Question

Given two images A,B I want a third image C which is as if B had transparency of t=0.5 and placed on top of A.

How is C calculated in reality and how n affects it ? I am not interested in any program or pseudo code I just want to know the basic rationale.

One way I think is C is nothing but alternating pixels of A and B. What are the other ways ?

Answer 1

The color and optionally transparency of each pixel of A and B is combined according to the weight.

If the transparency is 0.75, then typically 25% of the color values from B and 75% of the color values from A (the underlying image) would be used.

Basically, the red, green, blue, and optionally the alpha channels are each calculated like this, and then recombined to form one resulting pixel.

Example:

A = [1 0 0]  <-- red
B = [0 1 0]  <-- blue
a = 0.75 (which means B is more transparent than it is opaque)

C = [
    0.75 = 1 * 0.75 + 0 * 0.25 (red component)
    0.25 = 0 * 0.75 + 1 * 0.25 (green component)
    0.00 = 0 * 0.75 + 0 * 0.25 (blue component)
] //       A          B

If the images have alpha channels on their own, the calculation becomes more complex.