Hipmunk tutorial video
Abstract
This post describes a minimal Hipmunk program, a Haskell binding to the Chipmunk 2D physics engine, featuring a simple falling circle with circle collision shape. The goal was to exemplify how a Hipmunk program is structured corresponding to a FRP game engine model (see The Yampa Arcade) and testing a sophisticated 2D physics engine (for simplicity) with Haskell. A physics engine could actually be completely pure, investigations retrieved Hpysics, a pure functional physics engine. Didn’t use it, looks very basic, yet complete, but it’s an abandoned project.
Hpysics simple demo
Description
Most rigid body physics engine define the following objects:
- Space: a physical simulation space independent from other spaces, describes gravity. When a body is added, the object becomes dynamic. When the shape is added too, collisions are also handled.
- Body: a point which describes the position, using mass, moment of inertia (rotation), velocity, force, torque etc.
- Shape: a geometrical figure which describes collision shapes and optionally has an offset to the body (f.e. when using multiple shapes)
- Constraints: describe different restrictions on the movement of Bodies, like Pins, Springs, Ropes…
The scene definition thus is pretty straight forward: setup scene, run simulation, handle collisions, loop. What is unfortunate though is that – like all FFI bindings – the binding to the external Chipmunk engine is fundamentally unsafe, so there are lots of IO Monads to do basic stuff. Using Hipmunk in a FRP environment like Yampa would require lots of “task-” (or “message-”) definitions and communication from the logic all the way to the physics engine, only to apply forces. Note the many unnecessary IO and State Monads. Here is a small code snipped which does 3 simulation steps:
import Data.IORef
import Data.StateVar -- defines $=
import qualified Physics.Hipmunk as H
main :: IO ()
main = do
H.initChipmunk -- initChipmunk :: IO ()
space <- H.newSpace -- newSpace :: IO Space
H.gravity space $= H.Vector 0 (-10) -- gravity :: Space -> StateVar Gravity
body <- H.newBody mass momentOfInertia -- newBody :: Mass -> Moment -> IO Body
-- (+:) is a convenient function for vector construction defined in Playground.hs
-- compare with H.Vector above
H.position body $= 0 +: 0 -- position :: Body -> StateVar Position
-- class Entity a
-- instances: Shape, Body, StaticShape, (Constraint a)
H.spaceAdd space body -- spaceAdd :: Space -> a -> IO ()
-- newShape :: Body -> ShapeType -> Position -> IO Shape
shape <- H.newShape body (H.Circle 5) offset
-- step :: Space -> Time -> IO ()
H.step space elapsedSeconds -- IO () => y = 0
H.step space elapsedSeconds -- IO () => y = -10
H.step space elapsedSeconds -- IO () => y = -30
pos <- get . H.position $ body
putStrLn . show $ pos
where
mass = 1.0
momentOfInertia = H.infinity
offset = H.Vector 0 0
elapsedSeconds = 1.0
– | Constructs a Vector.
(+:) :: H.CpFloat -> H.CpFloat -> H.Vector
(+:) = H.Vector
infix 4 +:
When I try to run the example code, I get an error:
$ ghc –version
The Glorious Glasgow Haskell Compilation System, version 7.0.3
$ runhaskell hipmunkexample
hipmunkexample.hs:16:26: Not in scope: `+:’
I get the Not in scope `+:’ error on both Mac OS X 10.6.7 and Windows 7.
Mac OS X can’t run this due to GLFW problems, but Windows can with two modifications.
Insert before main:
– | Constructs a Vector.
(+:) :: H.CpFloat -> H.CpFloat -> H.Vector
(+:) = H.Vector
infix 4 +:
Then compile (runhaskell doesn’t work due to required C libraries):
C:\>ghc -o hipmunkexample.exe –make hipmunkexample.hs
[1 of 1] Compiling Main ( hipmunkexample.hs, hipmunkexample.o )
Linking hipmunkexample.exe …
C:\>hipmunkexample.exe
Vector 0.0 (-30.0)
added, thanks!
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