Values above one subtract a portion of in0 while scaling in1. Values below 0.0 have the opposite effect. Extrapolating away from a blurred image increases sharpness. In the following examples, a single alpha value is used per image. However other processing is possible, for example where alpha is a function of X and Y, or where a brush footprint controls alpha near the cursor.
Interpolation darkens the image, and extrapolation brightens it.
In both cases, brighter pixels are affected more. Changing Contrast Contrast can be controlled using a constant gray image with the average image luminance. Interpolation reduces contrast and extrapolation boosts it. Negative alpha generates inverted images with varying contrast. In all cases, the average image luminance is constant.