Following a body with the camera in Box2D – The smart way

May 4, 2010 • Actionscript 3, Box2D, Flash, Game design by Emanuele Feronato • 9 Comments

We are about to create an intelligent camera that will follow a body in Box2D. What’s the difference between an intelligent camera and a dumb one?

Simple… the intelligent camera follows the object keeping it in the middle of the screen as long as there are enough game area to fill the screen. The dumb one just keeps the object in the middle of the screen, sometime causing to have just a little part of the game on the screen.

Let me make things clearer with a real world example:

In this map, taken from Crystal Kingdom Dizzy, the cherry is followed by a smart camera, because the cherry is centered and the game area fills the entire screen.

Now look at this:

The cherry moved up and the camera followed it in a dumb way: the camera wanted to keep the cherry in the middle of the screen, but doing it will make the game scroll down even if there aren’t enough game area to fill the entire screen.

An intelligent camera should show the cherry this way:

So when the cherry moves in a direction that would make the game scroll away from the screen, the camera has to release it and don’t keep it in the middle of the screen until there is enough gaming area to fill the entire screen.

How do we apply this concept to the catapult prototype?

This way:

package {
	import flash.display.Sprite;
	import flash.events.Event;
	import flash.events.KeyboardEvent;
	import Box2D.Dynamics.*;
	import Box2D.Collision.*;
	import Box2D.Collision.Shapes.*;
	import Box2D.Common.Math.*;
	import Box2D.Dynamics.Joints.*;
	public class main extends Sprite {
		public var world:b2World=new b2World(new b2Vec2(0,10.0),true);
		public var world_scale:int=30;
		var the_cannonball_itself:b2Body;
		var catapult_chassis_body:b2Body;
		var catapult_arm_body:b2Body;
		var rear_wheel_body:b2Body;
		var front_wheel_body:b2Body;
		var arm_revolute_joint:b2RevoluteJoint;
		var front_wheel_revolute_joint:b2RevoluteJoint;
		var rear_wheel_revolute_joint:b2RevoluteJoint;
		var left_key_pressed:Boolean=false;
		var right_key_pressed:Boolean=false;
		var following_catapult:Boolean=true;
		var bg:background_mc = new background_mc();
		public function main():void {
			addChild(bg);
			debug_draw();
			the_ground();
			the_catapult_body();
			the_catapult_arm();
			the_catapult_motor();
			the_wheels();
			the_wheel_motors();
			the_cannonball();
			addEventListener(Event.ENTER_FRAME, update);
			stage.addEventListener(KeyboardEvent.KEY_DOWN, key_down);
			stage.addEventListener(KeyboardEvent.KEY_UP, key_up);
		}
		public function the_cannonball():void {
			var cannonball:b2BodyDef= new b2BodyDef();
			cannonball.position.Set(90/world_scale, 90/world_scale);
			cannonball.type=b2Body.b2_dynamicBody;
			var ball:b2CircleShape=new b2CircleShape(10/world_scale);
			var cannonball_fixture:b2FixtureDef = new b2FixtureDef();
			cannonball_fixture.shape=ball;
			cannonball_fixture.friction=0.9;
			cannonball_fixture.density=20;
			cannonball_fixture.restitution=0.5;
			the_cannonball_itself=world.CreateBody(cannonball);
			the_cannonball_itself.CreateFixture(cannonball_fixture);
		}
		public function the_wheel_motors():void {
			var front_wheel_joint:b2RevoluteJointDef = new b2RevoluteJointDef();
			front_wheel_joint.enableMotor=true;
			front_wheel_joint.Initialize(catapult_chassis_body, front_wheel_body,new b2Vec2(0,0));
			front_wheel_joint.localAnchorA=new b2Vec2(80/world_scale,0);
			front_wheel_joint.localAnchorB=new b2Vec2(0,0);
			front_wheel_revolute_joint=world.CreateJoint(front_wheel_joint) as b2RevoluteJoint;
			front_wheel_revolute_joint.SetMaxMotorTorque(1000000);
			//
			var rear_wheel_joint:b2RevoluteJointDef = new b2RevoluteJointDef();
			rear_wheel_joint.enableMotor=true;
			rear_wheel_joint.Initialize(catapult_chassis_body, rear_wheel_body,new b2Vec2(0,0));
			rear_wheel_joint.localAnchorA=new b2Vec2(-80/world_scale,0);
