Question

i have a 3d world where i have several 2d circles laying on the ground facing to the sky.

how can i check if a line will intersect one of those circles frop top-to-down?

i tried to search but all i get is this kind of intersection test:
http://mathworld.wolfram.com/Circle-LineIntersection.html

but its not what i need, here is image what i mean:
http://imageshack.us/m/192/8343/linecircleintersect.png

Answer 1

If you are in a coordinate system, where the ground is given by z = c for c some constant, then you could simply calculate the x, y coordinates of the line for z = c. Now for a circle of origin x0, y0 and radius R, you would simply check if

(x – x0)^2 + (y – y0)^2 <= R^2.

If this is true, the line intersects the circle.