved-syntax">int</span><span class="plain-syntax"> </span><span class="function-syntax">ImageFiles::get_JPEG_dimensions</span><button class="popup" onclick="togglePopup('usagePopup1')">...<span class="popuptext" id="usagePopup1">Usage of <b>ImageFiles::get_JPEG_dimensions</b>:<br>none</span></button><span class="plain-syntax">(</span><span class="reserved-syntax">FILE</span><span class="plain-syntax"> *</span><span class="identifier-syntax">JPEG_file</span><span class="plain-syntax">, </span><span class="reserved-syntax">unsigned</span><span class="plain-syntax"> </span><span class="reserved-syntax">int</span><span class="plain-syntax"> *</span><span class="identifier-syntax">width</span><span class="plain-syntax">, </span><span class="reserved-syntax">unsigned</span><span class="plain-syntax"> </span><span class="reserved-syntax">int</span><span class="plain-syntax"> *</span><span class="identifier-syntax">height</span><span class="plain-syntax">) {</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">unsigned</span><span class="plain-syntax"> </span><span class="reserved-syntax">int</span><span class="plain-syntax"> </span><span class="identifier-syntax">sig</span><span class="plain-syntax">, </span><span class="identifier-syntax">length</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">int</span><span class="plain-syntax"> </span><span class="identifier-syntax">marker</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (!</span><a href="6-bf.html#SP1" class="function-link"><span class="function-syntax">BinaryFiles::read_int16</span></a><span class="plain-syntax">(</span><span class="identifier-syntax">JPEG_file</span><span class="plain-syntax">, &</span><span class="identifier-syntax">sig</span><span class="plain-syntax">)) </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="constant-syntax">FALSE</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">sig</span><span class="plain-syntax"> != </span><span class="constant-syntax">0xFFD8</span><span class="plain-syntax">) </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="constant-syntax">FALSE</span><span class="plain-syntax">; </span><span class="comment"> </span><code class="display"><span class="extract-syntax">0xFF</span></code><span class="comment"> (marker) then </span><code class="display"><span class="extract-syntax">0xD8</span></code><span class="comment"> (SOI)</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">do</span><span class="plain-syntax"> {</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">do</span><span class="plain-syntax"> {</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">marker</span><span class="plain-syntax"> = </span><span class="identifier-syntax">getc</span><span class="plain-syntax">(</span><span class="identifier-syntax">JPEG_file</span><span class="plain-syntax">);</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">marker</span><span class="plain-syntax"> == </span><span class="identifier-syntax">EOF</span><span class="plain-syntax">) </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="constant-syntax">FALSE</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> } </span><span class="reserved-syntax">while</span><span class="plain-syntax"> (</span><span class="identifier-syntax">marker</span><span class="plain-syntax"> != </span><span class="constant-syntax">0xff</span><span class="plain-syntax">); </span><span class="comment"> skip to next </span><code class="display"><span class="extract-syntax">0xFF</span></code><span class="comment"> byte</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">do</span><span class="plain-syntax"> {</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">marker</span><span class="plain-syntax"> = </span><span class="identifier-syntax">getc</span><span class="plain-syntax">(</span><span class="identifier-syntax">JPEG_file</span><span class="plain-syntax">);</span>
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<span class="plain-syntax"> } </span><span class="reserved-syntax">while</span><span class="plain-syntax"> (</span><span class="identifier-syntax">marker</span><span class="plain-syntax"> == </span><span class="constant-syntax">0xff</span><span class="plain-syntax">); </span><span class="comment"> skip to next non </span><code class="display"><span class="extract-syntax">FF</span></code><span class="comment"> byte</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (!</span><a href="6-bf.html#SP1" class="function-link"><span class="function-syntax">BinaryFiles::read_int16</span></a><span class="plain-syntax">(</span><span class="identifier-syntax">JPEG_file</span><span class="plain-syntax">, &</span><span class="identifier-syntax">length</span><span class="plain-syntax">)) </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="constant-syntax">FALSE</span><span class="plain-syntax">; </span><span class="comment"> length of marker</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">switch</span><span class="plain-syntax">(</span><span class="identifier-syntax">marker</span><span class="plain-syntax">) {</span>
