duplicating a pixel layer in Photoshop gradually decreases its quality

[jeremyp1470049]

I recently had to duplicate a pixel layer and iterate it around an artistic composition in Photoshop.  By the time I had duplicated it 100 times it was almost unrecognizably bad.  Like making a photocopy of a photocopy back in the '80`s.  

 

Why can't photoshop literally perfectly copy a layer when it duplicates? 

Photoshop 23.0.2 

Mac os 12.0.1

M1 macbook pro 

[S_Gans]

That is SERIOUSLY odd - Photoshop DOES perfectly and exactly duplicate a layer, pixel to pixel. Can you walk us through what you're precisely doing (how you're doing it) that makes this happen?

[jeremyp1470049]

I have been working on a digital composition of - one diamond - hundreds of times in a single document.

 I took a real diamond photo and imported it to the new document, then rotated its layer and pressed Command J, dragged and rotated again (but never transformed its size). Then I would press Command J again when I liked the new placement, so I could place the next iteration around the composition.  

After about 100 of these iterations the file was very noticably bad... and in my screenshot you can see the original few generations looked ok (on the left), but the ~180th iteration (on the right side) looks like a marble notebook cover.

[Brad @ Roaring Mouse]

Your copied pieces look like they had an excessive anount of Unsharp Masking appled to it.

I'm wondering if you have a Smart layer with a sharpening effect applied that is being reapplied over an over again as you copy?

[J453]

Hard to tell what's going on without seeing the layers panel or the actual file to troubleshoot. Can you share a link to the PSD?

[scotwllm]

Picture this: you have a graphic three pixels wide by 1 pixel tall. The first pixel is black, then white, then transparent. If you move the black and white pixels one half pixel to the right, Photoshop has to calculate what goes into each pixel space. The pixel grid stayed put, but the colors moved. That's where the degradation comes from.

[JohanElzenga · Accepted solution]

Yep. Rotation resamples all the pixels, unless you rotate in 90 degree steps. The way to do this is to start with the first layer each time, so each layer was only rotated once. A lot of extra work, no doubt, but the only way to avoid this problem. Or use a smart object...

[Brad @ Roaring Mouse]

"unless you rotate in 90 degree steps"

Even then, if the copy doesn't fall precisely on pixels, it's resampled to fit.

[jeremyp1470049]

I didn't save this file.  I reworked it, duplicating from the same original layer each time individually, and that fixed it

[jeremyp1470049]

yeah, I think you've solved my mystery.  that makes perfect sense.  thank you 

[jeremyp1470049]

I never thought it baked in the new pixel locations when I duplicated.  I imagined Photoshop kept the original pixels in memory and just described and rendered the translation on the fly... but I guess that would be too cumbersome