Where can I find levels info

[thetexan]

I am looking for a place to go to learn more about the theory of levels, and other operations, for that matter.

For example, I want to know more about what happens to the distribution of pixel tones when you adjust the different sliders of levels. Much deeper than a simple tutorial. When I move the white slider down to say 180 from 255 I want to understand how those pixels are disposed of.

For example, above all pixels above 180 are clipped to 255 white, but how are the rest distributed. I believe the remaining tones (0-180) are distributed between 0 to 255. Thus 179 is now very close to 255. But the transistion between the redistribution and the even distribution above 180 must be something other than a sudden flattening out.

Well, these details and clinical technicalities interest me and I would like to know if there is a more technical exploration of how levels, curves, and other operations work, either in book form or on the internet.

thanks,
tex

 

[Steve]

Very complicated subject.
See if THIS gets you started.
Here's ANOTHER.

 

[Tom Mann]

This may be more mathematical than some may like, your other post on color --> transparency suggests you might appreciate the mathematical framework behind the change in shape of the histogram after application of a levels or a "curves" adjustment in PS.

Although the following method works for any of the histograms available in PS (eg, R, G, B, luminosity, C, M, Y, K, hue, sat, brightness, etc. etc.), to be concrete, for the sake of this discussion let's only consider the histogram of luminosity values.

The distribution of luminosity values can be considered a random variable. "Levels" or "Curves" are transformations (functions) applied to that random variable. What you are asking is one of the classic questions addressed in a junior or senior level college course in probability / statistics : "What is the distribution (or PDF - probability density function) of a function of a random variable?" BTW, note that the "levels" command is a monotonically increasing function -- when you read the material cited below, you'll see that this provides considerable simplification in the calculations.

The answer to the above question is available in many places on the web, but here are a few that I found on a quick search = {pdf of a function of a random variable} :

http://www.ece.uah.edu/courses/ee385/500ch4.pdf
- - - Take a look at Fig 4.2 in the above. This pretty much describes the case of a levels transformation in which you adjust all of the sliders, except you leave the middle (aka, "gamma") slider at its default setting. At its default setting, the curve in the middle of Fig 4.2 remains linear. If you move it, the shape of that section of the graph bows either up or down.

http://www.math.uiuc.edu/~r-ash/Stat/StatLec1-5.pdf

http://www.statlect.com/subon2/dstfun1.htm


HTH,

Tom