Setting the Box2D gravity for a pixel coordinate game?

  • Setting the Box2D gravity for a pixel coordinate game? jmasterx

    I'm making a game with Box2D with a top left coordinate system. I multiply positions by an M_TO_PX_RATIO of 10.0f to convert from meters to pixels.

    I noticed that when I set gravity to 9.8, the simulation is rather slow. When I set gravity to 9.8 * M_TO_PX_RATIO, the simulation runs at the correct speed, like normal gravity. However, the high gravity causes jittering.

    Am I doing it properly? When using a top left, pixel coordinate system, must I do anything else to account for meters to pixels?


  • Box2D is a meter-based physics engine, and as such, it would not behoove you to try to force it to "snap" to pixels. The easiest way to get around this is simply allow Box2D to calculate all of the physics in the background, and simply render all of your pixel sprites to the screen at the closest pixel coordinate (I like to floor my x and y value to prevent sprite jitter).

    Here's an extremely simple psudo-code version of what I'm trying to explain:

    // Draw function
    mySprite.render(Floor(calculatedXLocation), Floor(calculatedYLocation));

    This will give the illusion of the sprite being locked to the pixel grid, but will allow your physics to run however you want them to run without being hampered by an odd time-step or gravity.

c++ game-design box2d
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