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A 2D rigidbody physics system written for yoyoengine.

Features

• Broad phase collision detection, using a hashed spatial grid
• OOB and Circle collision detection and resolution
• Collision and trigger callbacks
• Easy synchronization with existing ECS
• Frustum-culled sleeping objects
• Spring and Hinge Joints
• Collision layers

Warning

Internal rotations are stored in degrees (not radians!) so make sure you convert properly.

Usage

#include <p2d/p2d.h>

// example collision callback
void collision_callback(struct p2d_cb_data* data) {
  // ...
}

// example trigger callback
void trigger_callback(struct p2d_cb_data* data) {
  // ...
}

// at some point during init
p2d_init(..., trigger_callback);

// create and register objects
// ...
struct p2d_object obj = {0};
obj.type = P2D_OBJECT_RECTANGLE,
obj.x = 100;
obj.y = 200;
obj.vx = 100;
obj.out_x = &YOUR_ECS_X;
obj.out_y = &YOUR_ECS_Y;
obj.out_rotation = &YOUR_ECS_ROTATION;
obj.mask = P2D_LAYER_1 & P2D_LAYER_2;
p2d_register_object(&obj);
// ...

// in your engine update loop: (run this at the hz you want your physics to run at)
p2d_step(physics_delta_time);

// before quitting, shutdown
p2d_shutdown();

Future work

Item	Description	Priority	Progress
Debug Resolution	The current basic resolution has edge cases	High	Done
Rotation Resolution	Implement rotation and torque	High	Done
Friction Resolution	Implement friction	High	Done
Joints	Constraints between objects	High	Mostly Done
Collision Layers	Specify what can collide with what	Medium	Mostly Done
Advanced Gravity	Allow seperate spatial fields of gravity	Low	Maybe Later
New Shapes	Implement planes for more complex shapes	Low	Maybe Later
Optimization	Micro-optimize for performance	Low	Maybe Later
Add "True" Sleeping	Standstill objects get optimized out	Low	Maybe Later

Testing

cmake -DBUILD_P2D_TESTS=ON ..

Resources

Here are some resources that helped me along the way, which can hopefully be useful to you too!

• (Book) Game Physics Engine Development
• (Book) Real Time Collision Detection (Book)
• (Youtube) Two-Bit Coding Physics Engine Playlist
• (Articles) Rigid Body Dyanmics - Chris Hecker
• (Reference Example) Box2D

Advice

If I were to write this again from scratch, I would:

• Avoid writing a physics engine at all, unless I was doing it for fun or learning (it's really really hard)
• define rectangles by half extents, because their x,y position should be the center, not the top left (mixing with circles that have x,y as center is a huge pain)
• use radians instead of degrees internally (the conversion is a big failure point)