A double exposure is a combination of two images into one where one image is overlaid onto another at less than full opacity.. In film photography, double exposures are made by exposing the same portion of film twice, resulting in an the second image being superimposed on the first.
A clipping mask is a group of layers to which a mask is applied. The bottom most layer, or base layer, defines the visible boundaries of the entire group. For example, suppose you have a shape in the base layer, a photograph in the layer above it, and text in the topmost layer
When you select part of an image, the area that is not selected is masked, or protected from editing. So, when you create a mask, you isolate and protect areas of an image as you apply color changes, filters, or other effects to the rest of the image.
