How do you raytrace the mouse screen position onto a world XY Plane?

[jwatte]

I can think of two options:

1. Create a new trace channel in edit global settings; call it “placement”
2. Put a new blocker volume in the world, set it to block “placement” and ignore all other tests
3. Use this trace channel in the test of position-under-cursor

or:

1. Do what you do now
2. Normalize the vector you get (from camera to whatever position you hit) to length 1
3. Project the vector to the plane where you want to get the location (divide by dot between vector and plane normal; multiply by distance-to-plane from camera center)