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10.4: Add Spotlight keywords to images with Preview
Authored by: dlong on Jun 16, '05 12:00:23PM

What I would _hope_ is that adding keywords wouldn't degrade (recompress) JPG files. Any idea whether this is the case, or whether multiple trips to the "Add one more keyword" wagon will start to junk up lossily-compressed files in Preview?



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10.4: Add Spotlight keywords to images with Preview
Authored by: q-wert on Jun 16, '05 07:47:09PM

From what I can tell from a quick test, the image is re-processed by Preview. (I used FileMerge to quickly compare the raw data). In my tests I've also noticed that Photoshop can also add keywords but without re-processing the image. However, it has some peculiarities when dealing with some metadata data (there's different metadata formats for JPG. IPTC is just one of them and Photoshop might mess up the others - or this one, I don't remember)



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10.4: Add Spotlight keywords to images with Preview
Authored by: network23 on Jun 17, '05 12:08:01PM

Something is definitely going on with the image when you add the keywords via Preview, but from what I can tell, the image is not being recompressed. I took a file, copied it, and added keywords to the copy multiple times, resulting in six additional saves. I then loaded them up in Photoshop and looked at them at 1600%. No pixels shifted or looked recompressed (the "layout" of pixels from the original compression looked identical), but I did notice some slight color shifting in some of the pixels. At 1200-1600%, the copied image looked more "rainbow-y" than the original, but when viewed at normal size, little, if any, change could be discerned.

So the image is being altered, but I do not think it is being recompressed.

I've posted my test image comparison.

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10.4: Add Spotlight keywords to images with Preview
Authored by: SeanAhern on Oct 07, '05 02:17:10PM

I think it's likely that it's being recompressed. Recall that JPEG compression involves color manipulation so as to remove colors that the eye "cannot see".

I took your two images that were side by side, brought them into Photoshop, and differenced them. The resultant image was almost black, but not quite. Once I adjusted the contrast to highlight the differences, you can easily see a variation. See here.



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