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Controllable player character with Box2D

Started by Syranide Dec 6, 2011 at 10:10 PM 2 replies 7.1k views

Original Post

Dec 6, 2011 at 10:10 PM

So I've been playing a bit with Box2D, the intention is to create a 2D physics platformer.

But I really can't figure out how to best control the player character via Box2D, I've look around the internet some and while there are some examples out there, all I can find are pretty crap and have major flaws or can't really be tweaked in any practical way. The idea is for the player character to have momentum, the world will have angled terrain that will slow down the movement a bit, and given steep enough angles the character should simply slip back down. The player should be able to drag and get pushed moving blocks. The player should not act like a vehicle but should stick to the ground. We definitely want to be able to tweak the feeling of the player character. The character feeling in LIMBO (http://www.youtube.c...h?v=t1vexQzA9Vk) is similar in nature to what we have in mind.

I've played with a dynamic body consisting of a sphere "leg", but it has rather erratic behavior, it will slide regardless of friction with no impulse applied when put on an angled surface. And sometimes it will instead get "stuck" on the very same surface instead, not sliding at all, but also not being able to go uphill. By simply going back and forth along some chain edges.

Of course I'm not expecting a solution, simply what "tools" I should use, there's something called kinematic bodies, but I can't really tell what they're actually meant for... it seems like I should be using dynamic bodies, or? I wouldn't really have an issue with just completely overriding the dynamic body physics for the player, but it must interact with the physics of the world to some degree; being "realistically" pushed by moving blocks (small ones should just bump you) and also be "realistically" affected when moving blocks (player should be affected by the momentum and weight of the block).

Any hints?

It's stripped of all graphics and such at the moment, and I doubt there's anything of interest to see (the player is the little sphere), but here it is anyway:
http://imageshack.us...812/testzs.jpg/

Ashaman73

Dec 7, 2011 at 10:12 AM

So I've been playing a bit with Box2D, the intention is to create a 2D physics platformer.

But I really can't figure out how to best control the player character via Box2D, I've look around the internet some and while there are some examples out there, all I can find are pretty crap and have major flaws or can't really be tweaked in any practical way. The idea is for the player character to have momentum, the world will have angled terrain that will slow down the movement a bit, and given steep enough angles the character should simply slip back down. The player should be able to drag and get pushed moving blocks. The player should not act like a vehicle but should stick to the ground. We definitely want to be able to tweak the feeling of the player character. The character feeling in LIMBO (http://www.youtube.c...h?v=t1vexQzA9Vk) is similar in nature to what we have in mind.

I've played with a dynamic body consisting of a sphere "leg", but it has rather erratic behavior, it will slide regardless of friction with no impulse applied when put on an angled surface. And sometimes it will instead get "stuck" on the very same surface instead, not sliding at all, but also not being able to go uphill. By simply going back and forth along some chain edges.

Of course I'm not expecting a solution, simply what "tools" I should use, there's something called kinematic bodies, but I can't really tell what they're actually meant for... it seems like I should be using dynamic bodies, or? I wouldn't really have an issue with just completely overriding the dynamic body physics for the player, but it must interact with the physics of the world to some degree; being "realistically" pushed by moving blocks (small ones should just bump you) and also be "realistically" affected when moving blocks (player should be affected by the momentum and weight of the block).

Any hints?

It's stripped of all graphics and such at the moment, and I doubt there's anything of interest to see (the player is the little sphere), but here it is anyway:
http://imageshack.us...812/testzs.jpg/

Character movement is always a pain in , you just can't simulate human movement with forces and some approximation object like a sphere,cylinder etc. I don't know your physics engine, but I handled by 'overwriting' the behaviour of the physics engine for controlled characters.

Here are some hints:
1. Do some raycasts (character center => down, character center => forward) to get the "context" in which a character moves. This way you can decide if the character touches the ground and if the character is infront of a wall.

2. Whenever the character touches the ground, you will manipulate the velocity directly, that is apply/neutralise the velocity , so that a character have a constant velocity in the desired direction. You can make a smooth transition, but don't use forces/torque to manipulate the velocity indirectly, this way is really error prone and a very frustrating experience.

3. An other tip is to neutralise (a part of) the velocity in the direction of a wall (see raycast), sometimes the physics simulation breaks or get "choppy" when you try to press a character with a constant velocity against a wall.

4. Still use a rigid body (sphere, cylinder etc.) to handle collision detection with other entities.

5. Neutralise a part of the velocity if the character touches the ground and is not directly controlled, this way you prevent sliding on swallow slopes.

Syranide

Dec 7, 2011 at 10:16 PM

I can't really picture it all in my head right now, but your suggestions actually seems to make sense.

Thank you!

Syranide

Dec 8, 2011 at 9:43 PM

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