JPEG Compression explanation
Jun 27, 2012 13:13:39 #
A few days ago we had a discussion on why/how multiple saves of a JPEG can degrade the image. The reason, as we saw, was that the compression takes areas of similar color and assigns those pixels all the SAME numbers. It keeps trying to simplify that image every time it saves. Eventually causing pixilation by having blocks of the same color.
Here are three examples of how jpg compression works. There are three images: complex, a gradient, and solid white. All three were 4x6 @ 300ppi (2,160,000 pixels).
I compressed each one at quality 12 and 6. The size differences are huge. So this is why all files are not the same size. The simpler the file, the less memory it takes for the file to be described. Not unlike having a canvas and equating the time it would take to paint it all white or paint a landscape.
The complex file at 12 is 2.7MB, the simple white file is only 164KB, yet both have exactly the same number of pixels.
So at least in theory, if we saved that complex file enough times, we would end up after many thousands of saves, with an image of all one color! Maybe hundreds of thousands of saves!
Here are three examples of how jpg compression works. There are three images: complex, a gradient, and solid white. All three were 4x6 @ 300ppi (2,160,000 pixels).
I compressed each one at quality 12 and 6. The size differences are huge. So this is why all files are not the same size. The simpler the file, the less memory it takes for the file to be described. Not unlike having a canvas and equating the time it would take to paint it all white or paint a landscape.
The complex file at 12 is 2.7MB, the simple white file is only 164KB, yet both have exactly the same number of pixels.
So at least in theory, if we saved that complex file enough times, we would end up after many thousands of saves, with an image of all one color! Maybe hundreds of thousands of saves!
Jun 27, 2012 18:25:37 #
Thanks CaptainC!
Have you ever considered writing a book?
Your explanations are so pleasantly pithy and succinct yet so easy to understand.
You are a natural teacher of things technical; a rare talent. Thanks for sharing it with us UHHs.
Have you ever considered writing a book?
Your explanations are so pleasantly pithy and succinct yet so easy to understand.
You are a natural teacher of things technical; a rare talent. Thanks for sharing it with us UHHs.
Jun 27, 2012 18:37:08 #
Festina Lente wrote:
Thanks CaptainC!
Have you ever considered writing a book?
Your explanations are so pleasantly pithy and succinct yet so easy to understand.
You are a natural teacher of things technical; a rare talent. Thanks for sharing it with us UHHs.
Have you ever considered writing a book?
Your explanations are so pleasantly pithy and succinct yet so easy to understand.
You are a natural teacher of things technical; a rare talent. Thanks for sharing it with us UHHs.
That is very nice off you to say....but no. That would be work!
Jun 28, 2012 16:21:52 #
Unless I'm too dense to get some other sense of what you said this seems to put to rest the agrument against using JPEG because of the quality lost when saving a JPEG that has been modified as a trivial stretch. Thank you, Sir!
Jun 28, 2012 16:26:59 #
BboH wrote:
Unless I'm too dense to get some other sense of what you said this seems to put to rest the agrument against using JPEG because of the quality lost when saving a JPEG that has been modified as a trivial stretch. Thank you, Sir!
Just to be clear...this only addresses MANY JPEG compressions. Three or four saves at quality 11 or 12 will not make a meaningful difference. Several saves at quality 5 or 6 can make a hash of an image.
If you shoot JPEG, save it as a TIFF or PSD for processing, then make one more conversion for printing, you will see no degradation.
Jun 28, 2012 21:08:48 #
This is why I love this place. Great info because someone with the knowledge cared enough to share.
Jun 28, 2012 22:46:37 #
Jun 28, 2012 23:09:20 #
Jun 29, 2012 01:37:36 #
That is the best explanation I have seen of this anywhere. The Captain is The Man! Thanks, Captain.
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