Box2d custom shape or bodies joint
Let's say i want to create simple physics object with a shape of "matryoshka" or banal snowman . As i see it , i have two options to do it: 1. To create 2 circle (or may be custom) bodies and connect them with weld joint , or 2. To create one body with two circle (or may be custom) shapes in it. So the question is: what is more expensive for CPU: bodies connected with joints or complicate-shaped bodies. If i have one object may be i don't feel difference in performance , but what if i have many object of that type? I know that joints are expensive , but may be custom shaped bodies is more expensiver? I'm working with Box2dFlash.
Since the question is about CPU use, joints use more CPU than shapes alone with no joint. Circles shapes can be more efficient than polygons in many cases, but not all. For CPU optimization, use as few bodies and as simple polygons as possible. For every normal defined in a polygon, a calculation may need to be performed if an object overlaps with another. For circles, a maximum of one calculation is needed.
As an aside, unless you are already experiencing performance problems, you should not worry about if your shapes are the idea CPU use. Instead, you should ask whether the simulation the create is the one you want to happen. Box2D contains many many special case optimizations to make it run smoothly. You can also decrease its accuracy per tick by setting the velocity and position iteration variables. This will have a far greater effect on efficiency than geometry unless your geometry is extremely complex.
If you don't want the two sections of the snowman's body to move in relation to each other, then a single body is the way to go. You will ideally be defining the collision shape(s) manually anyway, so there is absolutely no gain to be had using a weld.
Think of it like this: If you use (a) weld(s), the collision shapes will be no less complicated than if you simply made an approximation of the collision geometry for a single body; it would still require either multiple collision shapes, or a single complex collision shape regardless of your choice. The weld will simply be adding an extra step.
If you need the snowman's body to be dynamic in anyway (break apart or flex) then a weld or regular joint is the way to go.