Edit 2: http://youtu.be/KiCzUZ69gpA - as you can see in this video, the shaking effect is amplified when I also render some text for each body. Observe how the ground body (blue) shakes violently when each body has some text rendered near it, and how it does not when text rendering is commented out. This has to be connected!
Edit: I've made two important additions to the original question: I've added my rendering functions, and the camera (translation) methods, and I think that the error is actually there, not in JBox2D.
I'm trying to simulate and render a lot of random bodies (2-20) connected with RevoluteJoint
s. One body can be connected to multiple others, and there are no separate constructions, i.e. all the bodies are interconnected.
However, when watching the live rendering, it is very shaky and unstable. By that I mean that bodies' positions (or maybe angles) seem to be randomly fluctuating for no apparent reason, making the simulation look unstable.
Here's a video of what I'm observing:
http://youtu.be/xql-ypso1ZU
Notice the middle square and the rotating rectangle. The middle square is shifting its position back and forth slightly at seemingly random intervals, and the rotating rectangle is very jittery (take a look at the point it is rotating about).
What could this effect be due? Is it some known issue with (J)Box2D, or is it an issue with my rendering system? I think that I have somehow misconfigured the physics engine, but also some floating point math in the rendering system could be the culprit.
Here's how I'm creating the bodies and the joints:
private Body setPart(Part part) {
// body definition
BodyDef bd = new BodyDef();
bd.position.set(0f, -10f);
bd.angle = 0f;
bd.type = BodyType.DYNAMIC;
// define shape of the body.
PolygonShape Shape = new PolygonShape();
Shape.setAsBox(part.width / 2, part.height / 2);
// define fixture of the body.
FixtureDef fd = new FixtureDef();
Filter filter = new Filter();
filter.groupIndex = -1;
fd.filter = filter;
fd.shape = Shape;
fd.density = 0.5f;
fd.friction = 0.3f;
fd.restitution = 0.5f;
// create the body and add fixture to it
Body body = world.createBody(bd);
body.createFixture(fd);
body.setUserData(new PartUserData());
return body;
}
private void setJoint(PartJoint partJoint) {
Body bodyOne = partToBody.get(partJoint.partOne);
Body bodyTwo = partToBody.get(partJoint.partTwo);
RevoluteJointDef jointDef = new RevoluteJointDef();
jointDef.bodyA = bodyOne;
jointDef.bodyB = bodyTwo;
jointDef.localAnchorA = partJoint.partOne
.getAnchor(partJoint.percentOne);
jointDef.localAnchorB = partJoint.partTwo
.getAnchor(partJoint.percentTwo);
// rotation
jointDef.lowerAngle = GeomUtil.circle(partJoint.rotateFrom);
jointDef.upperAngle = GeomUtil.circle(partJoint.rotateTo);
jointDef.enableLimit = true;
jointDef.maxMotorTorque = 10.0f; // TODO limit maximum torque
jointDef.motorSpeed = GeomUtil.circle(partJoint.angularVelocity);
jointDef.enableMotor = true;
world.createJoint(jointDef);
}
The time step is 0.01f
.
Here is how I draw bodies:
private void drawBody(Body body) {
// setup the transforms
Vector position = camera.translate(body.getPosition());
currentGraphics.translate(position.x, position.y);
currentGraphics.rotate(body.getAngle());
// do the actual rendering
for (Fixture fixture = body.getFixtureList(); fixture != null; fixture = fixture
.getNext()) {
PolygonShape shape = (PolygonShape) fixture.getShape();
if (body.getUserData() instanceof PartUserData) {
fillShape(shape, partFillColor);
currentGraphics.setStroke(partOutlineStroke);
outlineShape(shape, partOutlineColor);
} else {
fillShape(shape, groundFillColor);
outlineShape(shape, groundOutlineColor);
}
}
// clean up
currentGraphics.rotate(-body.getAngle());
currentGraphics.translate(-position.x, -position.y);
currentGraphics.setColor(defaultColor);
currentGraphics.setStroke(defaultStroke);
}
I think that the issue might be the way I'm handling rendering of all the bodies.
This is the algorithm for each body:
1. Translate the Graphics2D
object to its position
2. Rotate it by body.getAngle()
3. Render the body
4. Rotate the graphics back
5. Translate the graphics back
Could it be that amongst all these transforms something goes wrong?
When I removed the calls to camera's methods, the effect seems to have been reduced. These are the relevant camera methods:
public Vector translate(Vec2 worldPosition) {
Vector point = new Vector();
point.x = (int) (worldPosition.x * pixelsPerMeter) - position.x;
point.y = (int) (worldPosition.y * pixelsPerMeter) - position.y;
point.x = (int) (point.x * zoom);
point.y = (int) (point.y * zoom);
point.x += renderer.getWidth() / 2;
point.y += renderer.getHeight() / 2;
return point;
}
public Vector translateRelative(Vec2 worldPosition) {
Vector point = new Vector();
point.x = (int) (worldPosition.x * pixelsPerMeter);
point.y = (int) (worldPosition.y * pixelsPerMeter);
point.x = (int) (point.x * zoom);
point.y = (int) (point.y * zoom);
return point;
}
But what part of them would cause an issue?