Curved Roads and Tracks

I love city building and transportation games, and ever since I worked on one, I’ve had a side interest in how to represent and draw roads and railroad tracks. After playing lots of Cities XL, I started to wonder how I’d represent curved roads.

The roads in Cities XL aren’t curves. They’re approximations to curves, using short line segments. I noticed in screenshots and videos of SimCity 5 that it also uses short line segments. Is there a way to represent curved roads without turning them into line segments? I wanted to avoid seeing roads like this:

“You see those roads in Hammerfell? They’ve got curved roads. Curved. Roads.”

So I wrote up my notes on circular arcs.

It took much longer than I had planned to write it. I started in October and was hoping to finish in December, but here it is two months later. Why did it take so long?

• I got distracted for several weeks learning about Geometric Algebra. This is a fascinating topic, and potentially useful for a future project, but I couldn’t figure out how to use it for this project.


• I’m quite rusty with math, and there’s some geometry and algebra involved here. The math for biarcs in particular took some time to figure out, with different approaches given in different papers, and none of them giving me everything I needed to know.


• Over the holidays I lost motivation, and ended up spending a month relaxing, hiking, and birdwatching.

Some things that could’ve gone better:

• The way arcs are defined in SVG seems really strange to me. The core definition is ambiguous so they include two extra flags to disambiguate. The way arcs are drawn in Canvas seems more intuitive but I’m guessing SVG’s approach must have some advantages.


• Arc radius goes to infinity for straight roads, which are common. I’m ignoring this to some extent but a real game will have to handle them. An alternative might be using curvature instead of radius, but I didn’t try this.


• IE9 doesn’t like d3.hsl() so I instead am using d3.hsl().toString(). This would’ve been easier to debug if IE had a nicer inspector.


• Android stock browser requires SVG clip paths to be defined inside <defs>; I’m not sure if that’s part of the standard but it was annoying to debug. Android’s Chrome and Firefox browsers are more forgiving.


• On iOS, large SVG arcs take a really long time to draw, and freeze up the phone. I have not put in a workaround.


• I’ve attempted to automatically adjust the layout for wide vs. narrow browsers, as well as mobile and tablet, but I’m unsatisfied with the results. It’ll have to do for now; I’ll take another look at this later.

It was lots of fun to play with this. However, I think that I may stick to grids for the next game.

Here’s the article about curved roads.