Adjustments : Alpha Function
 

Alpha is like Curves, with the exception that it operates on the alpha channel (transparency). It can be used to adjust the alpha channel of an image with a very high degree of control. The transfer function is represented as a graph of output value against input value.

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.