How to make an epllise collider ?

[JJJJJJJustin]

How do I create an ellipse collider in box2d?
I was recently using box2d and I created a standard circle collider for a circle, but once I changed the scale of the circle (to visually make it look more like an ellipse), I couldn't get realistic physics simulation for the ellipse because its collision mode was still based on the standard circle, and I couldn't figure out how to design an ellipse collider.
(from a novice programmer)

[Kira-PH]

You can't. Circles have the same radius all round and there is no Ellipse shape in Box2D.

The closest you could get would be a capsule, or a polygon - but be aware that the maximum number of polygon vertices is 8, so if you need it to be smoother than that, you'll need multiple polygons.

[JJJJJJJustin]

Thanks for your answer!
I have only tried using polygons to simulate ellipse collisions, and I did find the problem of the number of vertices being limited.
Multiple polygons might be a good suggestion, I will try it, and hope the accuracy is high enough.

[Kira-PH]

You can make a simple convex hull decomposer by adding a vertex in the centroid of a polygon with more than 8 points and then creating polygons by taking blocks of (up to) 7 vertices around the outside. The 2nd polygon's start edge is comprised of the centroid and the last vertex from the previous polygon.

Very roughly in pseudocode it would be something like this:

vertices[] v = { v1, v2, v3, ..., vx };

centroid = ComputeCentroid(v);
newHulls[] = { };
newHull = { centroid, v[0] };
p = 1
while (p < v.Length) {
  newHull.Add(v[p]);
  if (newHull.Length == 8) {
    newHulls.Add(newHull);
    newHull = { centroid, v[p] };
  }
  p++;
}
if (newHull.Length > 2)
  newHulls.Add(newHull);

Won't work for concave hulls.

[erincatto]

You can probably create something close to an ellipse using a rounded polygon shape. Maybe a diamond shape with a radius.

b2Vec2 points[8] = {
	{ 0.0f, -0.25f }, { 0.0f, 0.25f }, { 0.05f, 0.075f }, { -0.05f, 0.075f }, { 0.05f, -0.075f }, { -0.05f, -0.075f }, { 0.05f, 0.0f }, { -0.05f, 0.0f },
};
b2Hull diamondHull = b2ComputeHull( points, 8 );
b2Polygon poly = b2MakePolygon( &diamondHull, 0.3f );

[JJJJJJJustin]

Thanks !