I need help with WorldToScreenPosition!

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Look:

Getting the angle between 2 objects

I prefer: var something : Transform; // position of the other object var p : Vector3 = transform.InverseTransformPoint(something.position); Now p is the position of something in my local coordinate space. If p.y > 0, it's above me. If p.x < 0 it's to my left. If p.z < 0 it's behind me. Very convenient for writing any kind of AI code.

Podperson describes how to use InverseTransformPoint. In your UI-element Update method just have: uielement.enabled = point.z > 0. Then it gets deactivated when the object is behind camera.