6 Replies Latest reply on Mar 22, 2013 2:38 PM by conroy

    Levels preview on channel is different to result - sometimes! Causes?

    Dave S-C Level 1

      Hi,

       

      Some background (and possibly useful to some): In CS3 I use a Dust & Scratches filter on a layer in a high resolution (3200+ppi) 16bpc image, then traditionally change the layer blend mode to Difference, make a duplicate of a channel which has the clearest information of where the filtered changes were made, and use levels to increase the contrast on a copy of that channel. Occasionally I get this result though - the Levels adjustment preview shows spots clearly, but clicking OK results in something very different from the preview. This only seems to happen occasionally.

      So preview looks like this:

      levels01.jpg

      and result looks like this:

      levels02.jpg

      As the purpose of the channel is to make a mask, the result isn't as good as I'd like. I assumed that the problem might have been the grey starting colour of the channel, so I pushed the 0 up to 9 in an attempt to get a uniform K value of 100% on the darkest areas, and then ran a second Levels adjustment to make the white spots brighter. The second levels adjustment resulted in pretty much the same result as above, again, not what was shown in the preview. I also assumed that it may be down to numbers and that levels is occasionally too limited to deal with all the shades of grey on a 16 bit channel. Probably wrongly as this normally works exactly as required.

       

      This only happens occasionally, and isn't that big a deal - I can quite happily go and use a Curve instead. I'm just wondering why the preview in the adjustment window is different from the actual result. The fact that it only happens on some of my images is what concerns me - purging all beforehand does not make any difference.

       

      Anybody any idea what is causing this and how to get accurate results in the preview? Is the preview using a different method to show the results? Unfortunately I won't have a copy of CS6 to try this on until a couple of weeks from now, so not sure the best way for someone to try and replicate it on newer versions.

        • 1. Re: Levels preview on channel is different to result - sometimes! Causes?
          Dave S-C Level 1

          I've created a single channel document (28MB) and uploaded it to Dropbox - the values that I'd like to enter in Levels on this are 8, 1.00 and 10. You should see a clear difference between the preview and the result.

          https://www.dropbox.com/s/nsun0lbzxziozw2/example.psd

          • 2. Re: Levels preview on channel is different to result - sometimes! Causes?
            conroy Level 5

            You write about a problem with 16 bpc documents but provide a 8 bpc doc. Didn't you mean to supply a 16 bpc doc?

             

            Anyway, even with your 8 bpc doc and working in 8-bit mode, the reason for the difference between preview and actual adjustment is that when zoomed to less than about 64% and with Cache Levels greater than 1, the preview image is calculated with very low precision. Photoshop does very low precision display calculation with Adjustment Layers and layer blending in general, too, unless the zoom is at least 64% or Cache Levels is 1.

             

            Low precision display tends to be more disturbing in 16-bit mode than 8-bit mode because of the greater reduction in precision in the former mode.

             

            To prevent this lowering of display precision, you can set Cache Levels to 1 in Preferences > Performance then relaunch Photoshop, but some graphics cards can have problems in 16-bit mode when CL is limited to 1. Also, reduced cache levels can result in noticeably jaggy interpolation of the display pixels when zoomed out and reduction in Photoshop's speed.

             

            Ironically, Photoshop CS6 requires us to have a quite powerful modern computer for its newest features, while it still uses display shortcuts which I presume to have been deliberately made very imprecise so the program would work reasonably fast on antique equipment.

            • 3. Re: Levels preview on channel is different to result - sometimes! Causes?
              Dave S-C Level 1

              Thank you conroy, I suspected it may be something like that - the reason my Curve appeared to provide better results was that I was zoomed in at 100% when I ran it. I'll definitely remember in the future! Yes, the file provided was 8bit, sorry about that - I was getting the same result with that file because of the low Zoom level. Thanks for the clarification regarding cache, zoom, and previews - I'm assuming that this applies across the whole application, would that be correct?

              • 4. Re: Levels preview on channel is different to result - sometimes! Causes?
                conroy Level 5

                As far as I know, the low precision display affects previews of adjustments and the blending of layers in both 8-bit and 16-bit modes. Here is an 8 bpc example which compares a Subtract blend of two layers of noise with the merged layers at zoom levels of 100%, 64%, 63% and 33.33%. The left doc has the separate layers and the right has the single layer which results from merging the two layers. The transition from precise to imprecise display happens at just below 64% zoom when Cache Levels is greater than 1.

                 

                Screen-shot-2013-03-22-at-20.51.40.png

                 

                Screen-shot-2013-03-22-at-20.49.03.png

                 

                Screen-shot-2013-03-22-at-20.49.26.png

                 

                Screen-shot-2013-03-22-at-20.52.31.png

                1 person found this helpful
                • 5. Re: Levels preview on channel is different to result - sometimes! Causes?
                  Dave S-C Level 1

                  Thanks again - I had been trying to work out why most often I got accurate results with level adjustments on layers and it was only occasionally that the 'problem' occurred. Purely down to Zoom level when running the adjustment. The differences you illustrate above are very handy - between 64% and 63% they are immense, and very important to be aware of, especially when working on very large (in pixel dimensions) images.

                  • 6. Re: Levels preview on channel is different to result - sometimes! Causes?
                    conroy Level 5

                    It's strange that Photoshop provides 16 bpc document mode, supports 10 bpc display if you have Windows and a suitable graphics card and monitor, yet, much of the time, it's doing very low precision calculation of the image to be displayed.