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Revision as of 04:52, 23 January 2012 by WikiSysopPS (talk | contribs) (Visualizing color operations)
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Curves is the master adjustment, one ring to rule them all etc. Learn Curves if you learn nothing else. In this page I am going to introdice Curves to you and also try to introduce some fundamental facts about color and color adjustment.

Heres the Curves window:

Curves 01.png

It looks like a graph because it is. It graphs the value range change that you impose upon the image. Here's how you read it.

Curves 03.png

The Curve above is its default state and represents no change. Below is a curve adjustments that represents a multiplication value of 0.5.

Curves 02.png

Color and numbers

So... what is a multiplication? Well... they call it digital art because, at its root, it is derived from numbers. In the digital world black is 0 (zero) and white is 1 (one). The graph above can be read in the following manner:

  • If 1 is multiplied by 0.5 the result is 0.5.
  • If 0.5 is multiplied by 0.5 the result is 0.25
  • If 0 is multiplied by 0.5 (or indeed any other number) the result is 0.

So... color adjustments are mathematics. These maths are (for the most part) quiet simple.

I will drive this fact home with a small demo. In the image below the left half has had a layer of mid grey (50% brightness) placed above it and set to Multiplication Blend Mode. This will multiply the image by 0.5. The right half has had a 0.5 multiplication curve applied as an adjustment layer. As you can see, they are both identical.

Curves 04.png

Though its possible to be a digital artist without knowing too much about the underlying maths of what you are doing, I am telling you this for a reason: students often stab away at PS's color controls without knowing what they do. 'I just fiddle with them untilo it looks good' is what I often hear. Unless you know how you achieved your successes, then you will not be able to reproduce them.

Visualizing color operations

A good colorist will be able to visualize color operations not just through their effects upon the image, but in the shape of the curve. These shapes constitute 'thought forms' that are embodiments of their associated color edits.

Operation name AKA Use Thought form
Add Col add.png
Contrast S Shaped curve Increase contrast. In a scene it brings forms 'closer' to the viewer. Col contrast.png
Gamma Lightens the dark areas but does not affect the blacks. Corrects under-exposed images.

A gamma of different values across the three channels is the ideal color corrector: good for removing color-cast from a neutral area.

Col gamma.png
Lift Offset Lightens the darks and the blacks. Good for pushing distant hills into the distance. Col lift.png
Multiply Gain, Brightness, Exposure Will brighten or darken an image but not affect the blacks.

A multiply of different values across the three channels will neutralize color-cast whites.

Col multiply.png
Clamp Flattens the data at both ends of the value range. Not a whole heap of use. Col clamp.png
Saturation Well... what is this doing here? This is the logic:
  • The RGB values of a neutral value such as grey are all identical.
  • An image is color-neutral if its RG and B channel look the same.
  • A saturation is, in effect, an averaging of channel information.
Col sat.png
Posterize Eeeek! This forces the values of the image into sharp, black and white. It is usually done on masks to create a hard-edge mask but is sometimes useful for other purposes. An example of its use is on the Matching Elements Workflow page. Col arbitrary.png
Arbitrary Color lookup, LUT Sometimes, just sometimes, you want to pull around the information any old which way. BTW... the Curves in the very first version of Photoshop was called arbitrary and some pros still call a Curves-style adjustment an arbitrary look-up (a look-up being technically what is happening under the hood as the before and after values all live within a table whose values are 'looked-up'). Col arbitrary.png