Box2D: An Acquired Taste Guide to Physics Engine Capabilities
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The document provides an overview of Box2D physics engine conventions and best practices, including how to work with bodies, fixtures, joints, forces, collisions, queries, and loading RUBE worlds. Key recommendations are to use forces over manual positioning, implement debug drawing, and leverage filters to control specific collisions.
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Box2D: An Acquired Taste Guide to Physics Engine Capabilities
- 1. Box2D: An Acquired Taste
- 2. Tonight Capabilities Essentials Do's and Don'ts RUBE basics Brian May (reincarnation of Sir Isaac Newton)
- 3. Assumptions You know what Box2D does You've set up Box2D successfully You know high school-level physics Not assuming you're using RUBE
- 4. Important Resources Class documentation – Looks like a lot, but most you won't use Official manual (no, seriously) RUBE (it's not too late to get it!) – Useful even when not using it as a level editor iforce2d (Box2D tutorials by the RUBE author) The official testbed
- 5. World Conventions Uses meters, kilograms, and seconds for units Y-axis goes upwards Negative coordinates okay World is infinitely large Rigid bodies – No scaling or deformation
- 6. Bodies and Fixtures b2Bodys handle motion – Needs at least one b2Fixture b2Fixtures handle collision – Owned by one b2Body b2(Body|Fixture)Defs used to make similar bodies and fixtures Create bodies with b2World:: CreateBody() – Destroy only with DestroyBody() Create fixtures with b2Body:: CreateFixture() – Destroy only with DestroyFixture() Body Fixtures
- 7. Body/Fixture Properties Position/Angle Type – Static, dynamic, or kinematic bodies (Line|Angul)ar (Velocity|Damping) – Kinda like air resistance Fixed rotation Awake Sleep enabled Bullet – More precise (but slower) collision detection between dynamic bodies Active (Is it on?) – Not the same as awake Gravity scale User data Shape – Circle, convex polygon, line, or polyline Friction Restitution – Bounciness Density Sensor – Don't be pushed back by collisions (very useful) Filter User data
- 8. Rendering Meters are not pixels Meters are not pixels Meters are not pixels Meters are not pixels Meters are not pixels Meters are not pixels Meters are not pixelsMeters are not pixels Meters are not pixelsMeters are not pixels 100 meters (328 feet) tall
- 9. Rendering Pick a pixel ↔ meter conversion – I personally use 64px = 1 meter Scale only when rendering – Or turning screen input into world coordinates Game logic and design decisions should be in meters
- 10. Rendering Bodies/Fixtures govern sprite positions on screen When adding to the render list, take the world coordinates, scale by your factor, subtract Viewport coordinates No need for AnimatedSprite to maintain position any more
- 11. Joints Use b2w+Joint to join two b2Bodys together Create with b2w+JointDef Some kind of constraint is applied – Maximum distance – Welded together – Etc. Too many types to detail here
- 12. Game Logic Use b2ContactListener – Don't use the body/fixture list (Begin|End)Contact() is self-explanatory PreSolve() called every frame before the two fixtures are pushed back – For advanced contact manipulation PostSolve() after they're pushed away – For game logic based on physics response
- 13. Cleaning Up Use b2DestructionListener! SayGoodbye() called just before a b2(Fixture|Joint) is destroyed – Nullify pointers here Don't forget the b2Body! – When a b2Fixture is removed, check its body for remaining fixtures – If none exist, nullify pointers to the body “Nah, I won't need it”
- 14. Debug Drawing Hamza, come up here and write this on the board: – I will not debug physics blindly WTF? Oh...
- 15. Debug Drawing Implement b2Draw Given coordinates and colors, draw lines Make (DirectX|SFML)Graphics implement it – Use sf::C(onvex|ircle)Shape for SFML – Use ID3DXLine for DirectX Doesn't need to be fast, just needs to be correct and toggle-able
- 16. Making Things Move Use forces to move objects Use forces to move objects Use forces to move objects Use forces to move objects Use forces to move objects Use forces to move objects Use forces to move objects Use forces to move objects Missing the point Especially so Manual positionsManual positions Manual velocityManual velocity
- 17. Making Things Move Forces for gentle motion Impulses for quick jolts Velocity only for kinematic bodies F = ma is God (p = mv is cool, too) VelocityImpulseForce
- 18. Finer Control Use b2Filter.to prevent certain collisions Pick up to 16 “types” of things (walls, items, enemies, etc.) and give each a bit categoryBits (I'm a <thing>!) maskBits (I'll collide with <other things>!) groupIndex (Forget those bits, this is what I'm really doing) Rules (for b2Fixtures with b2Filters a and b): – If a.groupIndex == b.groupIndex ● If the group index is positive, collide ● Else if it's negative, don't collide – Else if (a.categoryBits & b.maskBits != 0) and vice versa, collide
- 19. Rays and Rectangles b2World::RayCast()for directional queries – What's in the way? – How will something bounce? – Subclass b2RayCastCallback b2World::QueryAABB() for area queries – For one-time checks – Subclass b2QueryCallback Implement these callbacks for game-specific logic Doesn't see Snake Sees Snake AABB Query Rain Flush (one-hit AoE attack) About to take huge damage
- 20. RUBE World editor for Box2D Not too late to get it!
- 21. Loading RUBE Worlds Use b2dJson b2dJson::readFromFile() – That's it Get custom properties with b2dJson::getCustomPropertiesForItem()
- 22. That's All, Folks Try not to fuck up