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Input/Output for Levels and Curves


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I have been fooling around with Levels and Curves a little bit. I have tried to figure out what input and output [channels] refer to. I cannot figure it out. Can anyone give a thorough but not too long explanation of what the difference between input and output is? I understand the channels (rgb). For instance, in the Curves dialog, the help on the website says the horizontal lines represent the intensity of the input and the vertical the output. I can see what it does, but cannot understand enough to use it and know what I am actually doing. Can anyone help? Help on the "Levels Adjustment" dialog would also be of help to me. Thanks.

"He is no fool who gives what he cannot keep to gain that which he cannot lose."

-Jim Elliot

John 3:36

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Honestly, I'm with him.^

I use curves, and I kind of know what I'm doing with it, but not enough. I've read the help thing about it, even searched elsewhere a bit.

I'm going to go reread the help thing though, it has been a while...

EDIT:

Basically... What I get out of it is, for luminosity, the right-up area of the grid represents lighter pixels, while the left-down area represents darker pixels. When the curve is raised on the left (darker pixels) it makes them lighter.

So, the 'x-axis' can be considered the input pixel brightness, and the 'y-axis' can be considered the output pixel brightness. Correct me if I'm wrong...

*goes to read about RGB curves*

EDIT:

I'm not sure about this, but I believe it works the same way for RGB. For example, if you have the red portion checked the input (x axis) of the R value changes to the output (y axis) R value.

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The Levels dialog mimics Photoshop very closely, the difference being that the input and output controls are switched (for some reason, unknown to me).

Do a search online (Google should work) for explanations of Photoshop's Levels functions and that will give you a good idea of how it works...

 

Take responsibility for your own intelligence. ;) -Rick Brewster

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The difference between input and output is exactly my problem. I figured out also that placing a dot on the left side of the diagonal line causes what effect you have selected (luminosity/RGB) to get brighter. Placing a dot on the right side of the diagonal line makes the pixels darker. However, a big hole in my understanding is what is the difference between placing a dot on the bottom left of the line and the top left of line (diagonal line, that is) or both? The Levels adjustment is simply an unknown to me. That is, the graphs and all. I did notice one thing. The horizontal and vertical line have values between 0 and 255, which corresponds with the RGB color values respectively that I find in the expanded colors dialog. That has something to do with it. What, I don't know. I know the pair of numbers (I call a coordinate pair, from high school :!: ) have 0-255 respectively and refer to the place on the grid/graph.

"He is no fool who gives what he cannot keep to gain that which he cannot lose."

-Jim Elliot

John 3:36

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Here's the quickest summary I can give you:

Move the curve down, the picture gets darker.

Move the curve up, the picture gets lighter.

Oh, and for the record, the 255 is actually 256. The numbers go by multiples of 4 and 8. For example, "128,128" is the exact middle of the curve, and 64 is the 25% mark. The same goes for transparency (i.e. the opacity slider in the "Layer Properties" window). I can understand if you don't get what I'm saying, as I had no idea how to work with curves over the first month or two.

"The greatest thing about the Internet is that you can write anything you want and give it a false source." ~Ezra Pound

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My point in bringing up Photoshop is that it is a program with a very large user base. I have personally seen the the Levels dialog discussed in detail in a few PS tutorials.

'Levels' in PS and 'Levels' in PdN have the same rudiment function. As I stated before, the only obvious difference is that the controls for input and output are reversed. The values that are referenced for 'input' and 'output' remain the same.

That said, any info you find on PS's Levels (with its immensely large knowledge/user base) would help you understand how the same function works in PdN.

I would look into this but I am not the type that has to know how/why something works to appreciate its capability.

 

Take responsibility for your own intelligence. ;) -Rick Brewster

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I would look into this but I am not the type that has to know how/why something works to appreciate its capability.

Yes, but understanding why/how something works may allow you to make it work in a way that better suits you. That's why I always want to understand how stuff works... lol.

Oh, and for the record, the 255 is actually 256.

It's 256 because it includes 0. The values go from 0-255.

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