Box2D in 3D space

Question

I am trying to combine a perspective camera with a box2D world using libgdx and I am having some problems getting the two to synch up. I want to do this so I can use 3D models but keep the game in a fixed plane like a 2D platformer and I will not move, angle or zoom the camera. I set the Box2D world to a fixed size, say 35x20 meters. The problem I have is calculating how much I need to move the 3D models so they synch up with the Box2D world. Lest say I move the Box2D body of a character 1 meter to the left how much should I move the 3D model then?

I tried to simply calculate pixels per meter and use that to move the 3D models. But I guess that I need to take the distance from the camera into account? So if someone have a formula or can point me in the right direction on which formula to use I would really appreciate it, thanks!

Answer 1

It should be pretty much the same as if you were using sprites. I would just keep it simple and say that one Box2D meter is one game unit.

Answer 2

The way I find the 3D units from pixels is by using ratios. If you want to find how far the 3D object's x is from the center of the screen then use a ratio like:

x2D = 10.0; // pixels of object from center
width2D = 60.0; // pixel width of view from center
width3D = 10.0; // 3D unit width of view from center

x3D = (x2D * width3D) / width2D

To find the 3D width use glFrustum()'s xLeft and zNear and find the z position from the modelview matrix like so:

width3D = (xLeft * z) / zNear

You can use the a / b = c / d ratio to solve for a lot of other graphics problems too, like scroll bars and radians to degrees.

Answer 3

Start with a bouncing ball on the ground. If it feels like moving slow, you have to increase your multiplier, say 10x. Multiply position in physics with 10 to calculate the display position and size of the ball, if that makes it feel better, it is the right constant. As long as your orthographic camera shows all of your sprite display, that can count as your resolution.