Unit collision avoidance for RTS

Question

I'm developing an RTS, and having a little difficulty with collision detection. From what I understand RTS generally don't bother with collision detection and just try to avoid collision, and that is what I am trying to accomplish.

My game uses a grid for static obstacles, but units are not bound to the grid for movement. How do I allow for units to avoid collisions, both with static obstacles and with other units? Right now my workaround is to redirect units around static obstacles every time they move from point to point (and every time a new obstruction is placed on map, units replan their routes). This doesn't seem like an ideal solution, but it works. I have no idea how to handle unit-unit collisions.

Answer 1

You are on the right track.

I have re-implemented collision avoidance for my RTS taking Recast as example. General approach is quite simple: Each agent (that's how they call units in collision-avoidance) checks for neighbours and obstacles and chooses path depending on its desired movement direction, other agents desired directions and allowed space. Complications arise from making this efficiently.

All agents are usually implemented as circles. Each agent keeps a list of 6-8 closest agents and sees if they are in their way. Obstacles are usually circles or NavMesh boundaries. Agents emit rays and check if they collide with any obstacles edges.

There are many pitfalls in implementation, so unless you want to make your game specifically-unique collision-avoidance based, better take an existing library.

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I have researched the topic and have made a list of available libraries (actualized in 2014), that implement agents navigation and collision avoidance:

• Recast Navigation/Detour/Crowd
• ORCA: Optimal Reciprocal Collision Avoidance
• RVO2
• HRVO
• ClearPath: Highly Parallel Collision Avoidance
• OpenSteer
• SimplePath

You might want to see their sites, some had explanatory videos and texts.