The stuff I do

An attempt at procedural race generation

- 2874 words -

← Posts

For several weeks now I have been working on and off on an ambitious project in which I want to procedurally generate a racetrack to create autonomous vehicles which will learn to drive on it thanks to a genetic algorithm. I did a lot of different experiments and even got some cool passable results but I am hitting a point where there is too much refactoring to be done to complete the project with the current code base. As I don't have the motivation to put more work to do things properly and as I also have other projects in mind that I want to start working on I'm writing this post to keep a trace of what I have done and what I would like to do differently. This writing will be messy and probably not very interesting for anyone else than myself but I hope that it will be helpful in a few months when I decide to build something on top of this experiment.

Generating a racetrack 🔗

My goal was to generate a racetrack which would be a loop with several turns with angles sufficiently difficult to be interesting. To do that I took a lot of inspiration from these two articles:

In the first post Gustavo Maciel describes an idea to generate a racetrack in three simple steps:

The second post is a second implementation of Maciel's idea, and my project is yet another implementation of the same idea but with my very own bugs and design flaws which makes it very special to my eyes 🤩.

Generating points 🔗

The first step was fairly easy, I decided to start prototyping on codepen with p5.js to get a quick feedback loop.
So I created a Point class which would hold a position as a P5.Vector as well as a numerical ID and the drawing logic. The idea of using a class to supercharge a simple vector is probably not a bad one, but in the following steps I ended up mixing my point for some uses with pure P5.Vector for other uses and that inevitably lead to some friction. Next time I work on this project I think it's a good idea to put more efforts in the design of this class so that I can use it exclusively. That will avoid having to write utils functions which have to handle both types as inputs which quickly becomes frustrating.

In later iterations this class also has some methods like move() that I use to change the position of the point (for example when two points are too close to form a proper hull) or constrain() that I use to make sure the point is moved back to the screen if I pushed it too far away with the move() function. This became a major pain: with this model the point is aware of itself and does some corrections which should only be done by the code responsible for generating the hull (separation of concerns much?).

Once this class exists I also created a Terrain class responsible for generating a few points and pushing them away when they are closer than an arbitrary distance. This pushing away operation will have to be repeated several times until the configuration stabilizes:

See the Pen Polygon generator (spacing points) by Adrien Fabre (@statox) on CodePen.

Generating a hull 🔗

This is where things get tricky: Generating a bunch of points is fairly easy, generating a polygon from this set is a much larger problem. A great resource to understand the problem at hand is the Convex hull algorithms wikipedia page. This page is a big rabbit hole and it's easy to get lost in all its links for hours, however I wanted to get things done so I decided to implement the Gift wrapping algorithm which is one of the simplest ways to find the hull of a polygon.

The Wikipedia page explains it better than I do but the basic idea is to pick up a point in your list of point and look for the point for which all the other points are on the right of the line formed by your two selected points and to continue doing that until you have reached your initial point again.

Fortunately the page has a pseudo code implementation so I was able to port that to javascript.

See the Pen Polygon Convex Hull with Jarvis March algorithm by Adrien Fabre (@statox) on CodePen.

After this successful attempt I decided that the hulls generated were too boring: I want some deadly turns!

The gift wrapping algorithm generates a convex hull which is a simple, boring envelope of the points. A concave hull however is a polygon with at least one angle between 180 and 360 degrees exclusive, which means that this polygon has a much more interesting shape with much more complex turns. It is however more complex to generate. So I explored the web a bit more and ended up on Concave hull: A k-nearest neighbours approach for the computation of the region occupied by a set of points. This paper describes an algorithm to generate a concave hull using the same idea as the gift wrapping but at each step, instead of choosing the next point among the complete list of generated point, we restrict the search the nearest neighbors of the current point.

This looks much more interesting:

See the Pen Polygon Concave Hull with Custom algorithm by Adrien Fabre (@statox) on CodePen.

However with concave hull come two big issues:

There are several ways to get rid of these issues:

I think that the important thing here anyway is to have your hull as clean as possible before you move on to the next step otherwise it will inevitably bite you.

I'm putting the previous sentence in bold because of course I was too impatient to get to the next step so, of course, after thinking about the possible solutions for half an hour I decided to ignore the problem until it's too late.

Interpolating the points 🔗

Earlier this year I did an experiment with Bezier curves when this project was just a vague idea in the back of my mind. The basic idea is to generate a bunch of points (hence the two previous parts) to get a polygon and use the formula created by Bezier to add more points to this polygon to make the curves smoother.

So I kept my two hull algorithms (the gift wrapping and the k-nearest neighbors), plugged my code generating Bezier curves and got this result:

See the Pen Bezier curves for polygon hull by Adrien Fabre (@statox) on CodePen.

