Prop tractor beam
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Excursion funnels are a game mechanic in Portal 2 which can be thought of as tractor beams. When a physics prop or the player enters an excursion funnel, they are held inside the funnel and slowly pushed through the funnel until they reach the end (excursion funnels are eventually cut off, typically by a brush or a static prop) at which point they remain still until the funnel is disabled. The player can leave the funnel at any time by moving towards the edge until dropping out of the beam. The player can also remove physics props from the funnel by grabbing them with the Use key.
Like light bridges, excursion funnels can be extended by placeing a portal at one end and the other portal somewhere else in the map.
Creating an Excursion Funnel
Creating an excursion funnel is a fairly straightforward process. Each funnel consists of a point entity called prop_tractor_beam. Valve typically puts a player clip brush in front of the prop_tractor_beam model as well as a light to give a soft blue glow to the area around the model (brightness level of approximately 50 120 250 50 is appropriate).
To control the flow of an excursion funnel, change the LinearForce keyvalue of the prop_tractor_beam. You can do this via the input SetLinearForce. LinearForce determines how much force is applied to objects in the funnel. A negative value will reverse the flow. Two good values are 250 and -250 for forward and backward flow, respectively.