Curves can be used to adjust the colors of an image with a very
high degree of control. It allows you to freely fine tune the intensity of every
shade by adjusting the transfer function, which is represented as a graph of
output value against input value.
Unlike the other adjustments, curves can be used to freely
adjust exactly how bright each shade of intensity in an image can be, in
particular, it can be used to highlight or dim only certain ranges of intensity
present in an image in a way that the others can't.
You adjust the graph using control points on a graph. Control points are added by clicking
anywhere that there currently is no control point, and may be removed by right-clicking on the desired control point.
To move a control point, simply click and drag it to a new location.
It is
not possible to have two control points vertically in line with each other, as
this would mean that a given input value maps to multiple
output values. It’s a logical impossibility, as it translates to a one-to-many mapping.
