"Blend If" Merge Fails - Changes Image [Locked for abusive comments]

Bottom line:

This is a bug that Adobe has known about to a point where forums have a canned response that is provided so quickly that most users don't even read the post they are responding to with boilerplate. 

It's unclear why Adobe hasn't fixed the bug, but most likely explanation is the overwhelming presence of apologists (as evidence in this thread) ready to write off the bug and pretend it doesn't exist while making fun of those struggling with it as if the ones struggling with it simply haven't understood the boilerplate. 

You clearly have very little experience— whether with Photoshop or with literacy is not clear.

I'm not going to repeat everything multiple times.

If anyone else is interested in learning how these things work, it's all explained above by davescm and myself.

Further, when a 720ppi document is viewed at 100% zoom level, it looks NOTHING LIKE the output product will look. At no point will you ever be looking at 4.8" blown up 5 times. 

VERY RELEVANT.

>> Ppi is entirely irrelevant to this discussion. That you insist it is, shows that you haven't fully understood the nature of pixel images and how they're represented on screen, on paper, and any other media.

Once again you show that you aren't paying attention. 

No one has said that PPI is relevant to viewing pixels at 100%. 

What was said is that PPI is relevant to the amount of output information that can be viewed on screen at a given time. Working at 720ppi means that very little of the output content is visible on any screen at 100% zoom. For a 24" document on a screen 3456px wide (much wider than most people's resolution) that is still only 4.8" => 20% of the image. 

So you're claiming it's not relevant that you can only preview 20% of an image when making changes?

You might want to adjust your attitude a little bit. We're only trying to explain what actually happens.

Dave explained it very well, and absolutely accurately. I don't really have anything to add. Read it all again. That's how it is.

Ppi is entirely irrelevant to this discussion. That you insist it is, shows that you haven't fully understood the nature of pixel images and how they're represented on screen, on paper, and any other media.

On screen, one image pixel is mapped to one screen pixel. The image pixel grid is aligned to the screen pixel grid. That's 100%. At other zoom ratios, the data are resampled for screen. That resampling is precisely the underlying issue here.

No ppi number ever enters into this. It's not needed on screen, because there already is a screen pixel grid that defines the resolution. On paper there is a need for it, because paper doesn't have a native pixel grid. The ppi number defines one.

Pixels per inch is a way to translate pixels into physical sizes where it's needed.

>> PPI is irrelevant to this discussion, it is metadata used when printing, only the number of pixels matters, and yes I frequently use very large pixel images when compositing 3D renders

PPI is absolutely not irrelevant— it corresponds to the density of pixels in relation to the scale where viewing the pixels makes sense. Zoomed in to pixels at 100%, it is almost impossible to see much of any image at 720dpi. If the goal is to see what a change does to the appearance of the image, viewing this close to the pixels makes the goal impossible.

That amounts to a bug. 

>> Thank you - I will not be participating in this thread any further

Thank you. Expert input is much preferred. 

The preview method has been discussed many times in these forums.

When zoomed out, cached versions of each layer are used to create the on screen preview depending on the zoom level. These cached layers do not contain the full number of pixels in every image, instead groups of pixels form a cahed pixel and it is these that are blended. There used to be an option to force cache level 1, which disabled cahing and did blend every image pixel, but if you try and set that it does not now stick, as it was removed due to other performance issues.

In the past, there was a second effect in that at zoom levels less than 66.7% the preview was made using 8 bits/channel , even when the image was 16 bits/channel. That was changed recently as compositing was switched to the GPU rather than the CPU.

Have you ever worked at 720dpi?
PPI is irrelevant to this discussion, it is metadata used when printing, only the number of pixels matters, and yes I frequently use very large pixel images when compositing 3D renders

'This part of the answer amounts to trolling....'
Thank you - I will not be participating in this thread any further

Dave

>> The preview is not a bug.

If it is not technically "a bug" in some restricted programming sense, it is functionally a bug in that it prevents WYSIWYG interaction with Photoshop. 

>> It is the way previews are created in Photoshop for speed when zoomed out.

As I understand it, they are just created at the scaled screen size— not with some special algorithm that skips pixels. 

> All pixels are not blended individually.

Evidence / citation please. 

>> Imagine the delays viewing a zoomed out version of a multi-layer  300,000 x 300,000 pixel image if they were. With most images there is no visible difference, but occasionaly with certain blend combinations or fine noise, a difference jumps out.

This is due to differences of how a given algorithm applied to the image functions at a different scale. 

>> I've not seen an issue on export - again please detail the exact steps so we can try and reproduce.

Here is the PSD File corresponding to the posted photos. Export as... at 600x800 to see export re-create the artifacts. 

>> When working with large files - check blending by using Ctrl+1 to view at 100% or add a temporary merged layer to the top of the workstack to quickly check actual blending when zoomed out (Shift+Ctrl+Alt+E) then delete it again

I'm well aware how to zoom in or check merged results— these are not solutions or even particularly helpful actions with the described context. Have you ever worked at 720dpi? This part of the answer amounts to trolling and does not address the bug. 

The preview is not a bug. It is the way previews are created in Photoshop for speed when zoomed out. All pixels are not blended individually. Imagine the delays viewing a zoomed out version of a multi-layer  300,000 x 300,000 pixel image if they were. With most images there is no visible difference, but occasionaly with certain blend combinations or fine noise, a difference jumps out.

I've not seen an issue on export - again please detail the exact steps so we can try and reproduce.

When working with large files - check blending by using Ctrl+1 to view at 100% or add a temporary merged layer to the top of the workstack to quickly check actual blending when zoomed out (Shift+Ctrl+Alt+E) then delete it again

Dave