Gaming Your Way

May contain nuts.

How the MTR car AI works ...

... or should, once I find the time to finish that slog. 

I must admit it was one of my worst ideas to take a stab at a racing game (and sell it to a client). The only luck so far is that it's a "when it's done" deal, so there's no deadline on this except my own wish to get it done and collect some cash for the client version.

The basic idea to do a "quick" little racing game wasn't that bad, but I really, really underestimated the amount of work I had (or rather still have) to poor into it. One of the most time eating tasks still is writing the car AI, which completely didn't appear on my radar until it was way to late.
I've written my share of AI code so far, so I thought this one wouldn't be much different. I think I was wrong with that.

The main big different with what I have written so far is that character can move freely, if something blocks the way, turn left or right and continue moving until you can resume the old path (or just run the a* again to find a new way to the player). For cars this doesn't quite work this way (now that's a surprise).

So cars need to use throttle to speed up and breaks to slow down you need to use steering angles to change direction and you must obey the limits (otherwise it'll looks like cheating). Moving the car along the track is quite easy if you have a fixed track, just draw a spline and let the car move along it. Too bad MTR has a built in track editor, so we need to construct the path for every track. (I covered this in an earlier post).

Waypoints for the AI.

Trouble enters the equation when we add other cars to the track and we cannot follow the waypoints anymore if some other car blocks the path. I calculate the throttle / steering values in steps, blending the values in the end (and then blend the desired values with the current ones).

First step is looking at the waypoints ahead and decide which lane is the shortest (using floating waypoints), then the desired throttle is calculated based on the distance between the last waypoint we've passed and the next. The shorter the distance the slower the car should go, I use 100% for the max distance and 75% for the smallest distance). 

Next step is deciding what angle we turn the wheels in order to head towards the next waypoint and if the angle is greater than the allowed one, we reduce speed further.

After this I have a basic throttle value (say 90%) and a steering value (say 20%, of the max steering angle).


Damn obstacles.

Now it gets a bit more complicated, we need to look for obstacles that might block the path we've just selected. The image above shows the problem, the red line shows the desired next spot we want to reach, the yellow line shows the current throttle, with the dark blue line showing the max throttle (but that's only for debugging). Important is the cyan line that is shown at the back of the green car, as it shows where we would hit the green car if we would follow the current path.

Right now, if this happens I offset the waypoint until we can pass the green car (adjusting steering to -20%), but only if there is enough room (there are also checks to see if we can pass on the left or right side of the obstacle), if we cannot pass we need to reduce speed to stay behind it.

Now just blend the values and pass them to the car and repeat this every time we hit a waypoint or another car is close enough to overtake.

If I ever get that video grabber to run I'll post a video next week.

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