Posted: Tue Sep 07, 2010 11:28 pm Post subject: 16-bit levels and curves flakiness
Hi all.
I'm having a bit of a problem with inconsistencies between running the Levels or Curves adjustment directly to a 16-bit grayscale image, versus doing so with an adjustment layer.
I want to be able to edit and make small cleanups to a very dark portion of a 16-bit image which is actually quite difficult to see (it's actually part of a digital elevation model, if anyone is curious). If I do the adjustment directly (via Adjustments->Levels), when I drag the sliders around... bringing the white end very close to the black, I see fairly dramatic grey level quantization as a preview, typical of if I was working with an 8-bit image. When I click OK, everything smooths itself out, thanks to the 16-bit data.
The problem is that I want to edit these dark regions (brightening them up enough to see what I am doing) but not actually affect the overall grey levels of the image. So I figure that the solution *should* be to just use an Adjustment Layer, do the same adjustment so I can visualize the changes, then when done, remove the adjustment layer and re-save. The trouble is that *most* of the time, adjusting Levels (or Curves - they behave the same) in the Adjustment Layer just visualizes the data as though it was an 8-bit image - it behaves just like the direct "Levels" command does while previewing, before I click "OK". The greyscale quantization that happens as a result is such that I can't see the details I'm looking for.
Am I doing something wrong? Is there a way to get the Adjustment Layer Levels to preview "better", using the full 16-bit image? Is there a different technique that someone might suggest to accomplish the same thing?
What makes it even stranger is that very rarely (10% of the time, and I haven't found any pattern behind it), Levels applied with an Adjustment Layer actually works properly, and the apparent 16 to 8 bit quantization in previewing the image doesn't happen.
If anyone has any idea that can shed light on this, needs more details on what I'm doing, or has any suggestions of something I can try, I've love to hear from you. Thanks!
Joined: 28 Aug 2010
Posts: 114
Location: Brisbane, Australia
Posted: Wed Sep 08, 2010 12:39 am Post subject: Re: 16-bit levels and curves flakiness
syzygy wrote:
If I do the adjustment directly (via Adjustments->Levels), when I drag the sliders around... bringing the white end very close to the black, I see fairly dramatic grey level quantization as a preview, typical of if I was working with an 8-bit image. When I click OK, everything smooths itself out, thanks to the 16-bit data.
I've never witnessed this behaviour. My Levels previews (either direct or adjustment layer) have always been accurate.
Posted: Wed Sep 08, 2010 1:51 am Post subject: Re: 16-bit levels and curves flakiness
I'm using CS5, though I've experienced the same issues with CS4 before.
One thing that's funny - this is an issue I've experienced for a long time, and I've spent a lot of troubleshooting time on trying to solve... and only an hour or two after making the posting (and continuing to troubleshoot), I discovered something significant, if rather unexpected.
It turns out that, at least on my machine, if I'm viewing the image at a zoom level of 67% or 100%, the full 16-bits are used in my extreme levels with an adjustment layer, and I don't get greyscale quantization. But if I'm viewing the same document at 50% or smaller, the quantization rears its ugly head. And of course, as luck would have it, the digital elevation model files I've been working with are quite large, so I've always been viewing them scaled at 50% or smaller. Now I've discovered that when I zoom in, everything looks good and the full 16 bits are used even with the Levels in an adjustment layer.
At least I now know a solution (I just need to zoom in enough when viewing the document). Kind of a strange and unexpected behaviour, but at least there's a workaround. I wonder if there is any reason why 50% seems to be the cutoff point... I couldn't find a preference setting anywhere that seemed remotely relate to the problem.
I just tried going into Photoshop->Preferences->Performance and turned off "Enable OpenGL Drawing", then restarted. But still the same (wrong) behaviour - quantization of the 16 bit grey levels when I'm viewing at zoom levels < 66%.
A rather obscure bug, if it can be properly called that, though it is now repeatable (at least for me) and the workaround of zooming in is doable, though unintuitive.
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