RoboLee here and proud to present the one and only... ASHPD (AKA the Portal Gun, full name: Aperture Science Hand-held Portal Device)
above you can see that in the first portal jump he goes in a portal and comes out the other with his momentum conserved, but his velocity in the Y direction is reversed (unfortunately I captured this after I thought I got cos and sin mixed up, so the X and Y values effectively got swapped, gif below shows how it should work). In the second portal jump his momentum and direction is conserved (this is the default portal behaviour, press X to swap to the previous behaviour and press Z to change back to this behaviour)
(note all the gifs loop, and I'm too lazy to make them loop nicely, so it jumps at the end of the gif to the start)
here's a better gif of the old portal algorithm:
when you press the R key both portals disappear:
and when you shoot a new portal the old one of the same type will disappear:
featuring:
portal goodness on any map
and portal everything and anything from any mod
relative portal enter/exit positions
momentum conserved
when you fire new portals, old portals are removed from the scene
press R to remove both portals
two portal modes, the default new one which does not change direction of the portal-ing objects and the old one which was based on the line/surface momentum changing, change with Z (new) and X (old) keys.
buy from the "Tools" tab in the buy menu.