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Asked: 2025-04-08T00:14:27+05:30

Why aren’t the circles colliding with each other or with rectangles in my Box2D simulation?

I’m diving into Box2D for the first time, trying to get a basic physics simulation going with Raylib, and I hit a snag that I just can’t seem to figure out. I’ve set up the collision between static and dynamic rectangles already, which is awesome, but now I’m trying to add circles into the mix, and they just won’t collide with anything—not even with each other. It’s completely baffling!

So, I’ve got this minimal reproducible example that I put together, and I’m hoping someone can spot what I might be missing. Here’s the gist of what I’m doing:

I start off by creating a Box2D world and set gravity so that everything falls down. I then add a static ground rectangle, which works fine. After that, I’ve got a dynamic square, and its collisions seem to work like a charm—it’s falling and bouncing off the ground as expected.

But then there’s the circle. I create a dynamic circle and position it away from the ground and the square, and when I run the simulation, it just falls straight through the ground without any collision happening at all! I even made sure to set its density and friction in the shape definition, similar to how I did for the rectangle.

I thought perhaps the issue was how I’m defining or creating the circle shape, so I double-checked that part too. I’m using default settings for the shape definitions – perhaps that’s where something’s going awry?

I also noticed that I’m using the Box2D functions for creating shapes and bodies, but I’m not entirely sure if I’m implementing them correctly, especially for the circles. It’s all feeling a bit overwhelming.

I’m basically stuck and cannot figure out if it’s a setup issue or if there’s just a step I’ve missed. Has anyone else run into this problem? I’d really appreciate any pointers or insights into what might be going wrong here. Thanks a ton in advance!

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    2 Answers

    1. Box2D Circle Collision Issue

      Sounds like you’re having a tough time getting your circles to cooperate with Box2D. It can definitely be a bit confusing when you’re just starting out!

      Here are a few things you might want to check:

      • Circle Shape Definition: Make sure you’re defining the circle correctly. When you create a circle shape, you typically do something like this: 
        b2CircleShape circleShape;
circleShape.m_radius = yourRadius; // Set the radius
      • Body Type: Ensure that the body for your circle is set to dynamic so that it can respond to collisions and gravity: 
        b2BodyDef circleBodyDef;
circleBodyDef.type = b2_dynamicBody;
      • Positioning: Check the position where you’re placing the circle. If it’s way below the ground or overlapping with the ground, it might fall straight through. Coordinates can be tricky!
      • Density and Friction: Just like you did with the rectangle, make sure you’re setting the density and friction for the circle correctly: 
        b2FixtureDef circleFixtureDef;
circleFixtureDef.density = yourDensity; // Set density
circleFixtureDef.friction = yourFriction; // Set friction
      • Debugging: Sometimes it’s helpful to visualize the shapes and bodies using Raylib’s debugging tools. This can give you insight into whether they’re being created properly.

      If everything else looks good and it’s still not working, you might want to share a snippet of your code for the circle. Other folks here might be able to spot something you’ve missed!

      Keep at it, and you’ll get it sorted out!

    2. The most common reason your circle isn’t colliding in Box2D is due to incorrectly setting up the circle’s fixture or missing certain required parameters. While rectangles use b2PolygonShape, circles must utilize b2CircleShape, defining both position and radius explicitly. Ensure you’re correctly initializing the circle shape with something like: b2CircleShape circleShape; circleShape.m_radius = desiredRadius;. Then, make sure you’re correctly attaching this shape to a fixture through b2FixtureDef, including necessary properties like density, friction, and restitution, similar to how you’ve handled rectangles.

      Another possibility is simply forgetting to set the shape’s radius or mixing units (pixels vs. meters)—make sure you consistently use Box2D’s unit system (often 1 unit = 1 meter). Also, ensure you’re not accidentally defining the circle shape within a temporary or local scope that gets destroyed prematurely after creating the fixture. Finally, inspect your fixture attachment carefully: verify you’re actually invoking CreateFixture appropriately on your circle body’s definition. Correcting these potential mistakes will help your circles start properly colliding with other objects.