Semi-transparent overlay

The semi-transparent overlay pattern consists in laying two data layers or images on each other. The upper one is semi-transparent, allowing to view the one under. It differs from the layer pattern : overlays typically contain data covering common parts of the screen, while layers are meant to visualize data that covers complementary areas (e.g. green areas and transport lines). Sometimes, the pattern is complemented by an opacity slider allowing to set the opacity level. See also:

• http://benschmidt.org/mta/