The results are quite nice we can see something which could look like a racetrack, that's the right path! However our issues with interesctions and narrow angles are still here and once again I decided that fixing them was a problem for future me 🤷.

Adding some width 🔗

So now I have a racetrack with a few hundreds points but they still form a simple line. There are a lot of options to make this line an actual road, my goal is to have cars detecting their position on the road with a ray tracing algorithm that I'll describe more later. To run this ray tracing I need to have a bunch of segments which represent the borders of my road.

So I created a Track object which get the points of the hull. For each point two more points are generated perpendicularly to the direction of the hull at the point. These two points are stored in two arrays which materialize the left and right borders of the track.

Here you can see in blue the initial hull, in green the right border and in red the left border:

See the Pen Race track generator by Adrien Fabre (@statox) on CodePen.

It's also at this point that I finally decided to start fixing the intersection issue. And of course I did it in a not very bright way: When the angle between three point is less than an arbitrary threshold I simply remove the middle point. That give this weird correction algorithm that you see in action in the previous codepen. And that introduces a worst problem which is that by doing that I change the width of the track on some portions.

While I was trying to fix this issue I also wondered how I would paint the road because p5.js doesn't have a way to create concave shapes. I came up with a simple solution which is to change the way I draw the line of the hull: By making the stroke weight much bigger I get something which looks even more like a racetrack:

See the Pen Race track generator (better smoothing) by Adrien Fabre (@statox) on CodePen.

With this implementation I thought I could display the track and have not-very-precise-but-still-good-enough border representation for the cars to run but realized that this is not good enough. So I came up with a solution which I find quite interesting.

Detecting that a car is not on the road 🔗

At this point I started merging everything I had done before into a real Github project with typescript, a real linter and all the right tools to develop this project properly. I tagged my commits properly at each steps so that I could showcase the progress of my project, but now I'm just too lazy to do that so I won't show demos.

One of the interesting ideas I had in the project was to find a way to easily check if an object is on the track or not. What I do is that I generate the track I described before and draw it on a HTML canvas with p5.js. I then use p5.js capabilities to read all pixels on the screen and keep a trace of its color. After that I can reduce the position of my car to a single pixel and check its color, if it's the track color we are good to continue otherwise the car is crashed.

I think this is a pertinent approach as once the track is stored in memory the check is in constant time however my implementation was not the smartest:

Making smart cars 🔗

Ray tracing 🔗

As I am impatient and stubborn I started creating my vehicles before my racetrack was fully ready, which unsurprisingly made things harder. At first things were going well: I had a codepen, I reused some ray tracing code made by the amazing Daniel Shiffman for one of his coding challenges, I plugged that on a car that is controlled with the arrow keys, WASD or ZQSD and here we are:

See the Pen Ray casting again by Adrien Fabre (@statox) on CodePen.

Evaluation function 🔗

Once I know how where my track is I also need to understand how far a car is on the track. My initial idea was to follow a "surface filling" algorithm. I can't find the source which gave me this idea but the principle is simple: You take your starting point (here it's a pixel on the track), you score it 0, then you take its adjacent neighbors and score them 1 and you can recursively score all the pixels of your track. This way, the father your point is on the track the higher is its score. This approach is not completely straight forward on a circular racetrack:

First, you need to decide of a starting point and a direction for your track. All your cars needs to go in the same direction, otherwise it is not a race anymore. Deciding how to store this direction was surprisingly painful, I tried different approach and they all ended up inconvenient because of how messy my data model was and how strongly coupled all the components were.

Once this starting position and direction are decided you need to create a border that you surface filling algorithm should not cross, otherwise the pixels will be scored proportionally to their distance to the starting point which will just maximize the scores in the middle of the track.

The evaluation function also has an issue to keep track of laps: It is super important to be able to say "this car ended up on a very high score pixel but I shouldn't score it very high because it went there by turning around at the start and just being stuck there for its whole race".

All the rest 🔗

At this point I started to frantically code more and more technical debt features in my code:

Thinking of the future 🔗

At this point I've been working on this project for several weeks and I know I've reached my time limit for a side project: I know how I want to do the next iteration but that will require a lot of rework and I'm starting to get too tempted by other new shinny projects to put the efforts necessary to make something good. So I will stop here with a demo that is hosted on github page.

Despite not being a real success this project is not a total failure neither: I had a lot of fun working on it and that's the most important part of my side projects. But I also learned a lot and I think that the mistakes that I made will really guide me in a next iteration probably next year!

← Posts


Related posts

Posts in the same category: [procedural]


Comments