			rear_wheel_joint.localAnchorB=new b2Vec2(0,0);
			rear_wheel_revolute_joint=world.CreateJoint(rear_wheel_joint) as b2RevoluteJoint;
			rear_wheel_revolute_joint.SetMaxMotorTorque(1000000);
		}
		public function the_wheels():void {
			var rear_wheel:b2BodyDef= new b2BodyDef();
			rear_wheel.position.Set(250/world_scale, 200/world_scale);
			rear_wheel.type=b2Body.b2_dynamicBody;
			var rear_wheel_shape:b2CircleShape=new b2CircleShape(40/world_scale);
			var rear_wheel_fixture:b2FixtureDef = new b2FixtureDef();
			rear_wheel_fixture.shape=rear_wheel_shape;
			rear_wheel_fixture.friction=0.9;
			rear_wheel_fixture.density=30;
			rear_wheel_fixture.restitution=0.1;
			rear_wheel_body=world.CreateBody(rear_wheel);
			rear_wheel_body.CreateFixture(rear_wheel_fixture);
			//
			var front_wheel:b2BodyDef= new b2BodyDef();
			front_wheel.position.Set(450/world_scale, 200/world_scale);
			front_wheel.type=b2Body.b2_dynamicBody;
			var front_wheel_shape:b2CircleShape=new b2CircleShape(40/world_scale);
			var front_wheel_fixture:b2FixtureDef = new b2FixtureDef();
			front_wheel_fixture.shape=front_wheel_shape;
			front_wheel_fixture.friction=0.9;
			front_wheel_fixture.density=30;
			front_wheel_fixture.restitution=0.1;
			front_wheel_body=world.CreateBody(front_wheel);
			front_wheel_body.CreateFixture(front_wheel_fixture);
		}
		public function the_catapult_motor():void {
			var arm_joint:b2RevoluteJointDef = new b2RevoluteJointDef();
			arm_joint.enableMotor=true;
			arm_joint.enableLimit=true;
			arm_joint.Initialize(catapult_chassis_body, catapult_arm_body,new b2Vec2(0,0));
			arm_joint.localAnchorA=new b2Vec2(-80/world_scale,-90/world_scale);
			arm_joint.localAnchorB=new b2Vec2(60/world_scale,0);
			arm_revolute_joint=world.CreateJoint(arm_joint) as b2RevoluteJoint;
			arm_revolute_joint.SetMotorSpeed(1000);
			arm_revolute_joint.SetLimits(-Math.PI,Math.PI/3);
			arm_revolute_joint.SetMaxMotorTorque(1);
		}
		public function the_catapult_arm():void {
			var catapult_arm:b2BodyDef = new b2BodyDef();
			catapult_arm.allowSleep=false;
			catapult_arm.position.Set(210/world_scale,110/world_scale);
			catapult_arm.type=b2Body.b2_dynamicBody;
			var arm_part:b2PolygonShape = new b2PolygonShape();
			arm_part.SetAsOrientedBox(150/world_scale, 10/world_scale, new b2Vec2(0,0),0);
			var arm_part_fixture:b2FixtureDef = new b2FixtureDef();
			arm_part_fixture.shape=arm_part;
			arm_part_fixture.friction=0.9;
			arm_part_fixture.density=5;
			arm_part_fixture.restitution=0.1;
			var stopper:b2PolygonShape = new b2PolygonShape();
			stopper.SetAsOrientedBox(10/world_scale, 20/world_scale, new b2Vec2(-140/world_scale,-30/world_scale),0);
			var stopper_fixture:b2FixtureDef = new b2FixtureDef();
			stopper_fixture.shape=stopper;
			stopper_fixture.friction=0.9;
			stopper_fixture.density=10;
			stopper_fixture.restitution=0.1;
			catapult_arm_body=world.CreateBody(catapult_arm);
			catapult_arm_body.CreateFixture(arm_part_fixture);
			catapult_arm_body.CreateFixture(stopper_fixture);
		}
		public function the_catapult_body():void {
			var catapult_body:b2BodyDef = new b2BodyDef();
			catapult_body.position.Set(350/world_scale,200/world_scale);
			catapult_body.type=b2Body.b2_dynamicBody;
			var main_part:b2PolygonShape = new b2PolygonShape();
			main_part.SetAsOrientedBox(125/world_scale, 20/world_scale, new b2Vec2(0,0),0);
			var chassis_fixture:b2FixtureDef = new b2FixtureDef();
			chassis_fixture.shape=main_part;
			chassis_fixture.friction=0.9;
			chassis_fixture.density=50;
			chassis_fixture.restitution=0.1;
			var fixed_arm:b2PolygonShape = new b2PolygonShape();
			fixed_arm.SetAsOrientedBox(20/world_scale, 60/world_scale, new b2Vec2(-80/world_scale,-40/world_scale),0);
			var fixed_arm_fixture:b2FixtureDef = new b2FixtureDef();
			fixed_arm_fixture.shape=fixed_arm;
			fixed_arm_fixture.friction=0.9;
			fixed_arm_fixture.density=1;
			fixed_arm_fixture.restitution=0.1;
			catapult_chassis_body=world.CreateBody(catapult_body);
