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Index › Programming Questions & Help › Other Libraries › JBox2D and collisions

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JBox2D and collisions (Read 1248 times)

cr0ybot

JBox2D and collisions
Feb 16^th, 2010, 3:27pm

Is there a way to turn off collisions on specific objects while keeping the other physics simulations, ie velocity, gravity, and joint constraints?

I'm making an avatar with arms, and in order to have its arms bend, i had to create an "elbow" body to connect to the main body and hand with 2 distance joints. I couldn't use a revolute joint because it does not allow both ends of the joint to rotate freely. So it would be nice to turn off collisions on the hidden elbow body so it doesn't get stuck behind things, and everything gets weird.

I thought about using the revolute joint the other way, so the non-rotating side was on the hand, but the hand is not actually a physics body, it is controlled with the mouse, and I can't have the hand colliding with anything either.

Any thoughts would be appreciated.

-cr0ybot

cr0ybot

Re: JBox2D and collisions
Reply #1 - Feb 16^th, 2010, 3:49pm

So I just answered my own question, and I figured I'd post it in case someone else has this problem.

You have to define a groupIndex for every ShapeDef you want to *not* collide with each other and set it to a negative number. ie:

Code:

BodyDef bd = new BodyDef();
bd.position = box2d.screenToWorld(x,y);
Body body = box2d.createBody(bd);

PolygonDef sd = new PolygonDef();
sd.setAsBox(box2d.scaleScreenToWorld(w/2), box2d.scaleScreenToWorld(h/2));
sd.density = 1.0f;
sd.friction = 0.3f;
sd.restitution = 0.5f;

// Here is where you set the groupIndex
sd.filter.groupIndex = -1;

body.createShape(sd);
body.setMassFromShapes();

You must do this for every ShapeDef that you define so all bodies that you want to never collide are in the same negative-numbered groupIndex.

By the way, I am using PBox2D to interface with JBox2D.

-cr0ybot

cr0ybot

Re: JBox2D and collisions
Reply #2 - Feb 16^th, 2010, 8:53pm

Alternatively, categories & masks can be used.

When defining a ShapeDef or its various subclasses (PolygonDef, CircleDef, etc.) you can define a "categoryBit" to identify a group of shapes with the "maskBits" field.

If you want no collisions on an object whatsoever, set its maskBits to 0x0000; the default, aka "collide with everything," is 0xffff:

Code:

// No collisions
shapeDef.filter.maskBits = 0x0000;

// Collide with all (except for those with collisions turned off)
shapeDef.filter.maskBits = 0xffff;

If you want a specific group to collide only with another specific group, define each groups' categoryBits separately, then point each groups' maskBits to the others' category:

Code:

// Categories are defined for each shape in each group
group1shapeDef.filter.categoryBits = 0x0001;
group2shapeDef.filter.categoryBits = 0x0002;

// This will only allow the two groups to collide with each other and nothing else
group1shapeDef.filter.maskBits = 0x0002;
group2shapeDef.filter.maskBits = 0x0001;

// This will allow the two groups to collide with everything EXCEPT the other group
group1shapeDef.filter.maskBits = 0xffff & ~0x0002;
group2shapeDef.filter.maskBits = 0xffff & ~0x0001;

I hope this helps other people understand this better. The original Box2D manual doesn't explain it very well. Thanks to this post at the box2d forums for helping me understand it:

http://www.box2d.org/forum/viewtopic.php?f=3&t=197

-cr0ybot

PhiLho YaBB Moderator Offline Posts: 4190 Near Paris (France)	Re: JBox2D and collisions Reply #3 - Feb 17^th, 2010, 2:14am Thanks for sharing this, as it highlights advanced and, as you said, relatively obscure points of the library.

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