Question

I want to get Screen Position from World Position in DirectX.

I can get my ball’s world position. But I can’t know how can I convert it to screen position.

Answer 1

You have a view/eye transform V (the one that “places” your “camera”) and a projection transform P.

Clip space coordinates are reached by

clip_position = P * V * world_space_position

From clip space you reach NDC space by dividing the clip space coordinates by the 4th clip space coordinate w, i.e.

ndc_x = clip_x / clip_w
ndc_y = clip_y / clip_w
ndc_z = clip_z / clip_w
ndc_w = clip_w / clip_w = 1

The viewport XY coordinates are then reached by mapping the range [-1,1] to the viewport dimensions. The difference between OpenGL and DirectX is, that in OpenGL the depth range [-1,1] is mapped to [0, DEPTH_BUFFER_RESOLUTION], while in DirectX it’s the depth range [0, 1] that maps to the depth buffer value range.