Using The Box2D Polygon "Set()" function?

Question

I'm using the Box2D physics engine. And there's a type of shape for box2D called b2PolygonShape.

In this class, you can create polygons. There is also a Set() function that takes an array of points and a vertex count.

Box2D has an example like this:

// This defines a triangle in CCW order.

b2Vec2 vertices[3];

vertices[0].Set(0.0f, 0.0f);

vertices[1].Set(1.0f, 0.0f);

vertices[2].Set(0.0f, 1.0f);

int32 count = 3;



b2PolygonShape polygon;

polygon.Set(vertices, count);

This works. Yet when I tried to practice and mess with this function, I did this:

  b2Vec2 vertices[4];

  vertices[0].Set(0, 0);
  vertices[1].Set(0,10);
  vertices[2].Set(10,10);
  vertices[3].Set(10,0);

  int32 count = 4;

  b2PolygonShape polygon;

  polygon.Set(vertices, count);

When I compiled and ran this, it crashed after the Set() function. Wouldn't this create a square?

Also in the console I got this:

Assertion failed: s > 0.0f

What did I do wrong?

Answer 1

That's because the four vertices you provided are in clockwise order and Box2D assumes they're in counter-clockwise, like in your first example. So it should work fine if you change it to:

  b2Vec2 vertices[4];

  vertices[0].Set(0, 0);
  vertices[1].Set(10,0);
  vertices[2].Set(10,10);
  vertices[3].Set(0,10);

  int32 count = 4;

  b2PolygonShape polygon;

  polygon.Set(vertices, count);

And by the way, if you're just creating a box-shaped polygon, you can use the convenience function

void b2PolygonShape::SetAsBox(float32 hx, float32 hy, const b2Vec2& center, float32 angle)

Answer 2

You could determine the winding of the polygon, and then reverse it if it is anti-clockwise.

One common method is to calculate the area of the polygon using the algorithm below. It will return a +ve number for clockwise winding, and a -ve number for anticlockwise winding.

Here is a Python example:

def poly_area(verts):
    """
    Return area of a simple (ie. non-self-intersecting) polygon.
    Will be negative for counterclockwise winding.
    """
    accum = 0.0
    for i in range(len(verts)):
        j = (i + 1) % len(verts)
        accum += verts[j][0] * verts[i][1] - verts[i][0] * verts[j][1]
    return accum / 2