Re: `Lossless' retroactive embedding of color profile (avoid JPEG recompression); basic color profile theory
Re: `Lossless' retroactive embedding of color profile (avoid JPEG recompression); basic color profile theory
- Subject: Re: `Lossless' retroactive embedding of color profile (avoid JPEG recompression); basic color profile theory
- From: bruce fraser <email@hidden>
- Date: Wed, 2 Aug 2006 09:04:25 -0700
At 12:26 AM +1000 8/3/06, Graeme Gill wrote:
If you don't want to understand how things work, then fine.
Some people would like to use the right tool for the right task.
For those interested, the point is that the subsequent changes
are minor (ie. probably not visible), and ultimately stop all
together.
To quantify it, if I use quality 4 in photoshop with my test image,
then on the first save to jpeg 49% of the pixels change value, with
an average change of 0.7 bits.
Between the first jpeg and the 6th, 1.64% of the pixels change value,
and the average change is 0.03 bits. After the sixth save, no
pixels change value.
So the idea that decompressing and compressing a JPEG degrades the
image each and every time, is a myth.
Would that that were so. In fact, it's heavily dependent on the
specific JPEG implementation. If the JPEG is resaved with exactly the
same settings-the same subsampling, the same quantization tables, the
same DCT-no further change occurs, as you rightly point out. But I've
seen plenty of implementations that don't do that. Photoshop's is
fairly good in this regard, some others are much, much worse.
So while I agree with you in principle, I'd be wary of making
sweeping generalizations based on one single JPEG implementation.
--
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