Physics - smooth rolling on a half-pipe shape?

I'm starting to look into physics for the first time, and since I've ventured out of the familiar territory of platformers, I've reverted back to newb status. <img src="smileys/smiley36.gif" border="0" align="middle" />

The first thing I tried to do is make a half-pipe shape, like a bowl, and have a ball roll along the inside. However, in the example I posted below, it doesn't roll smoothly. Its collision mask is set to circle, so I assume the problem is with the half-pipe itself.

Speaking of which, I ignored the "You should only have 8 points in the collision polygon!" rule and bumped it up to... *cough*... 19, just to see if that would make it roll smoother. It sort of did, but it's still not quite there. I assume I shouldn't be adding even more points. <img src="smileys/smiley17.gif" border="0" align="middle" /> You'll also notice the ball kind of goes through the half-pipe on the right side, I have no idea why that happens either.

db.tt/HKn2g4sK

Any advice?

You could splice up the half pipe and redistribute it over several objects of 4 to 8 points max each. (avoid making it as animation frames of the same object, there's a bug preventing it (using only the collision mask of the first animation frame))

But to be honest, apart from the right-side bug, it looks pretty ok.

Agreed! I personaly think you should just ignore the litte "bumps" and claim it on the virtual material of the pipe and the ball ;-)

Yeah, just discovered that animation frame bug myself, haha. I just tried splitting it into two halves, each with 8 points, but it still has some issues. I'll try splitting it up a bit more.

The bumpiness is mostly taken care of, but the bug on the right side is still there. That sprite is just a flipped clone of the one on the left, why would this happen?

It appears that the bug is because the collision polygon for the halfpipe is concave. According to this the Box2d engine needs the collision polygons to be convex to function correctly.

Why dont you just use 1 invisible block (multiple copies) for detection ?

I used this method in quite a few projects....without a performance hit.

<img src="http://dl.dropbox.com/u/22173473/detector.png" border="0" />

The pyhsic engine seems to be quite buggy - as I found out and also reported. Depending on the shape of the collision polygone the enginge shows very "unpredictable" behaviour