Do it with the GIMPOverview of FeaturesLayersAlpha

2.7.1. Alpha

Every layer contains a special layer called the "alpha layer" that specifies its opacity. This layer is treated as a black and white bitmap that can be manipulated with all the normal tools, and in which black is considered full transparency (zero opacity) and white is full opacity between this layer and the layer below it.

The Resultant pixel of the two layers is calculated using the formula:

Result = (1-alpha) * PixelBelow + alpha * PixelAbove

Written by Shlomi Fish