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Bodies bounce on corners of static geometry and other strangeness

Topics: User Forum
Mar 20, 2009 at 5:47 PM
To reproduce, create a static rectangular body. Create some circle bodies (32 segments in my case). Let the circle bodies drop onto the rectangular body and watch as they slide over it's edges. They bounce upwards slightly, and sometimes even rebound.

Can anyone explain this to me and (hopefully) tell me what I can do about it?

Also, I've played with the collisionGridCellSize parameter and doesn't seem to affect the corner bounce issue. It does however seem to reduce the accuracy of my geometries. If I build a large static geometry with lots of vertices (~100 and many of them have 90 degree edges) and set the grid size quite large (or leave it as zero), the circle bodies will slide over the corners as if it my verts had been optimized out and it was just a slope. Reducing the grid size seems to nicely fix this issue however the performance is impacted quite badly.

Maybe I need to split the large static geometry into smaller ones and use some sort of broad phase detection? Can somebody point me towards a good source of information about the tradeoffs of the grid size and how to utilise the broad phase routines?

As you would've guessed by now, I'm quite a Farseer noob. Feel free to tell me to RTFM, but please include a link to said manual  :)

P.S, what a cool name for the engine. I assume it's a reference to Robin Hobb's books.

Mar 21, 2009 at 9:46 AM
Edited Mar 21, 2009 at 9:49 AM
The collision grid size determines the accuracy of the collisions. If it works better when you use a low collision grid size, then that's probably the cause. It is indeed a good thing to use small geometries, they have several benefits, one of them being that the geometries are not included that often in the broad phase collision system. You can read about geometry chunking in our manual (It's right here:

I actually don't know where the name originated from. I've been wondering the same thing. Maybe it's time to ask Jeff.

Edit: Would have created a link, but someone stole my link button :)
Mar 21, 2009 at 4:01 PM
Thankyou for the helpful reply - the URL did not diminish it. I must be blind to have missed that documentation link on the home page  :o

I'll definitely have to play with the grid cell size. Perhaps trying to get the grid to line up with the vertices is worth a try for the bouncy corner issue... time to play.