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copied!<p><em>Units</em>, <em>Xdensity</em> and <em>Ydensity</em> members of JPEG file header specifies unit of measurement used to describe <strong>physical dot density</strong> when a file is printed.</p> <ul> <li>If <em>Units</em> is 1, <em>Xdensity</em> and <em>Ydensity</em> are dots per inch.</li> <li>If <em>Units</em> is 2, <em>Xdensity</em> and <em>Ydensity</em> are dots per cm.</li> </ul> <p>The point is that dot resolution (the scaled printing resolution) stored in an image file simply does not matter on the screen. Thus, Windows programs will always show you 96 logical ppi on the screen for any file. Note, some applications prefer using 72 logical ppi to display pictures on the screen, e.g. Adobe applications.</p> <p>Graphics applications such as ACDSee, Adobe Photoshop, CorelDRAW, simply ignores <em>Units</em>, <em>Xdensity</em> and <em>Ydensity</em> members when displaying JPG files on the screen, but graphics applications consider the value of those members when printing JPG files if they exist. In case, a JPG file does not have <em>Units</em>, <em>Xdensity</em> and <em>Ydensity</em> members, graphics applications use their custom default values (usually 150 dpi) to print the JPG file.</p> <p>So, for the question about a Delphi code that can read all JPEG header files, the answer is simple, just read JPG file header information; in case the optional members did not exist in a file, just ignore the optional members or tell end-users that they were currently not specified in the file.</p> <h3>Further Reading on DPI and PPI confusions</h3> <ul> <li><a href="http://www.scantips.com/no72dpi.html" rel="nofollow">Say No to 72 dpi</a></li> <li><a href="http://www.scantips.com/no72dpib.html" rel="nofollow">A few scanning tips</a></li> </ul> <h3>References on JPEG File Format Specification</h3> <ul> <li><a href="http://www.fileformat.info/format/jpeg/egff.htm" rel="nofollow">JPEG File Interchange Format File Format Summary</a></li> <li><a href="http://www.martinreddy.net/gfx/2d/JPEG.txt" rel="nofollow">JPEG File Interchange Format, v1.02</a></li> </ul>

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