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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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Definitely maybe -- it depends entirely on what you want it to do. What do you want it to do? –  Anko Jul 12 '13 at 9:48

1 Answer 1

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).

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Thanks Mario! great think! Here is what I will implement this, to consider two conditions: A.Add isHorize flag. You can use the b2ContactListener to getManifold which include the contact point position info. –  user25959 Jul 13 '13 at 7:54
    
B: Add left or right flag; This we will compare the contact point position relative to sprite center to set flag then do decide which direction force to apply. Thank you! –  user25959 Jul 13 '13 at 7:56

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