			catapult_chassis_body.CreateFixture(chassis_fixture);
			catapult_chassis_body.CreateFixture(fixed_arm_fixture);
		}
		public function the_ground():void {
			var ground:b2BodyDef= new b2BodyDef();
			ground.position.Set(2500/world_scale, 400/world_scale);
			var my_box:b2PolygonShape = new b2PolygonShape();
			my_box.SetAsBox(2500/world_scale, 15/world_scale);
			var ground_fixture:b2FixtureDef = new b2FixtureDef();
			ground_fixture.shape=my_box;
			ground_fixture.friction=0.9;
			ground_fixture.restitution=0.1;
			//
			var my_box2:b2PolygonShape = new b2PolygonShape();
			my_box2.SetAsOrientedBox(15/world_scale, 350/world_scale, new b2Vec2(2485/world_scale,-335/world_scale),0);
			var right_wall_fixture:b2FixtureDef = new b2FixtureDef();
			right_wall_fixture.shape=my_box2;
			right_wall_fixture.friction=0.9;
			right_wall_fixture.restitution=0.1;
			//
			var my_box3:b2PolygonShape = new b2PolygonShape();
			my_box3.SetAsOrientedBox(15/world_scale, 350/world_scale, new b2Vec2(-2485/world_scale,-335/world_scale),0);
			var left_wall_fixture:b2FixtureDef = new b2FixtureDef();
			left_wall_fixture.shape=my_box3;
			left_wall_fixture.friction=0.9;
			left_wall_fixture.restitution=0.1;
			//
			var my_box4:b2PolygonShape = new b2PolygonShape();
			my_box4.SetAsOrientedBox(2500/world_scale, 15/world_scale, new b2Vec2(0,-670/world_scale),0);
			var roof_fixture:b2FixtureDef = new b2FixtureDef();
			roof_fixture.shape=my_box4;
			roof_fixture.friction=0.9;
			roof_fixture.restitution=0.1;
			//
			var the_ground_itself:b2Body=world.CreateBody(ground);
			the_ground_itself.CreateFixture(ground_fixture);
			the_ground_itself.CreateFixture(right_wall_fixture);
			the_ground_itself.CreateFixture(left_wall_fixture);
			the_ground_itself.CreateFixture(roof_fixture);
		}
		public function key_up(event:KeyboardEvent):void {
			switch (event.keyCode) {
				case 39 :
					right_key_pressed=false;
					break;
				case 37 :
					left_key_pressed=false;
					break;
			}
		}
		public function key_down(event:KeyboardEvent):void {
			switch (event.keyCode) {
				case 39 :
					right_key_pressed=true;
					left_key_pressed=false;
					break;
				case 37 :
					left_key_pressed=true;
					right_key_pressed=false;
					break;
				case 32 :
					arm_revolute_joint.SetMaxMotorTorque(10000);
					following_catapult=false;
					break;
			}
		}
		public function debug_draw():void {
			var debug_draw:b2DebugDraw = new b2DebugDraw();
			var debug_sprite:Sprite = new Sprite();
			addChild(debug_sprite);
			debug_draw.SetSprite(debug_sprite);
			debug_draw.SetDrawScale(world_scale);
			debug_draw.SetFlags(b2DebugDraw.e_shapeBit|b2DebugDraw.e_jointBit);
			debug_draw.SetFillAlpha(0.5);
			world.SetDebugDraw(debug_draw);
		}
		public function set_motor_speed():void {
			var current_speed:Number;
			if (right_key_pressed) {
				current_speed=1;
			}
			if (left_key_pressed) {
				current_speed=-1;
			}
			if (! right_key_pressed&&! left_key_pressed) {
				current_speed=rear_wheel_revolute_joint.GetMotorSpeed()*0.9;
				if (Math.abs(current_speed)<0.1) {
					current_speed=0;
				}
			}
			rear_wheel_revolute_joint.SetMotorSpeed(current_speed);
			front_wheel_revolute_joint.SetMotorSpeed(current_speed);
		}
		public function update(e:Event):void {
			var pos_x:Number;
			var pos_y:Number;
			set_motor_speed();
			world.Step(1/30,10,10);
			if (following_catapult) {
				pos_x=catapult_chassis_body.GetWorldCenter().x*world_scale;
				pos_y=catapult_chassis_body.GetWorldCenter().y*world_scale;
			} else {
				pos_x=the_cannonball_itself.GetWorldCenter().x*world_scale;
				pos_y=the_cannonball_itself.GetWorldCenter().y*world_scale;
			}
			pos_x=stage.stageWidth/2-pos_x;
			if (pos_x<0-4500) {
				pos_x=-4500;
			}
			if (pos_x>0) {
				pos_x=0;
			}
			x=pos_x;
			pos_y=stage.stageHeight/2-pos_y;
			if (pos_y<0-15) {
				pos_y=-15;
			}
			if (pos_y>285) {
				pos_y=285;
			}
			y=pos_y;
			world.ClearForces();
			world.DrawDebugData();
		}
	}
}

