"Calculate A More Accurate Histogram Icon"

[swimpix]

I posted this on the Adobe forums and came up dry, so I thought I would try here.

CS5 Version 12.0.4 Mac OS X 10.10.5

1) The "Calculate A More Accurate Histogram Icon" in my Curves Adjustment panel is no longer visible.

This may be file specific as I can open new image files and previous work and the icon is displayed and works as expected. Then there are a couple of files where it will not display. Currently I am only seeing this with some jpeg files, although 98% of my files regardless of type or bit depth will display the Calculate A More Accurate Histogram Icon in the Adjustment Panel when I create a Curve Adjustment Layer.

2) As long as we are discussing this, does clicking on this actually recalculate the existing histogram to fill in any gaps or is it simply updating the view of the histogram from 8 bit to the actual 16 bit view assuming you are editing in 16 bit.

[klev]

Re: "Calculate A More Accurate Histogram Icon"

CS5 is no longer supported, so you aren't likely to get a response from Adobe. What's your actual question. Number 2 has an implied question. Number 1 is practically a statement. I don't know exactly what you're asking.

As for number 2, you can always check by trying in 8 and 16. Updating to fill gaps wouldn't make any sense. It's not how a histogram works, so it wouldn't make sense to use extra cpu cycles for something that is basically nonsense.

A histogram partitions the data. In this case each partition is a group of pixel values, presumably <r,g,b> triplets. The height of the histogram at each partition is determined by the percentage of pixels in the image that fall into that group. When you click the button for a more detailed histogram, it should recompute any partition that may have changed or at least retrieve the data (it might be updated by another thread in the background). There is no reason for photoshop to simply smooth it out. That would be a completely stupid approach for image data, and it would make the entire thing invalid.

If you're really worried, check by making screenshots of the resulting histograms on 8 and 16 bits per channel. I suspect if anything, the quick previews are accomplished by applying the adjustment to a greatly downsampled version of the image. This is a fairly common technique in image processing. See mip map if you're interested in reading about that.

[andrewrodney]

Re: &quot;Calculate A More Accurate Histogram Icon&quot;

#2 just updates the Histogram to be more 'accurate' as doing so for each edit and move of an adjustment would slow the show down. So Adobe provides a sub sampled histogram and gives you the opportunity to update it from the full data based on the most recent edit.

The Histogram can be useful and can be a huge waste of time depending on where you see it and what you hope it will tell you:

Everything you thought you wanted to know about Histograms

Another exhaustive 40 minute video examining:

What are histograms. In Photoshop, ACR, Lightroom.
Histograms: clipping color and tones, color spaces and color gamut.
Histogram and Photoshop’s Level’s command.
Histograms don’t tell us our images are good (examples).
Misconceptions about histograms. How they lie.
Histograms and Expose To The Right (ETTR).
Are histograms useful and if so, how?

Low rez (YouTube): http://www.youtube.com/watch?v=EjPsP4HhHhE
High rez: http://digitaldog.net/files/Histogram_Video.mov