# Is it a good idea to use box2d to control sprite movement in a 2D game?

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I'm trying to use Box2D to control sprite movement.Some burning issues are these:

1.When I trying to move the sprite on a slope or a lean barrier. The sprite will move off the slope which is not what I want.

2.ALternatively, I could use setPosition or CCAction which provides by cocos2d-x to move the sprite and update the body position use SetTransform. But the body will lost it's physical limitation, it will passthrough other body.

Thanks for your time, if you guys got any solution?

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If you want the ball to "stand" still where the red circle is, you should apply forces that cancel out gravity, or (to get a more exact result) set velocity to 0 every frame until the ball should start moving.

If you want the ball to never lose contact with the hill as it rolls left to right, you should reduce its speed. At all times, the parabolic trajectory that the center of the ball would follow without obstacles and without external forces must lie within the Minkowski sum of the hill and the ball (meaning that the ball "tries" to sink underground and there is friction).

In both cases, setting position shouldn't be needed.

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This is a really hard question, because in the end it will depend on the kind of game you're trying to create. In most cases however, using a physics engine like Box2D might be overkill.

Especially for simple 2D platformers that don't include any physics puzzles, I'd go with doing the math on your own. It's not that hard to be honest.

To be able to stand on terrain and slopes, you just have to implement some very basic movement:

• Movement: When hitting left/right, just move your character to the left or right, don't worry about collision. Either move your character absolute distances or adjust his velocity.
• Gravity: Check to see whether there's something solid at the character's feet (depending on your implementation you might skip this check as you'll check it next anyway). If there isn't, move or accelerate him downwards.
• Collisions: After moving your character, verify he's not entering anything solid:
• Move the character to the right while his left part is in something solid.
• Move the character to the left while his right part is in something solid.
• Move the character upwards while his bottom part is in something solid.
• Move the character downwards while his top part is in something solid.
• If two opposite sides of the character are stuck in something solid (left and right or top and bottom), your character is squeezed. In games like Super Mario Bros this results in your character being killed.

Back to your initial question with the slope: Using the above logic, your character won't follow the slope at first. It will essentially move "into" the slope. But thanks to the logic under Collisions he'll be adjusted upwards, essentially following the slope up (or down).

Marios give useful idea about the this situation.

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I suggest you keep to using a 2d physics library.  You should be able to shape the world and the object definitions so they do what you want.  It's going to make your life much easier.

The statements in the above post are short-sighted., especially about collisions.  What happens when 2 objects collide, but when you handles the collision responses, one of the objects collide with a 3rd object that's already been checked for collisions?  And, you'll have to properly handle all types of angles and surfaces for these collisions.  Physics engines handles these for you, and are VERY efficient at doing it.

Edited by BeerNutts

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I suggest you keep to using a 2d physics library.  You should be able to shape the world and the object definitions so they do what you want.  It's going to make your life much easier.

The statements in the above post are short-sighted., especially about collisions.  What happens when 2 objects collide, but when you handles the collision responses, one of the objects collide with a 3rd object that's already been checked for collisions?  And, you'll have to properly handle all types of angles and surfaces for these collisions.  Physics engines handles these for you, and are VERY efficient at doing it.

Thank you BeerNutts. The diffrerent contact vary from different position. I define 4 states to decide which force to apply. So I can control it now.