100

101

102

103

104

105

106

107

108

109

110

111

112

113

114

115

116

117

118

119

120

121

122

123

124

125

126

127

128

129

130

131

132

133

134

135

136

137

138

139

140

141

142

143

144

145

146

147

148

149

150

151

152

153

154

155

156

157

158

159

160

161

162

163

164

165

166

167

168

169

170

171

172

173

174

175

176

177

178

179

180

181

182

183

184

185

186

187

188

189

190

191

192

193

194

195

196

197

198

199

200

201

202

203

204

205

206

207

208

209

210

211

212

213

214

215

216

217

218

219

220

221

222

223

224

225

226

227

228

229

230

231

232

233

234

235

236

237

238

239

240

241

242

243

244

245

246

247

248

249

250

251

252

253

254

255

256

257

258

259

260

261

262

263

264

265

266

267

268

269

270

271

272

273

package {

import flash.display.Sprite;

import flash.events.Event;

import flash.events.KeyboardEvent;

import Box2D.Dynamics.*;

import Box2D.Collision.*;

import Box2D.Collision.Shapes.*;

import Box2D.Common.Math.*;

import Box2D.Dynamics.Joints.*;

public class main extends Sprite {

public var world:b2World=new b2World(new b2Vec2(0,10.0),true);

public var world_scale:int=30;

var the_cannonball_itself:b2Body;

var catapult_chassis_body:b2Body;

var catapult_arm_body:b2Body;

var rear_wheel_body:b2Body;

var front_wheel_body:b2Body;

var arm_revolute_joint:b2RevoluteJoint;

var front_wheel_revolute_joint:b2RevoluteJoint;

var rear_wheel_revolute_joint:b2RevoluteJoint;

var left_key_pressed:Boolean=false;

var right_key_pressed:Boolean=false;

var following_catapult:Boolean=true;

var bg:background_mc = new background_mc();

public function main():void {

addChild(bg);

debug_draw();

the_ground();

the_catapult_body();

the_catapult_arm();

the_catapult_motor();

the_wheels();

the_wheel_motors();

the_cannonball();

addEventListener(Event.ENTER_FRAME, update);

stage.addEventListener(KeyboardEvent.KEY_DOWN, key_down);

stage.addEventListener(KeyboardEvent.KEY_UP, key_up);

}

public function the_cannonball():void {

var cannonball:b2BodyDef= new b2BodyDef();

cannonball.position.Set(90/world_scale, 90/world_scale);

cannonball.type=b2Body.b2_dynamicBody;

var ball:b2CircleShape=new b2CircleShape(10/world_scale);

var cannonball_fixture:b2FixtureDef = new b2FixtureDef();

cannonball_fixture.shape=ball;

cannonball_fixture.friction=0.9;

cannonball_fixture.density=20;

cannonball_fixture.restitution=0.5;

the_cannonball_itself=world.CreateBody(cannonball);

the_cannonball_itself.CreateFixture(cannonball_fixture);

