Let me give you an example: When I click the mouse in my app I spawn a star, but if you click and hold, the star isn't actually spawned yet and you can use the mouse to set up a line that specifies the star's initial velocity/trajectory. Once you let go the star is spawned. Right now I'm doing what you suggest, I'm drawing the trajectory line from the real world coordinates of the star's spawn location to the real world coordinates of the end of the line (how far the star will travel in one second). When you zoom in/out, this line either becomes much thicker than it was before, or very jagged and sometimes missing in parts as the distance increases between the viewer and the line. What I'd like to do is have the line only be drawn "screen-deep" and therefore always have the same thickness and clarity, but for that I need to be able to take the real world start and end coordinates of the star's trajectory and convert them into screen coordinates in order to draw the line outside of the influence of my current view.
