Photoshop CC 2017: Simple, basic question about image/layer degradation.

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We know that transforming images cause some degree of quality to be sacrificed depending on the degree that it's being altered. I fully understand and get this, but – I'm curious if undoing a transform will fully restore the the image/layer back to it's pristine, unaltered, original state? Seems like it would but it does alter the pixels when transforming and then alters them again in order to undo it, right? Or does PS reinstate a cached version of the image/layer or something like that?

Anyone know definitively "yes" or "no" to this? Really been curious about this for a while.
 
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grauenwölfe

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Posted 1 year ago

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David, Official Rep

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Howdy Grauenwölf,

No, simply applying an inverse transformation onto an already transformed image will NOT restore the original.  It might if you're super lucky, but the algorithm isn't that smart.  Instead, each transformation is like taking a photocopy -- the quality gets a little worse with each attempt.

Thanks,
David
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Jerry Syder

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I thought an "inverse" transformation was different to undoing. As in, going a step back before the transformation was applied? i.e. to a cached state? I understand that transforming something, then going back to transform to remove the transform that was applied would degrade. To be clear Grauenwölf, when you say "undo", do you mean going a step back in the history list panel? 
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grauenwölfe

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Jerry,
Yes, precisely what I meant, using the Undo command. Glad you called for more clarity.

David,
I think your answer relates to an example like this?
  • Step 1. Transform image to 200%
  • Step 2. Transform again to 50% to get you back where you started.
If so, I'm completely aware this is destructive but thank you for throwing it out there.

Jeff,
Sounds like you fully understood what I was getting at. So Undo/Step Backward does in fact put things back to a pristine, unaltered state. Great to know. Thank you.
(Edited)
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Jerry Syder

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Hehe I was actually clear but was "clarifying" based on the other response in the thread. Happy Friday :-)
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David, Official Rep

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I had assumed, as an experienced Photoshop user, that you were aware that Undo (<-- notice, capitalized to indicate a command and not just a noun in my sentence) will revert your document to the previous state.  Actually, it pretty much goes without saying that Undo will revert any changes you make to the document, particularly since nearly all commands you do to a layer (Filters, Transform, Adjustments, etc) actually make changes to the pixel data of that layer as well.  Undo returns the document to the previous state -- exactly as it was and (unless your system has custom settings) with last 20 potential states available, at least until you close PS.

Novice users will often scale an image down and then later blow it back up, only to be confused that the resulting image is of lower quality.  Another scenario is when a user transforms something, saves it, closes Photoshop, and then wants to remove the transformation later.  I thought you were looking for a non-trivial solution to these more complicated situations...
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Jeffrey Tranberry, Sr. Product Manager, Digital Imaging

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If you do a an Undo/Step Backward, it restores the state. If you transform a layer (scale down) and then transform it (scale up) to match the original size, then that would be lossy.

If you use Smart Objects, each transform is re-applied to the source file pixels.