}

public function the_wheel_motors():void {

var front_wheel_joint:b2RevoluteJointDef = new b2RevoluteJointDef();

front_wheel_joint.enableMotor=true;

front_wheel_joint.Initialize(catapult_chassis_body, front_wheel_body,new b2Vec2(0,0));

front_wheel_joint.localAnchorA=new b2Vec2(80/world_scale,0);

front_wheel_joint.localAnchorB=new b2Vec2(0,0);

front_wheel_revolute_joint=world.CreateJoint(front_wheel_joint) as b2RevoluteJoint;

front_wheel_revolute_joint.SetMaxMotorTorque(1000000);

var rear_wheel_joint:b2RevoluteJointDef = new b2RevoluteJointDef();

rear_wheel_joint.enableMotor=true;

rear_wheel_joint.Initialize(catapult_chassis_body, rear_wheel_body,new b2Vec2(0,0));

rear_wheel_joint.localAnchorA=new b2Vec2(-80/world_scale,0);

rear_wheel_joint.localAnchorB=new b2Vec2(0,0);

rear_wheel_revolute_joint=world.CreateJoint(rear_wheel_joint) as b2RevoluteJoint;

rear_wheel_revolute_joint.SetMaxMotorTorque(1000000);

}

public function the_wheels():void {

var rear_wheel:b2BodyDef= new b2BodyDef();

rear_wheel.position.Set(250/world_scale, 200/world_scale);

rear_wheel.type=b2Body.b2_dynamicBody;

var rear_wheel_shape:b2CircleShape=new b2CircleShape(40/world_scale);

var rear_wheel_fixture:b2FixtureDef = new b2FixtureDef();

rear_wheel_fixture.shape=rear_wheel_shape;

rear_wheel_fixture.friction=0.9;

rear_wheel_fixture.density=30;

rear_wheel_fixture.restitution=0.1;

rear_wheel_body=world.CreateBody(rear_wheel);

rear_wheel_body.CreateFixture(rear_wheel_fixture);

var front_wheel:b2BodyDef= new b2BodyDef();

front_wheel.position.Set(450/world_scale, 200/world_scale);

front_wheel.type=b2Body.b2_dynamicBody;

var front_wheel_shape:b2CircleShape=new b2CircleShape(40/world_scale);

var front_wheel_fixture:b2FixtureDef = new b2FixtureDef();

front_wheel_fixture.shape=front_wheel_shape;

front_wheel_fixture.friction=0.9;

front_wheel_fixture.density=30;

front_wheel_fixture.restitution=0.1;

front_wheel_body=world.CreateBody(front_wheel);

front_wheel_body.CreateFixture(front_wheel_fixture);

}

public function the_catapult_motor():void {

var arm_joint:b2RevoluteJointDef = new b2RevoluteJointDef();

arm_joint.enableMotor=true;

arm_joint.enableLimit=true;

arm_joint.Initialize(catapult_chassis_body, catapult_arm_body,new b2Vec2(0,0));

arm_joint.localAnchorA=new b2Vec2(-80/world_scale,-90/world_scale);

arm_joint.localAnchorB=new b2Vec2(60/world_scale,0);

arm_revolute_joint=world.CreateJoint(arm_joint) as b2RevoluteJoint;

arm_revolute_joint.SetMotorSpeed(1000);

arm_revolute_joint.SetLimits(-Math.PI,Math.PI/3);

arm_revolute_joint.SetMaxMotorTorque(1);

}

public function the_catapult_arm():void {

var catapult_arm:b2BodyDef = new b2BodyDef();

catapult_arm.allowSleep=false;

catapult_arm.position.Set(210/world_scale,110/world_scale);

catapult_arm.type=b2Body.b2_dynamicBody;

var arm_part:b2PolygonShape = new b2PolygonShape();

arm_part.SetAsOrientedBox(150/world_scale, 10/world_scale, new b2Vec2(0,0),0);

var arm_part_fixture:b2FixtureDef = new b2FixtureDef();

arm_part_fixture.shape=arm_part;

arm_part_fixture.friction=0.9;

arm_part_fixture.density=5;

arm_part_fixture.restitution=0.1;

var stopper:b2PolygonShape = new b2PolygonShape();

stopper.SetAsOrientedBox(10/world_scale, 20/world_scale, new b2Vec2(-140/world_scale,-30/world_scale),0);

var stopper_fixture:b2FixtureDef = new b2FixtureDef();

stopper_fixture.shape=stopper;

stopper_fixture.friction=0.9;

stopper_fixture.density=10;

stopper_fixture.restitution=0.1;

catapult_arm_body=world.CreateBody(catapult_arm);

catapult_arm_body.CreateFixture(arm_part_fixture);

catapult_arm_body.CreateFixture(stopper_fixture);