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<span class="plain-syntax"> </span><span class="comment"> all variant forms of "start of frame": e.g., </span><code class="display"><span class="extract-syntax">0xC0</span></code><span class="comment"> is a baseline DCT image</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">case</span><span class="plain-syntax"> </span><span class="constant-syntax">0xc0</span><span class="identifier-syntax">:</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">case</span><span class="plain-syntax"> </span><span class="constant-syntax">0xc1</span><span class="identifier-syntax">:</span><span class="plain-syntax"> </span><span class="reserved-syntax">case</span><span class="plain-syntax"> </span><span class="constant-syntax">0xc2</span><span class="identifier-syntax">:</span><span class="plain-syntax"> </span><span class="reserved-syntax">case</span><span class="plain-syntax"> </span><span class="constant-syntax">0xc3</span><span class="identifier-syntax">:</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">case</span><span class="plain-syntax"> </span><span class="constant-syntax">0xc5</span><span class="identifier-syntax">:</span><span class="plain-syntax"> </span><span class="reserved-syntax">case</span><span class="plain-syntax"> </span><span class="constant-syntax">0xc6</span><span class="identifier-syntax">:</span><span class="plain-syntax"> </span><span class="reserved-syntax">case</span><span class="plain-syntax"> </span><span class="constant-syntax">0xc7</span><span class="identifier-syntax">:</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">case</span><span class="plain-syntax"> </span><span class="constant-syntax">0xc9</span><span class="identifier-syntax">:</span><span class="plain-syntax"> </span><span class="reserved-syntax">case</span><span class="plain-syntax"> </span><span class="constant-syntax">0xca</span><span class="identifier-syntax">:</span><span class="plain-syntax"> </span><span class="reserved-syntax">case</span><span class="plain-syntax"> </span><span class="constant-syntax">0xcb</span><span class="identifier-syntax">:</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">case</span><span class="plain-syntax"> </span><span class="constant-syntax">0xcd</span><span class="identifier-syntax">:</span><span class="plain-syntax"> </span><span class="reserved-syntax">case</span><span class="plain-syntax"> </span><span class="constant-syntax">0xce</span><span class="identifier-syntax">:</span><span class="plain-syntax"> </span><span class="reserved-syntax">case</span><span class="plain-syntax"> </span><span class="constant-syntax">0xcf</span><span class="identifier-syntax">:</span><span class="plain-syntax"> {</span>
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<span class="plain-syntax"> </span><span class="comment"> fortunately these markers all then open with the same format</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">getc</span><span class="plain-syntax">(</span><span class="identifier-syntax">JPEG_file</span><span class="plain-syntax">) == </span><span class="identifier-syntax">EOF</span><span class="plain-syntax">) </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="constant-syntax">FALSE</span><span class="plain-syntax">; </span><span class="comment"> skip 1 byte of data precision</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (!</span><a href="6-bf.html#SP1" class="function-link"><span class="function-syntax">BinaryFiles::read_int16</span></a><span class="plain-syntax">(</span><span class="identifier-syntax">JPEG_file</span><span class="plain-syntax">, </span><span class="identifier-syntax">height</span><span class="plain-syntax">)) </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="constant-syntax">FALSE</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (!</span><a href="6-bf.html#SP1" class="function-link"><span class="function-syntax">BinaryFiles::read_int16</span></a><span class="plain-syntax">(</span><span class="identifier-syntax">JPEG_file</span><span class="plain-syntax">, </span><span class="identifier-syntax">width</span><span class="plain-syntax">)) </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="constant-syntax">FALSE</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="constant-syntax">TRUE</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> }</span>
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<span class="plain-syntax"> </span><span class="identifier-syntax">default:</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">fseek</span><span class="plain-syntax">(</span><span class="identifier-syntax">JPEG_file</span><span class="plain-syntax">, (</span><span class="reserved-syntax">long</span><span class="plain-syntax">) (</span><span class="identifier-syntax">length</span><span class="plain-syntax"> - </span><span class="constant-syntax">2</span><span class="plain-syntax">), </span><span class="identifier-syntax">SEEK_CUR</span><span class="plain-syntax">) != </span><span class="constant-syntax">0</span><span class="plain-syntax">) </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="constant-syntax">FALSE</span><span class="plain-syntax">; </span><span class="comment"> skip rest of marker</span>
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<span class="plain-syntax"> }</span>
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<span class="plain-syntax"> }</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">while</span><span class="plain-syntax"> (</span><span class="identifier-syntax">marker</span><span class="plain-syntax"> != </span><span class="identifier-syntax">EOF</span><span class="plain-syntax">);</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="constant-syntax">FALSE</span><span class="plain-syntax">;</span>
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<span class="plain-syntax">}</span>
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</pre><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="displayed-code all-displayed-code">
