Box2d shape confusion

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#1

Hello there,
I have set up a box2d body with the following shape:

	b2Vec2 verticies[7];
	verticies[0] = b2Vec2(0, 0);
	verticies[1] = b2Vec2(1, 1);
	verticies[2] = b2Vec2(2, 1);
	verticies[3] = b2Vec2(3, 3);
	verticies[4] = b2Vec2(4, 1);
	verticies[5] = b2Vec2(5, 1);
	verticies[6] = b2Vec2(6, 0);

	b2PolygonShape ps;
	ps.Set(verticies, 7);

I expected a shape like this(#paint :slight_smile: ):

paintskills123.png1152Ă—648 2.59 KB

But I get this:

There probably is a pretty simple and logical explanation for this and I would love to know that one. Could anybody please assist me. Thanks! Much appreciated! Btw. I have tried to do some research before asking

#2

Where did you get that result image from? A cocos2d-x project or a box2d editor? Maybe it’s just not renedered properly, you code seems fine, except the fact that you are creating a non-convex shape which is a problem; box2d doesn’t work well with concave shapes, but it should have drawn it right (though that may depend on your box2d version).

#3

Hi! First of all, thank you so much for your reply!! I use cocos2d-x 3.7 and the result image is from a cocos2d-x project using the box2d version that follows along. I get what your saying but still cannot figure out what exactly to do?

#4

Here is a little article about convex and concave shapes. Also, you might want to use the b2Vec2::Set function to set your vertices like this: verticies[0].Set(0, 0); etc.

And one more thing: the vertices order should be provided counter-clockwise, your vertices are in a clockwise order right now.

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#5

Points (2, 1) and (4, 1) are removed as they are not in the convex hull.

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#6

But I do need those points in order to achieve the shape i want correct? So it is not possible to create such a shape or? and @Boby thanks, that did help me a little but still not enough so that i can create the shape i want…

#7

You have to split your concave polygon into multiple convex polygons.

#8

Well true, thanks buddy! But doesn’t that take up more memory than when only have one polygon or? (1 < 2+ so probably yes but more like how)

#9

It is linear, i.e. two polygons will consume twice the amount of memory of one polygon.

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#10

guys thanks for it all. it helped me sorting it out but i still do not get why box2d just doesn’t support concave shapes… wouldn’t that make it all easier?

#11

Well I believe math problems :stuck_out_tongue: