I have been wondering if the maximum quality setting for JPEG really means zero compression loss. That would be great for myself and other hobbyists who are forced to use this file format when we bring pictures to an inexpensive photo kiosk. This could easily be the case based on my reading of the algorithm. The algorithm uses a Fourier Transform to convert an 8×8 pixel array per color into 64 frequencies; i.e. 64 variations in tone per color across the array. Since very few 8×8 pixel arrays will have that many frequency components, they come up as zeros which can then be discarded or get put into a pile with other zeros. One way or another, they no longer need a byte per pixel/per color. Does anyone here have communication with PS programmers? I think the answer would be of interest to many people.
There are dozens of postings in this forum on JPEG compression.
As far as I recall, the situation is that, since you decompress a JPEG file to open it, any change (even one pixel) will mean that, on saving it, there will be some degradation because even the highest quality setting involves compression. It is a "lossy" process.
I know this isn’t answering your question directly but it’s worth mentioning because many users wrongly think that max quality = no loss.
If "an inexpensive photo kiosk" will only accept JPEGs, you cannot expect laboratory quality prints anyway.
John, as you now realize, not even on pixel needs to be changed. The mere fact of resaving as JPEG, even without modifications at high quality, causes changes to the RGB pixel values. This is a result of the fact that JPEG does not store pixel values, but stores regional aproximations. The bit-level changes are very minor at high quality levels, and most likely would not be noticed, but they are there.
Hi everybody, Thanks for your responses. Those pictures of pixels tell the story. Still, it is too bad that PS does not have a zero loss setting. It seems that it would only require that PS only discard "frequencies" with zero amplitude. Then inverting those with non-zero amplitude would yield the original 8×8 pixel array. Then again, I have not dug into the JPEG specification itself.
Yet resaved once or twice, and at the highest level, I would defy anyone to tell the difference in the actual print-out, compared to the before and after tiff counterparts brought through the same edits.
So if the integrity of your image at the pixel level is important to you, never save as Jpeg.
Ever.
However, if the disk space saving of Jpegs is key for you, AND if you don’t mind a small amount of degradation that can only be descerned by some type of overlay or apply image process to show the pixels that changed, then go ahead and resave a Jpeg.
On the otherhand you could also save Zip or LZW compressed tiffs, those compression schemes are lossless.
And if you fall into the camp of "really getting bent out of shape by things that you know to exist, but you cannot see", you should probably start saving everything in 16 bit if your scanner will give you a high-bit capture.
Don’t get me wrong.
I’m not advocating Jpegs, but geez they aren’t that bad.
Still, it is too bad that PS does not have a zero loss setting
It is not PS that does this. It is the jpg compression algorithm. If you want zero loss, all you have to do is to save into a new format, like PSD or TIFF.
Then you can save away without degrading the image. Only at the final stage will you convert back to jpg. (And keep a good copy in the other format so you never have to make a copy of the jpg.)
If you want zero loss, all you have to do is to save into a new format, like PSD or TIFF. Then you can save away without degrading the image.
This brings up an interesting question: If a file is converted from JPEG to TIFF or PSD, and later converted back to JPEG, is there any additional loss?
But saving to a tiff or psd lets you save over and over without loss. You will lose again when you finally go to jpg, but only one set of losses is usually acceptable. The real problem is when you save over and over again in jpg.
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