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Box2D.go - Go port of Box2D - a 2D Physics Engine for Games.

License: Other

Go 100.00%

golang box2d physics-engine

box2d's Introduction

Byte Arena

arena-trainer release

GITHUB_API_TOKEN=TOKEN ./scripts/make-release-trainer.sh RELEASE

mq-cli release

./scripts/make-mq-cli.sh

box2d's People

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box2d's Issues

Collisions problem

Hello, guys
I'm trying to use your engine in my own game and found that the are no collisions between bodies occurs at all.
I created a small test with your code from cpp_compliance_test.go, and I see that there the same situation.

This is a small test - a small dinamic circle body above ground Edge. Standard gravity (0,-10), and you can see that this circle body fall down and pass through the edge to negative Y axis.

Is there something wrong in this code that I copied from your cpp_compliance_test.go, or collisions between dynamic/static are not supported yet?

package main

import (
	"fmt"
	"testing"

	"github.com/ByteArena/box2d"
)

func TestBox2dCollisions(t *testing.T) {

	world := box2d.MakeB2World(box2d.MakeB2Vec2(0, -10))

	// ground body (from cpp_compliance_test.cpp)
	{
		bd := box2d.MakeB2BodyDef()
		ground := world.CreateBody(&bd)
		shape := box2d.MakeB2EdgeShape()
		shape.Set(box2d.MakeB2Vec2(-20.0, 0.0), box2d.MakeB2Vec2(20.0, 0.0))
		ground.CreateFixture(&shape, 0.0)
	}

	// circle character (from cpp_compliance_test.cpp)
	bd := box2d.MakeB2BodyDef()
	bd.Position.Set(0, 10.0)
	bd.Type = box2d.B2BodyType.B2_dynamicBody
	bd.FixedRotation = true
	bd.AllowSleep = false

	body := world.CreateBody(&bd)

	shape := box2d.MakeB2CircleShape()
	shape.M_radius = 0.5

	fd := box2d.MakeB2FixtureDef()
	fd.Shape = &shape
	fd.Density = 200.0
	body.CreateFixtureFromDef(&fd)

	// Prepare for simulation. Typically we use a time step of 1/60 of a
	// second (60Hz) and 10 iterations. This provides a high quality simulation
	// in most game scenarios.
	timeStep := 1.0 / 60.0
	velocityIterations := 8
	positionIterations := 3

	for i := 0; i < 100; i++ {
		// Instruct the world to perform a single step of simulation.
		// It is generally best to keep the time step and iterations fixed.
		//runtime.Breakpoint()
		world.Step(timeStep, velocityIterations, positionIterations)
		fmt.Printf("circle character coordinates: %v,%v\n", body.GetPosition().X, body.GetPosition().Y)
	}
}

Output is:

=== RUN   TestBox2dCollisions
circle character coordinates: 0,9.997222222222222
circle character coordinates: 0,9.991666666666665
circle character coordinates: 0,9.983333333333333
circle character coordinates: 0,9.972222222222221
circle character coordinates: 0,9.958333333333332
........etc
........
circle character coordinates: 0,1.2222222222222219
circle character coordinates: 0,0.9999999999999999
circle character coordinates: 0,0.7750000000000001
circle character coordinates: 0,0.5472222222222226
circle character coordinates: 0,0.3166666666666673
circle character coordinates: 0,0.08333333333333426
circle character coordinates: 0,-0.15277777777777657
circle character coordinates: 0,-0.39166666666666516
circle character coordinates: 0,-0.6333333333333315
circle character coordinates: 0,-0.8777777777777757
circle character coordinates: 0,-1.1249999999999976
circle character coordinates: 0,-1.3749999999999973
circle character coordinates: 0,-1.6277777777777747
circle character coordinates: 0,-1.8833333333333298
circle character coordinates: 0,-2.1416666666666626
circle character coordinates: 0,-2.4027777777777732
circle character coordinates: 0,-2.6666666666666616```

Why don't have SetPosition() for box2d.Body?

fixture.GetBody().SetPosition(box2d.MakeB2Vec2(0, 0))

Runtime error: index out of range

I just got into Box2d but as I wrote a simple game engine it panicked.
Relevant files are here
https://pastebin.com/R0Yc6sCg
https://pastebin.com/QzKJeaDu

It is a very late evening for me so I might just have done a silly mistake

How to create joint?

box2d doc says joint must be created by create method in B2world.

func (world *B2World) CreateJoint(def *B2JointDef) B2JointInterface {

But CreateJoint method receives B2JointDef, not B2JointDefInterface.
I cannot pass *B2PrismaticJointDef as *B2JointDef and even passing &B2PrismaticJointDef.B2JointDef will not work because def.(type) is B2JointDef, not B2PrismaticJointDef.
So type asserting will fail.

How can I create various joints?

Simulation instability caused by some joints

It looks like some of the joints types don't work properly.

I'm implementing a loader for R.U.B.E scenes here https://github.com/maxfish/gojira2d/blob/master/pkg/physics/b2djson_scene.go and I'm trying a lot of different scenes.
I can see that the only joints which work well are revolute, rope, distance and wheel. Motor, prismatic, weld and friction seem to make the simulation explode after a while.

Here is one example of a weld join that is unstable: https://pastebin.com/dPB2xBfV
As soon as body3, while falling, touches body2, body2 sprints away from the screen at high speed

Another example simulating a R.U.B.E scene containing all types of joints:

This is how it's supposed to be https://www.youtube.com/watch?v=X6Unn3ybcKI
This is how the Box2D implementation simulate it https://www.youtube.com/watch?v=1wUdmBTDKLI

Any ideas of what could be the cause?

Creating a revolute joint?

Following the C++ API:

b2RevoluteJointDef jointDef;
b2RevoluteJoint* joint = (b2RevoluteJoint*)myWorld->CreateJoint(&jointDef);

I have tried:

jointDef := box2d.MakeB2RevoluteJointDef()
joint := (*box2d.B2RevoluteJoint)(world.CreateJoint(&jointDef))

which results in the error: cannot convert world.CreateJoint(&jointDef) (value of type box2d.B2JointInterface) to *box2d.B2RevoluteJoint

can you give an example on how to create a joint?

DynamicsB2JointFriction Dump method has a Pointer Reciever

I'm a noob to box2d (and Go), I was following a C++ example on creating joints.
In the example:

b2xxxJoint* joint = (b2xxxJoint*)world->CreateJoint( &jointDef );

They cast the returned joint to the one they wanted created. Doing the same in Go gives a compilation error

impossible type assertion:
box2d.B2FrictionJoint does not implement box2d.B2JointInterface (Dump method has pointer receiver)

var jointDef box2d.B2FrictionJointDef = box2d.MakeB2FrictionJointDef()
jointDef.BodyA = ball1
jointDef.BodyB = ball2
joint := world.CreateJoint(&jointDef).(box2d.B2FrictionJoint)

Looking at the source and you can see it's a pointer receiver B2JointFrictio Line 282

Some of the other joints are not B2JointDistance Line 337

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.