Stamp Visible yields different result [2020]

[cab993]

Beginning today, when I Stamp Visible [cmd-opt-shift-e] it yields a result different from what is visible.  Specifically, I have multiple pixel layers set to the lighten blend mode.  Same (problematical) result with Merge Visible directly from the menu,  I have used Stamp Visible 1000s of times over the last several years and it did what is was supposed to do.  I have PS 21.2.3 and Mac OS 10.15.6.  Help!  Thanks.

[JJMack]

Where is the layer  stack did you stamp the visible layers. Could what you see be effected by adjustment layer above it?

[cab993]

Thanks for the reply.  I stamped visible from the top layer in the stack.  There is nothing above it.

[D Fosse · Accepted solution]

You need to view at 100% for an accurate preview. The final stamped result is correct.

You probably have a very noisy image, or a "binary" image where pixels are either black or white, such as line art, halftone screen etc.

Adjustment previews are calculated on the basis of the on-screen version. If this is zoomed out and scaled, the inevitable softening of "hard" pixels introduce intermediate values that aren't really there in the full data.

[cab993]

Thanks!  I will investigate.   You are indeed correct that I first noticed this "problem" on a very noisy set of images.

[D Fosse]

With a "normal" photograph this isn't an issue, it's just these images where there are very hard pixel-level transitions. On/off, so to speak.

If Photoshop calculated all adjustment/blending previews on the full image data, this problem would go away. Then you'd always see previews that matched the final result. The official explanation is that it's done this way for performance reasons.

But the real problem is that it still wouldn't be a correct representation of these images. To see them correctly, you need to see every single pixel faithfully reproduced on screen. And that's what 100% does: it maps exactly one image pixel to exactly one screen pixel. Zoomed out, those pixels are blended and softened, and then you get a lot of intermediate pixel values that aren't real.

[cab993]

I have checked my images at 100% and you are absolutely correct.  Although I knew that interpolations were going on at < 100%, I did not realize the extent of the difference could be so great and that it depended on the extent of noise.  I can rest easier now.  Thank you very much for your help!