Forums » Help » Turning off gravity for entities with Box2D

From the Box2D document, it sounds like gravity is set at the world level, whereas Impact defines it as the entity level. I would like to use Box2D and have some entities that honor gravity and some that do not. Is this not possible?

I googled a bit - it seems to be a common problem with Box2D. In short, Box2D doesn't support "lighter than air" bodies directly, but you can apply a force to them for each frame to push them up. E.g. if you want to have entities that float, do this in your entity's update() method:

this.body.ApplyForce( new b2.Vec2(0, -ig.game.gravity * b2.SCALE), this.body.GetPosition() );

Note that when you apply a force to an entity it won't come to "rest". So having a lot of these entities will impact the performance of your game.

If you want your entities to stay fixed all the time, just skip the this.body.SetMassFromShapes(); in your createBody() method (have a look at it in lib/plugins/box2d/entity.js).

Or, if you don't need collision for these entities at all (e.g. for items that are just "collected" when touched) you can always subclass them from ig.Entity instead of from ig.Box2DEntity.

The 2nd option seems to fit me, I want certain objects to stay fixed all the time. So I'll give that a shot and if it works I'll post my code here. Should just be able to add it as an option in the init.

Yes, this works for me. I'm able to get around it by created a property in the Box2D entity.js file called "isGravitated" defaulted to true. Then I pass a value through the settings object and set isGravitated in the init. Then in the createBody function I do:

		if(this.isGravitated) {
			this.body.SetMassFromShapes();
		}

Works flawlessly. Thank you!

Ho do you do this for a non box 2d entity, so for example i don't want my coin entity to be affected by gravity changes in the level?

thx

:-D

this.gravityFactor =0;