rope.m

/////// RopeNode.h

#import "cocos2d.h"
#import "ObjectiveChipmunk.h"

struct RopeNodeParticle;

@interface RopeNode : CCNode {
	ChipmunkBody *_body1, *_body2;
	cpVect _offset1, _offset2;
	
	int _count;
	cpFloat _segLength;
	int _iterations;
	
	cpFloat _width;
	ccColor4B _color;
	
	cpFloat _damping;
	cpVect _gravity;
	struct RopeNodeParticle *_particles;
}

@property(assign) cpFloat length;
@property(assign) cpFloat width;
@property(assign) ccColor4B color;

@property(assign) cpFloat damping;
@property(assign) cpVect gravity;
@property(assign) int iterations;

- (id)initWithLength:(cpFloat)length segments:(int)count
	body1:(ChipmunkBody *)body1 body2:(ChipmunkBody *)body2 
	offset1:(cpVect)offset1 offset2:(cpVect)offset2;

- (void)step;

@end

/////// RopeNode.m

#import "RopeNode.h"

typedef struct RopeNodeParticle {
	cpVect pos, prev;
} Particle;

@implementation RopeNode

@synthesize gravity = _gravity, iterations = _iterations, damping = _damping, width = _width, color = _color;

- (id)initWithLength:(cpFloat)length segments:(int)count
	body1:(ChipmunkBody *)body1 body2:(ChipmunkBody *)body2 
	offset1:(cpVect)offset1 offset2:(cpVect)offset2;
{
	if((self = [super init])){
		_body1 = [body1 retain];
		_body2 = [body2 retain];
		_offset1 = offset1;
		_offset2 = offset2;
		
		_count = count;
		_iterations = 5;
		self.length = length;
		
		_width = 1.0f;
		_color = ccc4(255, 255, 255, 255);
		
		cpVect start = [_body1 local2world:offset1];
		cpVect end = [_body2 local2world:offset2];
		
		_particles = calloc(count + 1, sizeof(Particle));
		for(int i=0; i<=count; i++){
			cpVect p = cpvlerp(start, end, (cpFloat)i/(cpFloat)count);
			_particles[i] = (Particle){p, p};
		}
	}
	
	return self;
}

- (void)dealloc {
	[_body1 release];
	[_body2 release];
	
	free(_particles);
	
	[super dealloc];
}

-(void)setLength:(cpFloat)length {
	_segLength = length/_count;
}

-(cpFloat)length {
	return _segLength;
}

static inline Particle
stepParticle(Particle p, cpVect g, cpFloat damping)
{
	cpVect pos = p.pos;
	cpVect vel = cpvmult(cpvsub(pos, p.prev), damping);
	return (Particle){cpvadd(cpvadd(pos, vel), g), pos};
}

static inline void
contract(Particle *particles, int i, int j, cpFloat dSQ, cpFloat ratio)
{
	cpVect v1 = particles[i].pos;
	cpVect v2 = particles[j].pos;
	
	cpVect delta = cpvsub(v2, v1);
	if(cpvlengthsq(delta) < dSQ) return;
	cpFloat diff = 0.5f - dSQ/(cpvlengthsq(delta) + dSQ);
	
	particles[i].pos = cpvadd(v1, cpvmult(delta, diff*ratio));
	particles[j].pos = cpvadd(v2, cpvmult(delta, diff*(ratio - 1.0)));
}

- (void)step;
{
	int count = _count;
	Particle *particles = _particles;
	
	cpVect g = _gravity;
	cpFloat damping = _damping;
	for(int i=1; i<count; i++) particles[i] = stepParticle(particles[i], g, damping);
	
	particles[0].pos = [_body1 local2world:_offset1];
	particles[count].pos = [_body2 local2world:_offset2];
	
	cpFloat dSQ = _segLength*_segLength;
	for(int iteration=0; iteration<_iterations; iteration++){
		contract(particles, 0, 1, dSQ, 0.0f);
		for(int i=2; i<count; i++) contract(particles, i-1, i, dSQ, 0.5f);
		contract(particles, count-1, count, dSQ, 1.0f);
	}
}

static void
generateVertexes(Particle *particles, cpVect *verts, int count, cpFloat width)
{
	cpVect v0 = particles[0].pos;
	cpVect t0 = cpvperp(cpvnormalize_safe(cpvsub(particles[1].pos, v0)));
	verts[0] = cpvadd(v0, cpvmult(t0, width));
	verts[1] = cpvadd(v0, cpvmult(t0, -width));
	
	for(int i=1; i<count-1; i++){
		cpVect v1 = particles[i].pos;
		cpVect t1 = cpvperp(cpvnormalize_safe(cpvsub(v1, v0)));
		
		cpVect t = cpvmult(cpvnormalize_safe(cpvlerp(t0, t1, 0.5f)), width);
		verts[2*i+0] = cpvadd(v1, t);
		verts[2*i+1] = cpvadd(v1, cpvneg(t));
		
		v0 = v1, t0 = t1;
	}
	
	cpVect v1 = particles[count-1].pos;
	cpVect t1 = cpvperp(cpvnormalize_safe(cpvsub(v1, particles[count-2].pos)));
	verts[2*count-2] = cpvadd(v1, cpvmult(t1, width));
	verts[2*count-1] = cpvadd(v1, cpvmult(t1, -width));
}

- (void)draw {
	int count = _count + 1;
	
	cpVect verts[count*2];
	generateVertexes(_particles, verts, count, _width);
	
	glDisableClientState(GL_COLOR_ARRAY);
	glDisableClientState(GL_TEXTURE_COORD_ARRAY);
	
//	glBindTexture(GL_TEXTURE_2D, textureAtlas.name);
	glDisable(GL_TEXTURE_2D);
	
	glColor4ub(_color.r, _color.g, _color.b, _color.a);
	glVertexPointer(2, GL_FLOAT, 0, verts);
	glDrawArrays(GL_TRIANGLE_STRIP, 0, count*2);
	
	glEnableClientState(GL_COLOR_ARRAY);
	glEnableClientState(GL_TEXTURE_COORD_ARRAY);
	glEnable(GL_TEXTURE_2D);
	
	glColor4f(1.0f, 1.0f, 1.0f, 1.0f);
}

@end

thanks! I'm going to try your code and see it in action.