Curves

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Curves is the master adjustment, one ring to rule them all etc. Learn Curves if you learn nothing else innthe adjustments menu. In this page I am going to introduce 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. What is a multiplication? Have a look at the Color as Numbers page for a full answer but briefly... it means that all the color values are being multiplied by (in this instance) 0.5.

Curves 02.png

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 post.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

Curve window niceties

The principle of curves is, once you get used to it, easy enough. The Curves dialogue box itself is also easy enough to grasp, though it has one or two niceties.

  • Adding a point is just a matter of clicking on it.
  • The points on the curve can be moved around by hand or by using the Arrow keys. This enables much finer control.
  • A point can be selected and deleted with the Delete key.
  • A histogram is also visible in the curves window which can be used as a guide for moving the black and white points around.
  • Command clicking on a part of the image will place a point at the part of the curve that corresponds to that RGB value.

Combined operations

The default curve adjusts the lightness values of an image. The RGB channels each have there own curve, to be used for when you are adjusting the hue of the image.

I have said this before, but... caution do not attempt to adjust lightness and color in one pass. They are seperate things, to be thought about in different ways.

Eyedropper icons

The eyedropper icons underneath the curve window are for targeting the colors of your image: moving the values of one color to brand new values. In this way a slightly color-cast white can be moved to a neutral white. These are sometimes usefull, but a 'hand made'bre-targeting is often better. The target color can be edited by double-clicking on the eyedropper icon.