# Setting the Box2D gravity for a pixel coordinate game?

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?

Thanks

• What is a "pixel coordinate game"? Also, how do you define "rather slow"? Is this a performance problem, or are objects simply not moving as fast as you expect? – Nicol Bolas Jul 21 '11 at 0:30
• @Nicol It is not performance, they just do not move nearly as fast as real life. It only looks fast enough if I set gravity to 100 instead of 10. By Pixel I mean that when I draw something at 10,10 I see it at pixel 10,10 on the screen. I tried setting my ratio to 32 and gravity to 10 but it still has a smooth slo mo feel. – jmasterx Jul 21 '11 at 0:35
• Is there any particular reason you're not using metres, considering that a) Box2D is tuned for metres/kilograms/seconds and b) it's more understandable for both you and I than pixels as a measurement of distance, and it could immediately tell you what's going on (e.g. maybe it's going a magnitude slower because your scale is a magnitude too large, so you're seeing a fall take place that's ten times larger than you intend) – doppelgreener Jul 21 '11 at 0:47
• If your simulation looks fine with a Gravity of 100, why not do that? As for the jittering: Increasing the iterations in one step should fix that. Btw. How big are your objects (in Box2d units)? – bummzack Jul 23 '11 at 8:28

// Draw function