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Chipmunk Physics • View topic - Polygon collision

Polygon collision

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Polygon collision

Postby derula » Mon Feb 03, 2014 3:16 pm

Hi!

First of all, cool free physics engine, I like it so far and find it quite easy to handle with the Ruby integrations and Gosu.

I feel like this is a dumb question and has probably been asked by millions of noobs already, but I couldn't find anything about it anywhere.

So I made a static poly ramp and a circle to roll up and down the ramp, and it worked great. But if I make the circle a cube, it stops working. Apparently, the cube wants to collide with the ramp's inside edges rather than its outside edges.

I've tried a couple things, like turning off friction and not giving the cube infinite moment of inertia, but nothing worked. I've tried changing the winding but then it just crashes.

Any ideas?
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Re: Polygon collision

Postby slembcke » Mon Feb 03, 2014 7:10 pm

That's... very weird.

Do you know what version of Chipmunk it is? Off the top of my head, try reversing the polygon vertex order. You are supposed to do them with a certain winding direction, and depending on how Chipmunk is compiled there may or may not be an assertion if you do it wrong. I forget exactly what happens in older versions when the winding direction is wrong, but inside out polygons sounds like a reasonable way for it to break.
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Re: Polygon collision

Postby derula » Tue Feb 04, 2014 5:14 pm

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Re: Polygon collision

Postby slembcke » Wed Feb 05, 2014 10:56 am

O_o I have no idea then. A video might help I guess?

I feel like the only time I've ever seen inside out polygons happen is when I was developing the collision detection code and debugging it. One thing that can cause extremely weird collisions is if the moment of inertia of the body is off by a lot. If you set it to infinity it will not allow the object to rotate, but might shed some light on what is going on with the collisions.
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Re: Polygon collision

Postby derula » Wed Feb 05, 2014 3:00 pm

Huh. Well I did have a moment of infinity, so I changed it back to normal just to make sure and yeah it's pretty much the same, except then it turns. I turned off my collision handler, no change. There is no collision detected until the bottom of the square touches the bottom of the ramp, and then it's trapped in there. Hitting my jump button which applies an (upwards) impulse makes it teleport down by like 10 pixels. I can control it with the arrow keys (by manually changing the speed), and in the center of the screen where the two ramps meet, it moves slower (probably only moves there because I'm forcing speed on it without checking for collisions) as it inches through the wall between the ramps.
I tried to record a video, but it failed, I guess partly due to my low-end laptop.

I tried to make a simple testcase, but then nothing worked and after the second step, my body's y coordinate became NaN and stayed there.

...
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Re: Polygon collision

Postby dooxe » Wed May 13, 2015 1:43 am

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Re: Polygon collision

Postby derula » Wed May 13, 2015 4:46 am

This isn't the problem I was having... here's a mockup of what it would look like for me: http://imgur.com/kdF5oi2

Basically, the "ground" did act as a kind of "ground", however instead of standing on it, objects ended up hanging from it. So I guess it was more the "ceiling" type of ground.

(But only for other polygon bodies, IIRC.)
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Re: Polygon collision

Postby dooxe » Wed May 13, 2015 6:21 am

Oh ok I see ! Sorry about that.
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