The inputs to the blending operation are grayscale values from 0.0 to 1.0 from the underlying image (Y_u) and RGB values from 0.0 to 1.0 from the superimposed image (RGB_s), and an opacity value from 0.0 to 1.0 (A).

The output is a single image containing RGB values (RGB_o) blended as:

R_o = R_s * A + Y_u * (1-A)

G_o = G_s * A + Y_u * (1-A)

B_o = B_s * A + Y_u * (1-A)