}

public function the_catapult_body():void {

var catapult_body:b2BodyDef = new b2BodyDef();

catapult_body.position.Set(350/world_scale,200/world_scale);

catapult_body.type=b2Body.b2_dynamicBody;

var main_part:b2PolygonShape = new b2PolygonShape();

main_part.SetAsOrientedBox(125/world_scale, 20/world_scale, new b2Vec2(0,0),0);

var chassis_fixture:b2FixtureDef = new b2FixtureDef();

chassis_fixture.shape=main_part;

chassis_fixture.friction=0.9;

chassis_fixture.density=50;

chassis_fixture.restitution=0.1;

var fixed_arm:b2PolygonShape = new b2PolygonShape();

fixed_arm.SetAsOrientedBox(20/world_scale, 60/world_scale, new b2Vec2(-80/world_scale,-40/world_scale),0);

var fixed_arm_fixture:b2FixtureDef = new b2FixtureDef();

fixed_arm_fixture.shape=fixed_arm;

fixed_arm_fixture.friction=0.9;

fixed_arm_fixture.density=1;

fixed_arm_fixture.restitution=0.1;

catapult_chassis_body=world.CreateBody(catapult_body);

catapult_chassis_body.CreateFixture(chassis_fixture);

catapult_chassis_body.CreateFixture(fixed_arm_fixture);

}

public function the_ground():void {

var ground:b2BodyDef= new b2BodyDef();

ground.position.Set(2500/world_scale, 400/world_scale);

var my_box:b2PolygonShape = new b2PolygonShape();

my_box.SetAsBox(2500/world_scale, 15/world_scale);

var ground_fixture:b2FixtureDef = new b2FixtureDef();

ground_fixture.shape=my_box;

ground_fixture.friction=0.9;

ground_fixture.restitution=0.1;

var my_box2:b2PolygonShape = new b2PolygonShape();

my_box2.SetAsOrientedBox(15/world_scale, 350/world_scale, new b2Vec2(2485/world_scale,-335/world_scale),0);

var right_wall_fixture:b2FixtureDef = new b2FixtureDef();

right_wall_fixture.shape=my_box2;

right_wall_fixture.friction=0.9;

right_wall_fixture.restitution=0.1;

var my_box3:b2PolygonShape = new b2PolygonShape();

my_box3.SetAsOrientedBox(15/world_scale, 350/world_scale, new b2Vec2(-2485/world_scale,-335/world_scale),0);

var left_wall_fixture:b2FixtureDef = new b2FixtureDef();

left_wall_fixture.shape=my_box3;

left_wall_fixture.friction=0.9;

left_wall_fixture.restitution=0.1;

var my_box4:b2PolygonShape = new b2PolygonShape();

my_box4.SetAsOrientedBox(2500/world_scale, 15/world_scale, new b2Vec2(0,-670/world_scale),0);

var roof_fixture:b2FixtureDef = new b2FixtureDef();

roof_fixture.shape=my_box4;

roof_fixture.friction=0.9;

roof_fixture.restitution=0.1;

var the_ground_itself:b2Body=world.CreateBody(ground);

the_ground_itself.CreateFixture(ground_fixture);

the_ground_itself.CreateFixture(right_wall_fixture);

the_ground_itself.CreateFixture(left_wall_fixture);

the_ground_itself.CreateFixture(roof_fixture);

}

public function key_up(event:KeyboardEvent):void {

switch (event.keyCode) {

case 39 :

right_key_pressed=false;

break;

case 37 :

left_key_pressed=false;

break;

}

public function key_down(event:KeyboardEvent):void {

switch (event.keyCode) {

case 39 :

right_key_pressed=true;

left_key_pressed=false;

break;

case 37 :

left_key_pressed=true;

right_key_pressed=false;

break;

case 32 :

arm_revolute_joint.SetMaxMotorTorque(10000);

following_catapult=false;

break;

