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    <p>If you're really interested in the technical details, check out the source code: </p> <ul> <li><a href="http://code.google.com/p/page-speed/source/browse/lib/trunk/src/pagespeed/image_compression/png_optimizer.cc" rel="noreferrer">png_optimizer.cc</a></li> <li><a href="http://code.google.com/p/page-speed/source/browse/lib/trunk/src/pagespeed/image_compression/jpeg_optimizer.cc" rel="noreferrer">jpeg_optimizer.cc</a></li> <li><a href="http://code.google.com/p/page-speed/source/browse/lib/trunk/src/pagespeed/image_compression/webp_optimizer.cc" rel="noreferrer">webp_optimizer.cc</a></li> </ul> <hr> <p>For PNG files, they use OptiPNG with some trial-and-error approach</p> <pre class="lang-cpp prettyprint-override"><code>// we use these four combinations because different images seem to benefit from // different parameters and this combination of 4 seems to work best for a large // set of PNGs from the web. const PngCompressParams kPngCompressionParams[] = { PngCompressParams(PNG_ALL_FILTERS, Z_DEFAULT_STRATEGY), PngCompressParams(PNG_ALL_FILTERS, Z_FILTERED), PngCompressParams(PNG_FILTER_NONE, Z_DEFAULT_STRATEGY), PngCompressParams(PNG_FILTER_NONE, Z_FILTERED) }; </code></pre> <p>When all four combinations are applied, the smallest result is kept. Simple as that.</p> <p>(N.B.: The <code>optipng</code> command line tool does that too if you provide <code>-o 2</code> through <code>-o 7</code>)</p> <hr> <p>For JPEG files, they use <a href="http://code.google.com/p/jpeglib/source/browse/" rel="noreferrer">jpeglib</a> with the following options:</p> <pre class="lang-cpp prettyprint-override"><code> JpegCompressionOptions() : progressive(false), retain_color_profile(false), retain_exif_data(false), lossy(false) {} </code></pre> <hr> <p>Similarly, WEBP is compressed using <a href="http://git.chromium.org/gitweb/?p=webm/libwebp.git" rel="noreferrer">libwebp</a> with these options:</p> <pre class="lang-cpp prettyprint-override"><code> WebpConfiguration() : lossless(true), quality(100), method(3), target_size(0), alpha_compression(0), alpha_filtering(1), alpha_quality(100) {} </code></pre> <hr> <p>There is also <a href="http://code.google.com/p/page-speed/source/browse/lib/trunk/src/pagespeed/image_compression/image_converter.cc" rel="noreferrer">image_converter.cc</a> which is used to losslessly convert to the smallest format.</p>
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    1. POHow does Google's Page Speed lossless image compression work?
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    1. COSpecifically retain_color_profile(false) for JPEGs can be a bad idea. As extended color gamut displays become more common a clearly defined color rendition instruction becomes an ambiguous one. If the original image was sRGB encoded this will still work in some browsers (Safari, Firefox with about:config modification) while in others only actually tagged images might be color managed (default Firefox). Of course browsers without any color management capabilities won't care…
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