JPG and loss of quality

Started Nov 12, 2012 | Discussions thread
richardplondon Forum Pro • Posts: 10,531
Re: JPG and loss of quality

_Fotogrfr_ wrote:

I never realized that "save as" creates more deterioration than "save". Strange.

It is important to distinguish what happens within a PS session with a given image, from what happens between PS sessions with that image.

If you open a JPG into Photoshop, the image is now held in memory. Its pixels may show some evidence of previous degradation due to compression. But PS is working internally with it as a standard RGB bitmap now, just the same as it would with any other image type.

So while you have this image open, you can hit Save as many times as you like, and each time you do, PS takes its internal RGB bitmap and compresses that into a file, and overwrites what was on disk before. The hundredth save is no different from the first save within this session - you can save to quality 12, then 2, then change your mind back to 12 with no penalty - because these saves-witihin-a session have not been cumulative one on the next - each one has been independently made, working from the same ongoing RGB bitmap; and no save operation within the session has affected this working bitmap's contents.

Only if you save, close the image, re-open forcing PS to make a fresh internal RGB bitmap from the external file, save, close etc will you accumulate compression problems.

One reason repeated Save As may seem  faster toward degradation, is if it means different files are then being worked on rather than the same one remaining in memory. With varying filenames it will more often happen that a brand new separate bitmap has to be built - PS can less often refer to an image already persisting in memory.


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