Box2D Forums

At the moment I'm experimenting with character movement, using a rectangular body to represent a torso, and a 'feeler' extending below the body to check what the character is standing on. The feeler is a b2Segment, which is checked against objects in the world with the b2PolygonShape::TestSegment function.

Occasionally I noticed my character standing on nothing, and eventually narrowed down the problem to the case where the segment is perfectly parallel to one of the sides of the shape. Admittedly this is quite an unlikely occurence, except for at the beginning of the simulation when many objects still have their initial angle of a perfect zero.

To remedy this I added a third case to the if/else block in TestSegment as below. Perhaps there is a better way to address the problem, but it seems to be working well enough for me so far.

Before:

After:

That sounds like a valid robustness problem. I will look into this.

Thanks!

I think I was wrong about the case simply being "when the segment is parallel to one of the edges of the shape". I noticed that my little man could run along the falsely intersected ground quite happily until he got quite close to it, at which point TestSegment worked correctly again. Here is the original case where I noticed the problem:



Since the segment being parallel to the edge of the shape was what I noticed when stepping through the code, and changing it fixed the problem, I thought that was that. But later I found that when the character got near the shape, there was a point where the test worked ok. As you can see in the image below with little man ver.2 now sporting his new bevelled edge look and an extra 'leg', the leg segment on the right has correctly missed the shape, while the leg on the left has incorrectly intersected with it.



I can't figure out why this is, so considering it might be a harder thing to reproduce than I thought, here's the reproduction:

I'm noticing some slightly errant behavior in testSegment as well, when shapes and lines are perfectly perpendicularly aligned or something like that. I would have an intersection with a shape that's not nearby. It would be easy to work around this in my case since I could just apply a miniscule torque or impulse on something, to get rid of the perfect alignment.

I committed a fix for this to SVN. Let me know if you have any more trouble with it.

I updated my code to SVN 171. I can confirm that you have fixed this particular testSegment issue. I also notice that the solver and overall physics behavior seems even more robust. As for the change in the b2World::Step function, what should the iterations values be set to? Should there be a preference for different values?

dc443: The manual on SVN discusses this. You can often get good results with only one position iteration. The velocity iteration is the same as the old iteration count.

Oh i see. You hadn't updated the manual on the main box2d site, and that was the one I checked first.

Another quick question, may sound dumb, but does the iterations value affect performance/behavior when joints are not being used?

The iterations are also used for solving contacts and friction.

The site manual is synced with release versions, not SVN.

Works great! Thanks Erin.