sine function

edit

i want to get a window function which is

#define PI 3.141592653
window function = sin(pixel / (image width - 1) * PI)^2

However, when i tried to do it, the left side of '^' can not be double type. Is there any suggestion for this situation?

Also the function means hann window i guess. Is there any window function which can use from library like that equation??