<story><title>Flickr – A Year Without a Byte</title><url>https://code.flickr.net/2017/01/05/a-year-without-a-byte/</url></story><parent_chain><item><author>Niksko</author><text>No, I think you&amp;#x27;ve misunderstood.&lt;p&gt;What they&amp;#x27;re saying is that you can take a JPEG compressed image, decompress to raw pixels, and then recompress with JPEG more efficiently (if you&amp;#x27;re careful, no specifics on how this is done), and you save space.&lt;p&gt;That&amp;#x27;s why they mention that they&amp;#x27;re doing it very carefully, because you&amp;#x27;ve got to make sure that when you decompress the new optimized image that it pixel for pixel matches the original decompressed image.</text></item><item><author>Nition</author><text>Really interesting that (if I&amp;#x27;m reading the article right) you can take an already-compresed JPEG, recompress it losslessly using another technique to get &lt;i&gt;better&lt;/i&gt; compression than the original compressed JPEG, and then decompress it to the original JPEG again.&lt;p&gt;The concept makes sense but I&amp;#x27;d never thought of that before.</text></item></parent_chain><comment><author>camtarn</author><text>Nition is correct, I think.&lt;p&gt;As far as I can tell, &amp;#x27;raw pixels&amp;#x27; is not quite accurate. Both PackJPG and Lepton seem to decompress as far as frequency-domain coefficients, but no further. That means they don&amp;#x27;t repeat the lossy stages in JPEG encoding - the transformation of pixel data to the frequency domain, and the discarding of high frequency infomation. It looks like PackJPG and Lepton do some interesting tricks with the coefficients to essentially make them more amenable to the following lossless compression. Both PackJPG and Lepton have their own file format, so they&amp;#x27;re definitely not outputting standard JPEG images.&lt;p&gt;Here&amp;#x27;s the Lepton page:
&lt;a href=&quot;https:&amp;#x2F;&amp;#x2F;blogs.dropbox.com&amp;#x2F;tech&amp;#x2F;2016&amp;#x2F;07&amp;#x2F;lepton-image-compression-saving-22-losslessly-from-images-at-15mbs&amp;#x2F;&quot; rel=&quot;nofollow&quot;&gt;https:&amp;#x2F;&amp;#x2F;blogs.dropbox.com&amp;#x2F;tech&amp;#x2F;2016&amp;#x2F;07&amp;#x2F;lepton-image-compress...&lt;/a&gt;&lt;p&gt;And for more detail, here&amp;#x27;s the paper that PackJPG is based on:&lt;p&gt;&lt;a href=&quot;http:&amp;#x2F;&amp;#x2F;www.elektronik.htw-aalen.de&amp;#x2F;packjpg&amp;#x2F;_notes&amp;#x2F;PCS2007_PJPG_paper_final.pdf&quot; rel=&quot;nofollow&quot;&gt;http:&amp;#x2F;&amp;#x2F;www.elektronik.htw-aalen.de&amp;#x2F;packjpg&amp;#x2F;_notes&amp;#x2F;PCS2007_PJ...&lt;/a&gt;</text></comment>
<story><title>Flickr – A Year Without a Byte</title><url>https://code.flickr.net/2017/01/05/a-year-without-a-byte/</url></story><parent_chain><item><author>Niksko</author><text>No, I think you&amp;#x27;ve misunderstood.&lt;p&gt;What they&amp;#x27;re saying is that you can take a JPEG compressed image, decompress to raw pixels, and then recompress with JPEG more efficiently (if you&amp;#x27;re careful, no specifics on how this is done), and you save space.&lt;p&gt;That&amp;#x27;s why they mention that they&amp;#x27;re doing it very carefully, because you&amp;#x27;ve got to make sure that when you decompress the new optimized image that it pixel for pixel matches the original decompressed image.</text></item><item><author>Nition</author><text>Really interesting that (if I&amp;#x27;m reading the article right) you can take an already-compresed JPEG, recompress it losslessly using another technique to get &lt;i&gt;better&lt;/i&gt; compression than the original compressed JPEG, and then decompress it to the original JPEG again.&lt;p&gt;The concept makes sense but I&amp;#x27;d never thought of that before.</text></item></parent_chain><comment><author>Veratyr</author><text>This definitely isn&amp;#x27;t what they&amp;#x27;re saying.&lt;p&gt;Lepton (one of the examples they mentioned) losslessly compresses a JPEG to Lepton format, then losslessly decompresses it back to JPEG. The pixels are never decompressed and in fact a JPEG decompressed from Lepton is bit-exact the same as the original. I&amp;#x27;ve tested and verified this on several million images.</text></comment>