you want the player to be able to click anywhere on the screen and have it project a ray from the camera towards the point in 3D space that their cursor is over
do you:
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spend most of the day fucking around with pitch and yaw rotations, scribbling trig diagrams in your notebook, trying to figure out exactly how the engine handles FOV projection and aspect ratios and how the virtual screen dimensions of the GUI system relate to the actual screen resolution
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just pretend they're clicking on a tiny screen one unit away from their eyeballs and have it work on the first try
