Nothing Moves

[dragonfax]

I followed the instructions for the hello world program, in the PDF linked from the README. And nothing moves. Perhaps I did something wrong.

func main() {

	var gravity = d2.MakeB2Vec2(0.0, -10.0)
	var world = d2.MakeB2World(gravity)

	groundBodyDef := d2.B2BodyDef{}
	groundBodyDef.Position.Set(0, -10)
	groundBody := world.CreateBody(&groundBodyDef)
	groundBox := d2.B2PolygonShape{}
	groundBox.SetAsBox(50, 10)
	groundBody.CreateFixture(&groundBox, 0)

	bodyDef := d2.B2BodyDef{}
	bodyDef.Type = d2.B2BodyType.B2_dynamicBody
	bodyDef.Position.Set(0, 4)
	body := world.CreateBody(&bodyDef)
	dynamicBox := d2.B2PolygonShape{}
	dynamicBox.SetAsBox(1, 1)
	fixtureDef := d2.B2FixtureDef{}
	fixtureDef.Shape = &dynamicBox
	fixtureDef.Density = 1
	fixtureDef.Friction = 0.3
	body.CreateFixtureFromDef(&fixtureDef)

	timeStep := 1.0 / 60.0
	velocityIterations := 6
	positionIterations := 2

	for i := 0; i < 60; i++ {
		world.Step(timeStep, velocityIterations, positionIterations)
		position := body.GetPosition()
		angle := body.GetAngle()
		fmt.Printf("%4.2f %4.2f %4.2f\n", position.X, position.Y, angle)
	}
}

$ go run main.go 
0.00 4.00 0.00
0.00 4.00 0.00
0.00 4.00 0.00
0.00 4.00 0.00
0.00 4.00 0.00
0.00 4.00 0.00
...

Truthfully I tried incorporating this module into another project I'm working on and it failed there. Which is when I took a step back, tried the hello world, and found that didn't work either.

[jack-kirton]

The only difference I can see between mine and yours is that instead of using B2BodyDef{} use MakeB2BodyDef(). The same applies for all the structures you have created from Box2D.

[Chillance]

Yes, doing bodyDef := box2d.MakeB2BodyDef() seems to output something different. It also makes sense since these "constructors" do set certain default values that wouldn't be set otherwise.

[einthusan]

In addition to what is said, you need to use ...

groundBox := box2d.MakeB2PolygonShape()
fixtureDef := box2d.MakeB2FixtureDef()
dynamicBox := box2d.MakeB2PolygonShape()

instead of using

groundBox := d2.B2PolygonShape{}
fixtureDef := d2.B2FixtureDef{}
dynamicBox := d2.B2PolygonShape{}

[Jack3G]

Just in case this helps anyone else who is looking here: The same thing was happening to me and my problem was that I had

const TIMESTEP = 1 / 60

// Instead of
const TIMESTEP = 1.0 / 60.0

If you do the first one it rounds down to 0, therefore no time passes in the simulation.