cocos2d-x中使用box2d時的PTM_RATIO

cocos2d-x中使用box2d時的PTM_RATIO定義成多少和box2d本身的特性有關。别人已經解釋的很清楚了：

http://www.raywenderlich.com/28602/intro-to-box2d-with-cocos2d-2-x-tutorial-bouncing-balls

以下是其中重點部分的摘錄：

This is defining a ratio of pixels to “meters”. When you specify where bodies are in Cocos2D, you give it a set of units. Although you may consider using pixels, that would be a mistake. According to the Box2D manual, Box2D has been optimized to deal with units as small as 0.1 and as big as 10. So as far as length goes people generally tend to treat it as “meters” so 0.1 would be about teacup size and 10 would be about box size.

So we don’t want to pass pixels in, because even small objects would be 60×60 pixels, way bigger than the values Box2D has been optimized for. So we need to have a way to convert pixels to “meters”, hence we can just define a ratio like the above. So if we had a 64 pixel object, we could divide it by PTM_RATIO to get 2 “meters” that Box2D can deal with for physics simulation purposes.