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Technically, with the sand texture, this is the equivalent of an extremely noisy image. That's always tricky and requires some extra precautions.
In this case, all that "noise" gets downsampled in two iterations. What that means is that a group of pixels with very different values get averaged out and sent along as a single average pixel.
First of all, you are probably exporting at a much smaller pixel size. So that alone will alter the pixel values. To make a fair comparison, you should res
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Technically, with the sand texture, this is the equivalent of an extremely noisy image. That's always tricky and requires some extra precautions.
In this case, all that "noise" gets downsampled in two iterations. What that means is that a group of pixels with very different values get averaged out and sent along as a single average pixel.
First of all, you are probably exporting at a much smaller pixel size. So that alone will alter the pixel values. To make a fair comparison, you should resample the image first and then compare.
But the really tricky part is the on-screen representation of a complex pixel structure with layer blending effects. To get a correct representation of this on screen, you must view at 100%. This is extremely important!
100% in Photoshop has nothing to do with size. It means that one image pixel is represented on screen by exactly one physical screen pixel.
Adjustment and blending previews are calculated based on the on-screen image data. This is done for performance reasons. When you are zoomed out, that means you get a rendering based on averaged values, not true pixel for pixel values. And so the calculations for the preview will not be made from the true pixel data, but from a softer, averaged version.
Unless you view at 100 %! Then you get a true and accurate preview, which will match the actual output result.
This isn't important for a "normal" photograph. But when you deal with sharp pixel-level transitions, it becomes something you need to watch. That applies to very noisy images (sand, starry night sky), but also to binary images like line art or halftone dots, where each pixel is either black or white.