Do you have the [  ] Use OpenGL Drawing option turned off (renamed to [  ] Use Graphics Processor in CS6)?

-Noel

One of the things you gain with Graphics Processor operation is a far better zoom resampling.

Why do you have it disabled?  Did something not work properly?

-Noel

I can't explain that - OpenGL acceleration normally cleans up the resampling, making it more accurate, so that you don't see that kind of effect.  Do you have Layer Styles in your document here?

Could you put up an image showing how a part of your document looks at 100% zoom?

-Noel

Your post intrigued me, so I did a little more testing.

Part of the problem, beyond just the fact that the resampling is done with a speed-optimized bi-llinear method inside OpenGL, is that the light and dark pixels appear to be being combined in gamma-corrected space - e.g., the 2.2 gamma of the display.

I compared the preview display of my own plug-ins, which do all their work behind the scenes in linear space.

My plug-in also uses OpenGL to do the resampling of one of several pre-calculated cache levels, again, all in linear space (I use a shader to bring the gamma up to 2.2 for display on the monitor as the combined pixels are output to the display).

There's more than meets the eye with Adobe's implementation...  I specifically chose to use linear data for all my image operations in my implementation, because there is a quality level one can achieve with such operations that cannot be reached working directly on gamma-precompensated data.  But I didn't have all the contstraints Adobe likely had, coming from some 10 prior versions that had no knowledge of OpenGL and which didn't have the luxury of manipulating high bit depth data.

Also, Adobe's cache levels (pre-downsized images, which are then fine-tuned by OpenGL resampling) likely still use their old speed-optimized CPU-based sampling methods, which threw away adjacent pixels - hence the completely dashed appearance of the 50% downsample.

This represents an area where Adobe could further improve their preview display in the future.  For example, it's not hard to imagine a GPU resampling program (OpenCL or OpenGL shader) that could assist in preparing the cache levels.  It's also not a really long jump to imagine an even better (Bicubic) resampling process, also GPU-accelerated.  Conceivably, with today's computer power, one could imagine previews in real-time every bit as good as those provided by Image - Image Size (Bicubic) resampling.

-Noel

Curiouser and curiouser...

I only see that fragmentation in my Photoshop CS6 32 bit version, while it looks better in the 64 bit version.

I suspect this is because I have optimized the cache levels differently in each.  I've opted for a Cache Levels setting of 1 and OpenGL mode of Basic in my 32 bit version, because of wanting to use 16 bit layer compositing in astroimage processing, which I happened to be doing today.

Yes, I see it as even at 50%, just as you've showed above, with my 64 bit version.  Here are my settings differences:

32 Bit:

64 Bit:

As with most things in Photoshop, it's more complex than we first imagine.

-Noel

I did more testing.  I was able to cause the difference seen by changing Cache Levels.  There may be other ways as well.

-Noel

When you changed the settings, did you close and restart Photoshop?  That's important.

Make sure your Cache level is set higher than 1.

-Noel