@NEETzsche@marine Wrong, since momentum is relative towards the frame of reference.
Immagine, that instead of portal, it is a car window, and the box is a person standing still.
While outside of the window, the person is still, but once he is met with it, he can be seen flying through the car :D
Also, you can imagine looking through the blue portal, you will see the box flying towards you at great speed as it enters the portal, and once it passes, it has no reason to stop.
The box is not the object that is moving. It is the portal. The box has no velocity, so when it finds itself on the other side of the portal, it just falls down.
@NEETzsche@marine Neither are you moving when a car is aproaching, yet once you're in a car, the changed frame of reference will make you moving in the same speed through the car, as it was moving towards you.
The way they modeled this in the game is that the object itself has a velocity and a direction (a vector basically), and the magnitude of that vector doesn't change when passing through the portal, only its orientation. I'm pretty sure if we made a map that does what the image is describing, the object will just fall down, as the current magnitude of its velocity vector is 0.0.
And, while someone managed to re-write the game with their own phisics engine, you could easily (and correctly) claim, that it's a question of physics implementation then. https://www.youtube.com/watch?v=ao1qVi5Qp3Y
In the example you gave, the guy very pointedly rewrote the engine to make sure that the velocity of the object entering the portal is relative to that portal, specifically, which I suppose makes sense since the re-orientation is basically just making the velocity relative to the portal exiting.
However, it doesn't follow from the logic these engines actually use, which is that objects have a velocity that's not really relative to anything. It's basically a given value.