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trouble casting a ray

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Hi, I'm having trouble casting a ray from the camera. I'm trying cast a ray from the position of the mouse. I have calculated the camera axises and I've normalized the mouse coordinates so that both the x and y position go from -1 to 1. I'm trying to see where the ray intersects along the plan where y = 0. The camera is always 10 units above the ground. n = eye - look I've tried rotating my normalized n vector, by the offset of the tan(mousey * fov) and I've tried several other methods, most of which I've forgotten. Any help is appreciated.

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i'm assuming you use openGL.

take vector
[mouseX,mouseY,1,0] (with x,y from -1 to 1)
and compute
direction=inverse(projection_matrix_as_loaded_into_GL*modelview_matrix_as_loaded_into_GL)*[mouseX,mouseY,1,0]
You should get direction.
Idea is simple: points is transformed to screen that way:
(projection_matrix_as_loaded_into_GL*modelview_matrix_as_loaded_into_GL)*direction

What we need is essentially opposite,

direction=inverse(projection_matrix_as_loaded_into_GL*modelview_matrix_as_loaded_into_GL)*screen_point

(result needs to be normalized)

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