Archive for the 'box2d' Category

Verlet rope

Published by patrickC on October 29, 2010 in box2d and cocos2d. 27 Comments

Introduction

In this brief article we’ll be taking a look at an efficient way of creating ropes in our cocos2d games that use box2d, starting from a cocos2s+box2d template project.
We will be using box2d’s latest addition, the b2RopeJoint to constrict two bodies with a rope joint (a maximum distance joint, one might also call it), so as always big thanks to Erin Catto for all his hard work on box2d.
We will also be using a small set of classes I wrote, VRope, that use Verlet integration to calculate the points of the rope, using a CCSpriteBatchNode to visually draw the rope in our game.

Click for video

It’s worth mentioning that using Box2D isn’t strictly necessary, you could still use the VRope class to draw a rope between two given points in other scenarios (for example using Chipmunk, your own physics system, or simply updating the two points manually) and have it react under gravity. With that in mind, the class also has some helper methods to use it with CGPoints only. (somethingWithPoints methods)

Setting up the project

First things first, make sure you have the latest version of Cocos2D and create a new project with the “Cocos2d Box2d Application” template. You will then need to update Box2D to the latest committed version, which you can find at http://code.google.com/p/box2d/source/checkout
To update Box2D in your current project:

From within XCode, delete the Box2D folder group from the “cocos2d Sources” folder (Delete References)
From Finder, go to your project’s folder and delete the Box2D folder
Copy over the latest version of Box2D back into your project’s folder (make sure you only copy the most deep rooted “Box2D” folder, and delete CMakeLists.txt and Box2DConfig.cmake)
Back to XCode, Project -> Add to Project, select the Box2D folder, make sure create groups (not folder reference) is selected, Add

This should give you a working Cocos2D project, updated with the latest Box2D source that has the b2RopeJoint ready to use.
If you’re not running the latest version of cocos2d (0.99.5-rc0) but still want to use VRope, you’ll need to rename CCSpriteBatchNode back to CCSpriteSheet, the rest should work as is.

Using b2RopeJoint

Using the new b2RopeJoint is very easy, it’s similar to the b2DistanceJoint, but unlike other joints it doesn’t have an Initialize method.

//define rope joint, params: two b2bodies, two local anchor points, length of rope
b2RopeJointDef jd;
jd.bodyA=body1; //define bodies
jd.bodyB=body2;
jd.localAnchorA = b2Vec2(0,0); //define anchors
jd.localAnchorB = b2Vec2(0,0);
jd.maxLength= (body2->GetPosition() - body1->GetPosition()).Length(); //define max length of joint = current distance between bodies
world->CreateJoint(&jd); //create joint

Continue reading ‘Verlet rope’

VertexHelper: A physics body editor

Published by jfahrenkrug on March 23, 2010 in box2d, chipmunk, cocos2d and editors. 13 Comments Tags: box2d, chipmunk, physics.

VertexHelper is a nifty little tool I (jfahrenkrug) have put together while developing a cocos2d iPhone app that used a lot of different sprites. All those sprites needed a counterpart for the physics engine, namely vertices. After defining the vertices of the 2nd sprite by hand, I knew that I’d go nuts if I had to define all 40 sprite vertices like that. So I wrote a tool: VertexHelper.

It lets you graphically define vertices of sprites for Box2D and Chipmunk bodies/shapes. You can find it on github or you can download a tarball of the source.

It is really easy to use VertexHelper:

First, you need an evenly spaced sprite sheet (every sprite in the image file has to be of the same size). For example, if your sprite sheet image is 250 x 120 pixels and every sprite is 50 x 60 pixels, then you’d have 10 sprites on your sheet (2 rows and 5 columns). Perfect.
Simply drag the sprite sheet image onto VertexHelper and enter the number of rows and columns. You’ll see a grid appear.
Now check the “Edit Mode” checkbox and click “around” each sprite to define it’s vertices. It’s that easy.
VertexHelper will generate the code for either Chipmunk or Box2D that you can simply cut and paste into your project.

A word of caution: You have to make sure not to define concave polygons, since both Chipmunk and Box2D don’t support them. VertexHelper won’t warn you if you do. Also, you have to make sure that you define your coordinates in the correct order: For Box2D, they have to be counter-clockwise and clockwise for Chipmunk.

But enough of the talking, this video gives you a much better impression of what VertexHelper does and how it works:

I hope it’s useful for you and please feel free to fork it on github and make it better for everyone!

For further updates, follow me on Twitter or keep an eye on my blog.

Integrating cocos2d with Box2d

Published by riq on February 22, 2010 in box2d and cocos2d. 0 Comments

Box2d is rigid-body physics engine that can be used as a good complement of cocos2d. In fact, cocos2d comes with 3 Xcode templates, and one of them is a cocos2d + box2d template.