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<span class="reserved-syntax">int</span><span class="plain-syntax"> </span><span class="function-syntax">ImageFiles::get_PNG_dimensions</span><button class="popup" onclick="togglePopup('usagePopup2')">...<span class="popuptext" id="usagePopup2">Usage of <b>ImageFiles::get_PNG_dimensions</b>:<br>none</span></button><span class="plain-syntax">(</span><span class="reserved-syntax">FILE</span><span class="plain-syntax"> *</span><span class="identifier-syntax">PNG_file</span><span class="plain-syntax">, </span><span class="reserved-syntax">unsigned</span><span class="plain-syntax"> </span><span class="reserved-syntax">int</span><span class="plain-syntax"> *</span><span class="identifier-syntax">width</span><span class="plain-syntax">, </span><span class="reserved-syntax">unsigned</span><span class="plain-syntax"> </span><span class="reserved-syntax">int</span><span class="plain-syntax"> *</span><span class="identifier-syntax">height</span><span class="plain-syntax">) {</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">unsigned</span><span class="plain-syntax"> </span><span class="reserved-syntax">int</span><span class="plain-syntax"> </span><span class="identifier-syntax">sig1</span><span class="plain-syntax">, </span><span class="identifier-syntax">sig2</span><span class="plain-syntax">, </span><span class="identifier-syntax">length</span><span class="plain-syntax">, </span><span class="identifier-syntax">type</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="comment"> Check PNG signature</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (!</span><a href="6-bf.html#SP1" class="function-link"><span class="function-syntax">BinaryFiles::read_int32</span></a><span class="plain-syntax">(</span><span class="identifier-syntax">PNG_file</span><span class="plain-syntax">, &</span><span class="identifier-syntax">sig1</span><span class="plain-syntax">)) </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="constant-syntax">FALSE</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (!</span><a href="6-bf.html#SP1" class="function-link"><span class="function-syntax">BinaryFiles::read_int32</span></a><span class="plain-syntax">(</span><span class="identifier-syntax">PNG_file</span><span class="plain-syntax">, &</span><span class="identifier-syntax">sig2</span><span class="plain-syntax">)) </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="constant-syntax">FALSE</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> ((</span><span class="identifier-syntax">sig1</span><span class="plain-syntax"> != </span><span class="constant-syntax">0x89504e47</span><span class="plain-syntax">) || (</span><span class="identifier-syntax">sig2</span><span class="plain-syntax"> != </span><span class="constant-syntax">0x0d0a1a0a</span><span class="plain-syntax">)) </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="constant-syntax">FALSE</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="comment"> Read first chunk</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (!</span><a href="6-bf.html#SP1" class="function-link"><span class="function-syntax">BinaryFiles::read_int32</span></a><span class="plain-syntax">(</span><span class="identifier-syntax">PNG_file</span><span class="plain-syntax">, &</span><span class="identifier-syntax">length</span><span class="plain-syntax">)) </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="constant-syntax">FALSE</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (!</span><a href="6-bf.html#SP1" class="function-link"><span class="function-syntax">BinaryFiles::read_int32</span></a><span class="plain-syntax">(</span><span class="identifier-syntax">PNG_file</span><span class="plain-syntax">, &</span><span class="identifier-syntax">type</span><span class="plain-syntax">)) </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="constant-syntax">FALSE</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="comment"> First chunk must be IHDR</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (</span><span class="identifier-syntax">type</span><span class="plain-syntax"> != </span><span class="constant-syntax">0x49484452</span><span class="plain-syntax">) </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="constant-syntax">FALSE</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="comment"> Width and height follow</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (!</span><a href="6-bf.html#SP1" class="function-link"><span class="function-syntax">BinaryFiles::read_int32</span></a><span class="plain-syntax">(</span><span class="identifier-syntax">PNG_file</span><span class="plain-syntax">, </span><span class="identifier-syntax">width</span><span class="plain-syntax">)) </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="constant-syntax">FALSE</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">if</span><span class="plain-syntax"> (!</span><a href="6-bf.html#SP1" class="function-link"><span class="function-syntax">BinaryFiles::read_int32</span></a><span class="plain-syntax">(</span><span class="identifier-syntax">PNG_file</span><span class="plain-syntax">, </span><span class="identifier-syntax">height</span><span class="plain-syntax">)) </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="constant-syntax">FALSE</span><span class="plain-syntax">;</span>
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<span class="plain-syntax"> </span><span class="reserved-syntax">return</span><span class="plain-syntax"> </span><span class="constant-syntax">TRUE</span><span class="plain-syntax">;</span>
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<span class="plain-syntax">}</span>
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</pre><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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