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I am making an adaptation of the classic Boids simulation from the 80s in Java. It works well enough, but I am trying to add a new rule to the behavior that would force the agents to avoid rectangles (walls) and I am not sure how to go about this.

I have seen this thread :

https://gamedev.stackexchange.com/questions/45381/wall-avoidance-steering

But I am confused by the syntax used (like partsList[j] -> normal) in the final code presented and how to obtain the distance between the agent and the rectangle, as well as how to actually drive the agents away. The formula makes sense though. Could someone please explain it to me? Thank you very much!

P.S. I have been following this pseudocode and I also used this Java source code as a reference.

Edit: Okay, I see why I was confused with the syntax, but I am still in the dark when it comes to writing the wall avoidance rule.

EvilTak
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Alex Hendrik
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1 Answers1

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Ah, I remember the 80s, and the boids simulation…

Generally in boids-like steering behaviors the idea is to apply a behavioral “steering force” against the velocity (momentum) of the agent. So implementing any given steering behavior comes down to finding some geometric construction that generates a vector pointing in the direction you want to turn. Ideally these will be tangential steering forces (perpendicular to the current velocity) so that steering is independent of speed control.

In the case of avoiding a wall—and a rectangle can be thought of as four walls—the general idea is to take a vector pointing away from (normal to) the wall. Using projection (dot product) you can separate out the components of that force that are parallel to and perpendicular to the velocity vector. The component of the wall normal that is perpendicular to the agent’s velocity is a steering force that will turn the agent away from the wall.

The other aspect is knowing when to use this wall avoidance behavior. A useful approach is to choose a time horizon, say 2 seconds, and decide if the agent would hit the wall within that time. Using current position, velocity, and that time value, you can do a simple linear prediction of where the agent will be in 2 seconds. If it crosses the wall during that interval then it ought to be using its wall avoidance behavior.

For more information, look up “containment” in that GDC 99 paper, and/or look at these: http://natureofcode.com/book/chapter-6-autonomous-agents/ https://gamedevelopment.tutsplus.com/series/understanding-steering-behaviors--gamedev-12732

Craig Reynolds
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