}

public function debug_draw():void {

var debug_draw:b2DebugDraw = new b2DebugDraw();

var debug_sprite:Sprite = new Sprite();

addChild(debug_sprite);

debug_draw.SetSprite(debug_sprite);

debug_draw.SetDrawScale(world_scale);

debug_draw.SetFlags(b2DebugDraw.e_shapeBit|b2DebugDraw.e_jointBit);

debug_draw.SetFillAlpha(0.5);

world.SetDebugDraw(debug_draw);

}

public function set_motor_speed():void {

var current_speed:Number;

if (right_key_pressed) {

current_speed=1;

}

if (left_key_pressed) {

current_speed=-1;

}

if (! right_key_pressed&&! left_key_pressed) {

current_speed=rear_wheel_revolute_joint.GetMotorSpeed()*0.9;

if (Math.abs(current_speed)<0.1) {

current_speed=0;

}

rear_wheel_revolute_joint.SetMotorSpeed(current_speed);

front_wheel_revolute_joint.SetMotorSpeed(current_speed);

}

public function update(e:Event):void {

var pos_x:Number;

var pos_y:Number;

set_motor_speed();

world.Step(1/30,10,10);

if (following_catapult) {

pos_x=catapult_chassis_body.GetWorldCenter().x*world_scale;

pos_y=catapult_chassis_body.GetWorldCenter().y*world_scale;

} else {

pos_x=the_cannonball_itself.GetWorldCenter().x*world_scale;

pos_y=the_cannonball_itself.GetWorldCenter().y*world_scale;

}

pos_x=stage.stageWidth/2-pos_x;

if (pos_x<0-4500) {

pos_x=-4500;

}

if (pos_x>0) {

pos_x=0;

}

x=pos_x;

pos_y=stage.stageHeight/2-pos_y;

if (pos_y<0-15) {

pos_y=-15;

}

if (pos_y>285) {

pos_y=285;

}

y=pos_y;

world.ClearForces();

world.DrawDebugData();

}

And this is the result:

Left and right arrows to move the catapult, spacebar to fire

Download the example and wait for the fully commented source code.

Rate this post: 1 Star

(19 votes, average: 4.37 out of 5)

Loading ...

Flash Templates provided by Template Monster are pre-made web design products developed using Flash technology.
They can be easily customized to meet the unique requirements of your project.

Be my fan on Facebook and follow me on Twitter! Exclusive content for my Facebook fans and Twitter followers

This post has 9 comments

Bwakathaboom

on May 4, 2010 at 6:19 pm

Off topic – do you usually use a single-class when developing your games? When do you decide to break off into multiple classes?

I find other people’s coding styles fascinating.
lala

on May 5, 2010 at 11:51 am

very off topic – i wasn’t able to find your contact information. i have been following this blog for almost a year. previously related to 3d graphics now going to learn flash scripting. your blog has always been amazing / informational / rapid update resouce. now when i wanted to start doing exercises, i found it extremely hard to find what i have read at some point. e.g: flash game structure discussions, box2d tutorials, if somehow you can improve menu / navigation. catagorise, then it will be amazing.
specially if series of tuorials can come under a menu.
sorry if this disturbs or annoyed.
Lorenzo

on May 5, 2010 at 4:28 pm

altro off topic: in questi giorni continuo a ricevere errore 500 quando tento di accedere al tuo sito.
AlexRath

on May 9, 2010 at 8:42 pm

Hey,

how about some Ease?
An elastic following, not so hard…
You know, softer, so that the Ball is not always exact in the middle.
That shows the speed better.
Labici Danut

on May 10, 2010 at 2:35 pm

Hmmm abit hard for me but I’ll try and see what happen
Peter

on June 11, 2010 at 3:29 pm

I just wanted to say thanks. Your tutorials have been more help than all the other Box2D resources I’ve found put together. Keep up the good work!
Csomakk

on August 30, 2010 at 9:33 pm

hello Emanuele!
I want to ask you if there is a way to zoom in or out in a box2d application?
thanks
ronaldmac

on September 29, 2010 at 2:56 pm

whats stage,x,y?where is it declared?
Following a body with the camera in Box2D

on May 25, 2012 at 3:40 am

[...] http://www.emanueleferonato.com/2010/05/04/following-a-body-with-the-camera-in-box2d-the-smart-way/ [...]

Emanuele Feronato

italian geek and PROgrammer

Following a body with the camera in Box2D – The smart way

This post has 9 comments

Bwakathaboom

lala

Lorenzo

AlexRath

Labici Danut

Peter

Csomakk

ronaldmac

Following a body with the camera in Box2D