Phys2D Constructive Solid Geometry

Game Mechanics
#1

As I was thinking of ways to separate my destructible Body hack for Phys2D into something more robust, efficient, and less hacky, I thought that maybe Constructive Solid Geometry would be a good choice. I would add a new type of Body to Phys2D that would have a binary tree of operations (Body’s added or subtracted). The actual used collision would be the union of the added shapes minus the union of the subtracted shapes. This would allow for much more intelligent destructible terrain without removing any of Phys2D’s robustness or efficiency. It also allows absolutely any type of shape to be constructed with minimal fuss and a simplistic view of geometry.

I’m pretty sure I have the know-how to do this, but before I delve in… any reason why this might be a silly idea?

#2

It’s a sound idea, I use it for terrain in Mootox (though I use Slick’s geometry operations first then convert into a phys2d body afterwards).

Kev

#3

My one question for this would be how I could use the “reverse side” of a shape’s geometry. What that means, basically, is say there is a box and it has a circle subtracted out of it. I now want to use the inside of the circle for collision, rather than the outside.

Any obvious ways to do this or am I going to need to use instanceof in a lot of places?

#4

Sorry, couldn’t get that. Maybe a picture?

Kev

#5

Sure. If you take a look at the first image in this Wikipedia entry:

You can see that the curve along the inside of the cube is determined by the geometry of the sphere that subtracted from it. Normally, you would factor collision using the outside of the sphere, but once it is used as a subtracted shape you’re actually using its inside - basically it’s the opposite. I’ve messed around with Phys2D geometry a bit, obviously, and doing things like reversing the direction of the Polygons resulted in some very messed up collision. As such, I assume there’s something I need to change in order to collide with the inside of the sphere rather than the outside.

#6

Oh, right, you just want to get trapped inside a shape? If they’re polygons reversing the order of the vertices should do this - but only for polygons against polygons.

Kev

#7

Cool, I thought so.

I suppose I can make the CSG Body require Polygons in its binary tree, rather than generic Shapes. Then other people (or me, later) can do the math involved for calculating collision along the inside rather than the outside of the other Shapes.