Cocos2d: Changing b2Body x val every frame causes jitter
- by Joey Green
So, I have a jumping mechanism similar to what you would see in doodle jump where character jumps and you use the accelerometer to make character change direction left or right.
I have a player object with position and a box2d b2Body with position.
I'm changing the player X position via the accelerometer and the Y position according to box2d.
pseudocode for this is like so
-----accelerometer acceleration------
player.position = new X
-----world update---------
physicsWorld-step()
//this will get me the new Y according to the physics similation
//so we keep the bodys Y value but change x to new X according to accelerometer data
playerPhysicsBody.position = new pos(player.position.x, keepYval)
player.position = playerPhysicsBody.position
Now this is simplifying my code, but I'm doing the position conversion back and forth via mult or divide by PTM_.
Well, I'm getting a weird jitter effect after I get big jump in acceleration data.
So, my questions are:
1) Is this the right approach to have the accelerometer control the x pos and box2d control the y pos and just sync everthing up every frame?
2) Is there some issue with updating a b2body x position every frame?
3) Any idea what might be creating this jitter effect?
I've collecting some data while running the game. Pre-body is before I set the x value on the b2Body in my update method after I world-step(). Post of course is afterwards. As you can see there is definitively a pattern.
012-06-19 08:14:13.118 Game[1073:707] pre-body pos 5.518720~24.362963
2012-06-19 08:14:13.120 Game[1073:707] post-body pos 5.060156~24.362963
2012-06-19 08:14:13.131 Game[1073:707] player velocity x: -31.833529
2012-06-19 08:14:13.133 Game[1073:707] delta 0.016669
2012-06-19 08:14:13.135 Game[1073:707] pre-body pos 5.060156~24.689455
2012-06-19 08:14:13.137 Game[1073:707] post-body pos 5.502138~24.689455
2012-06-19 08:14:13.148 Game[1073:707] player velocity x: -31.833529
2012-06-19 08:14:13.150 Game[1073:707] delta 0.016667
2012-06-19 08:14:13.151 Game[1073:707] pre-body pos 5.502138~25.006948
2012-06-19 08:14:13.153 Game[1073:707] post-body pos 5.043575~25.006948
2012-06-19 08:14:13.165 Game[1073:707] player velocity x: -31.833529
2012-06-19 08:14:13.167 Game[1073:707] delta 0.016644
2012-06-19 08:14:13.169 Game[1073:707] pre-body pos 5.043575~25.315441
2012-06-19 08:14:13.170 Game[1073:707] post-body pos 5.485580~25.315441
2012-06-19 08:14:13.180 Game[1073:707] player velocity x: -31.833529
2012-06-19 08:14:13.182 Game[1073:707] delta 0.016895
2012-06-19 08:14:13.185 Game[1073:707] pre-body pos 5.485580~25.614935
2012-06-19 08:14:13.188 Game[1073:707] post-body pos 5.026768~25.614935
2012-06-19 08:14:13.198 Game[1073:707] player velocity x: -31.833529
2012-06-19 08:14:13.199 Game[1073:707] delta 0.016454
2012-06-19 08:14:13.207 Game[1073:707] pre-body pos 5.026768~25.905428
2012-06-19 08:14:13.211 Game[1073:707] post-body pos 5.469213~25.905428
2012-06-19 08:14:13.217 Game[1073:707] acceleration x -0.137421
2012-06-19 08:14:13.223 Game[1073:707] player velocity x: -65.022644
2012-06-19 08:14:13.229 Game[1073:707] delta 0.016603
