Comments on: JPEG Image Compression Degradation http://www.patricktaylor.com/1940 www.patricktaylor.com Fri, 03 Feb 2012 12:05:56 +0000 hourly 1 http://wordpress.org/?v=3.2.1 By: Patrick http://www.patricktaylor.com/1940/comment-page-1#comment-4794 Patrick Sun, 02 Jan 2011 10:41:37 +0000 http://www.patricktaylor.com/?p=1940#comment-4794 The article describes that when you open, save, and close a .jpeg in Photoshop some quality is lost. My reply to Calin simply clarifies that to save a file you must either edit it or give it a new filename. The article describes that when you open, save, and close a .jpeg in Photoshop some quality is lost. My reply to Calin simply clarifies that to save a file you must either edit it or give it a new filename.

]]> By: Ben http://www.patricktaylor.com/1940/comment-page-1#comment-4793 Ben Sat, 01 Jan 2011 00:54:22 +0000 http://www.patricktaylor.com/?p=1940#comment-4793 I think the article is a bit misleading as has already been pointed out by Calin. Patrick then states the actual facts in his reply, but then for some unknown reason states that the contradiction or at least correction agrees with what was written above in the article. In the article it clearly states that opening and saving the jpeg in photoshop without any editing will degrade the jpeg. This is misleading and the reality is noted in Patricks reply... either the image has to be edited or the "save as" not the save option has to be used for image degradation. I think the article is a bit misleading as has already been pointed out by Calin. Patrick then states the actual facts in his reply, but then for some unknown reason states that the contradiction or at least correction agrees with what was written above in the article. In the article it clearly states that opening and saving the jpeg in photoshop without any editing will degrade the jpeg. This is misleading and the reality is noted in Patricks reply… either the image has to be edited or the "save as" not the save option has to be used for image degradation.

]]> By: Patrick http://www.patricktaylor.com/1940/comment-page-1#comment-4512 Patrick Sat, 12 Jun 2010 21:03:22 +0000 http://www.patricktaylor.com/?p=1940#comment-4512 To clarify. I did the experiment two ways. You can't simply open and save an image (in Photoshop at least). Either (i) it has to be edited in some way before you can <strong>Save</strong> it with the same filename or (ii) you can open it and <strong>Save As</strong> with a new filename without any edits. The edits in the first experiment were flipping the image back and forth, and there were no edits in the second. The result in both cases was the same degradation effect, which tends to agree with what is <a href="http://www.faqs.org/faqs/jpeg-faq/part1/section-10.html">written here</a> when using a 'a regular image editor'. I've repeated the experiment in CS4. Same result. To clarify. I did the experiment two ways. You can't simply open and save an image (in Photoshop at least). Either (i) it has to be edited in some way before you can Save it with the same filename or (ii) you can open it and Save As with a new filename without any edits. The edits in the first experiment were flipping the image back and forth, and there were no edits in the second.

The result in both cases was the same degradation effect, which tends to agree with what is written here when using a 'a regular image editor'. I've repeated the experiment in CS4. Same result.

]]> By: Calin Rotaru http://www.patricktaylor.com/1940/comment-page-1#comment-4511 Calin Rotaru Sat, 12 Jun 2010 15:54:34 +0000 http://www.patricktaylor.com/?p=1940#comment-4511 I have tried to reproduce your results, without any effect. It seems to me that photoshop besides decompressing the image also applies a graining/smoothing step to reduce jpeg distortions or something similar. Normally there is no major degradation of the image by repeating the open/save operation again and again. The reason is that after the first compression step, the image colors are shifted to improve the jpeg image. After the second open the colors are already very close to the jpeg compressed data and there will be no significant change anymore. You should try other tooling, photoshop is sometimes kind of blackbox here. I worked here with an image converter, AZImage and set JPEG Average, the quality to 75 out of 100, DCT float, optimize for the jpeg library. I have tried also with int instead of float to exclude for numerical errors, but the same. I cannot say that this behavior comes from the jpeglib. I have tried to reproduce your results, without any effect. It seems to me that photoshop besides decompressing the image also applies a graining/smoothing step to reduce jpeg distortions or something similar.

Normally there is no major degradation of the image by repeating the open/save operation again and again. The reason is that after the first compression step, the image colors are shifted to improve the jpeg image. After the second open the colors are already very close to the jpeg compressed data and there will be no significant change anymore.

You should try other tooling, photoshop is sometimes kind of blackbox here. I worked here with an image converter, AZImage and set JPEG Average, the quality to 75 out of 100, DCT float, optimize for the jpeg library. I have tried also with int instead of float to exclude for numerical errors, but the same. I cannot say that this behavior comes from the jpeglib.

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