Barycentric coordinates
The point D has bartcentric coordinates a, b, c with respect to the points A,B,C, where:
* a is the ratio of the red area to the total shaded area
* b is the ratio of the green area to the total shaded area
* c is the ratio of the blue area to the total shaded area
We have D=aA+bB+cC, and a+b+c=1.