move one dynamic body relatively to other dynamic body

Question

I faced with some problem which I can't find solution for. I have swinging platform (body1) and hero (body2) standing on it. Platform is swinging so hero moving right and left on the platform.

In the beginning I want hero to just stand on the platform without changing it's position relatively to platform. I achieved that by adding SKPhysicsJointFixed joining the platform and the hero.

Then in some moment in the game I want hero to run along the platform which means that Y should remain the same relatively to platform and X should grow. The only solution that I found is to make hero body static and change it's position in the didSimulatePhysics. The problem with that approach is that position not changing accurately. Hero position lags to one frame in comparison with platform position.

After hero reaches the right end of the platform he should stop moving and again just stand on the platform. So I make hero body static and add fixed joint again. And because hero position changes with lags hero position relative to platform is not correct. Sometimes it stops some pixels below the surface of the platform and sometimes some pixels above the surface.

So is there some other approach to move the hero along the platform, hero body should remain dynamic all the time.

Answer 1

I have a game with moving floating platforms. The simplest and most dynamic approach to achieve correct relative velocity is to simply make adjustments to the velocity. Here is a simplified version of what I do to achieve this.

In my game loop I have the following:

if heroOnBridge {
    hero.extraMotion = floatingBridge.physicsBody!.velocity
} else {
    hero.extraMotion = CGVector(dx:0,dy:0)
}

Here what I do is constantly look to see if my hero is on a moving bridge, if so then I set my hero's "extra motion" property to the velocity of the bridge that I am on. If I am not on a bridge then I don't add on any extra velocity. Note that I don't show how heroOnBridge is calculated because it really depends on how you define whether you are on a platform. In my case I assume you are on a platform if you intersect the platform frame and are on the ground.

Then in the area where I calculate my hero's velocity depending on his direction (left, right, jumping etc), I simply add on the extraMotion to the hero's velocity.

If you don't already I suggest you have an area in the game loop for processing the motion of you characters. It will allow you to make changes to the velocity of your hero depending on certain factors (in the air, jumping, moving left/right, on a platform, etc.)

If you need more help with calculating the velocity part let me know and I will post more of my code. You may also want to read my answer here for smoothly and dynamically setting the velocity.

Remember, for the most part you never want to set the position of anything directly, always try to set the necessary velocity to achieve that position over some period of time. This prevents very jittery and unpredictable motion.