I have the world, view, and projection matrices. I have an object that is part of the UI and should be the same size no matter how far from the camera it is. I'm using this to create a gizmo similar to the "move" gizmos you see in 3D modelling program.
How can I extract a proper scale to do this from world/view/projection (or from the view frustrum if preferred, I have the six planes).
Bonus question: Using these same matrices, is there a decent way to figure out how much I'd need to move a 3D vector to make it move ONE PIXEL visually?