apecs-physics

2D physics library for apecs. Uses the Chipmunk physics engine. The apecs-gloss package provides a simple optional gloss-based renderer.

Feel free to create an issue or PR for suggestions/questions/requests/critiques/spelling fixes/etc. See TODO.md for suggestions if you want to help out with the code.

The examples directory contains a number of examples, each can be run with stack build && stack <examplename>:

helloworld

makeWorld "World" [''Physics, ''Camera]

Generate a world. The Physics component adds a physics space to the world. The Camera component tracks a camera position and zoom factor.

initialize = do
  set global ( Camera (V2 0 1) 60
             , earthGravity )

Globals can be set with any entity argument, global is just an alias for -1. earthGravity = V2 0 (-9.81), normal earth surface gravity if we assume normal MKS units. Note that the positive y-axis points upwards.

  let ballShape = cCircle 0.5
  ball <- newEntity ( DynamicBody
            , Position (V2 0 3)
            , Density 1
            , Elasticity 0.9
  )
  ballEntity <- newEntity (Shape ball ballShape)

Still in the initialize function, here we see our first object being instantiated. The type of ballShape is Convex, the apecs-physics format for shapes. Convex is a convex polygon, consisting of a number of vertices and a radius. In the case of a circle, the polygon consists of a single point with a non-zero radius. Both Chipmunk and gloss only support convex polygons, Convex is used to give them a common interface.

A DynamicBody is one of three types of bodies. It is a normal body, fully affected by physical forces. The elasticity of a collision is the product of the elasticities of the colliding shapes.

  let lineShape = hLine 6
  newEntity ( StaticBody
            , Angle (-pi/20)
            , Shape lineShape
            , Elasticity 0.9 )

Static bodies are not affected by physics, and generally rarely move. They are equivalent to bodies with infinite mass and moment, and zero velocity. Changing their position triggers an explicit rehash of their shapes, wish is relatively expensive.

main = do
  w <- initWorld
  runSystem initialize w
  defaultSimulate w

defaultSimulate is a convenience wrapper around gloss’ simulateIO. You can find its definition in Apecs.Physics.Gloss, in case you want to change the rendering behavior.

tumbler

initialize :: System World ()
initialize = do
  set global ( Camera 0 50
             , earthGravity )

  let sides = toEdges $ cRectangle 5
  tumbler <- newEntity ( KinematicBody
                       , AngularVelocity (-1) )

As previously stated, both Chipmunk and gloss exclusively have convex polygon primitives. Our tumbler, however, is obiously not convex. Fortunately, composing shapes is really easy. We use toEdges to turn a rectangle into an outline of one, and use foldMap to make a composite Picture.

A KinematicBody is halfway between a DynamicBody and a StaticBody. It can have an (angular) velocity, but will not respond to forces. It can be used for e.g. moving platforms, or in this case. Note that we did not add any shapes to the tumbler yet.

  forM_ sides $ \line -> newEntity (ShapeExtend (cast tumbler) $ setRadius 0.05 line)

The time has come to talk about the destinction between shapes and bodies. A body can have multiple shapes. Shapes belonging to the same body cannot move relative to one another, i.e. a body is a fixture for multiple shapes. When using the normal Shape data constructor to add a shape to a body, we actually create two Chipmunk structs; one for the body, and one for the shape, even though they are addressed by the same entity in apecs.

When we want to add multiple shapes to a body, however, we need to make new entities for each individual shape. The reason for this is that this way, we can still easily change the properties of each individual shape. Shape actually just represents a special case of ShapeExtend, the case in which the body has the same entity as the shape.

When you use a tuple of components in apecs, they are added in the order you list them in the tuple. This is important to realize, as adding a shape to an entity wihout a body is a noop. Always make sure you first add a body, and then the shapes. This also comes up when e.g. setting a shape’s properties: you can only set a shape’s Mass or Density when there is a shape in the first place. If you don’t, you will get a runtime error about simulating zero-mass DynamicBodies.

  replicateM_ 200 $ do
    x <- liftIO$ randomRIO (-2, 2)
    y <- liftIO$ randomRIO (-2, 2)
    r <- liftIO$ randomRIO (0.1, 0.2)
    let ballshape = cCircle r
    let c = (realToFrac x+2)/3
    ball <- newEntity ( DynamicBody
              , Position (V2 x y)
              , Density 1 )
    newEntity (Shape ball ballshape)

  return ()

Finally, we randomly add a bunch of balls.

constraints

The final example is a gallery of (some of) the available constraints. Drag shapes around with the left mouse button, create a new box with the right.

This example is too large to fully include here, but if you have made it this far, I recommend looking at the source. Aside from demonstrating constraints, queries and interaction it also contains some neat tricks like:

let rubber = (Friction 0.5, Elasticity 0.5, Density 1)
newEntity ( DynamicBody
          , rubber )

Nesting tuples creates composable and reusable pieces of configuration (this is an apecs thing, not an apecs-physics thing). This can also be useful if you find yourself needing bigger tuples than the current maximum.

Constraints are a lot like shapes, but instead of having one associated Body, they have two. It also comes in the varieties Constraint and ConstraintExtend.

Dragging an object with the mouse is also done using a constraint. The mouse position actually controls the position of a static body without shapes, and we use a PinJoint to attach whatever we are dragging to it.

Rendering

A basic rendering system, drawBody :: (Body, Transform, ShapeList) -> System m Picture is provided for debugging and prototyping purposes. Inside your gloss draw function you can combine this with foldDrawM to draw every physics body. For example

draw = foldDrawM drawBody