Circle to Polygon Collision Detection (Solved)

Scott Jensen · 2009-09-10T19:28:47

Hi i hope this is the right place for this. First a detail about me i am utterly horribly lost when it comes to anything above basic algebra. So half of what i have for the collision does not make 100% sence to me and i am being host about that. Ok on to the point over the last .... 4 days i have piece together a Poly to Poly collision system. I wanted to add Circle to Poly so i could have stuff roll down hills and all that kind of neat stuff. right now my code is broken some where and i am not sure where as most of what i have read for tutorials or random snippets of code here and there have goten me to where i am now but i am stuck and at a total lose. Here is what i do understand. Using Sat ( Separating Axis theorem )I have been able to Do poly to poly collision detection and as i understand it takes the points of your poly and projects them onto an axis and if there is a separation on any axis there can be no collision. The concept should be some what the same with a Circle you need to project it on to the axis some how and well that is the part i think is busted and does not work right. Here is where i think my probably may lie in one or two of these here functions. //Some Vector Accessorie functions float VectorLength(Vector2 a) { float length_squared = (a.X * a.X) + (a.Y * a.Y); return (length_squared * length_squared); } Vector2 vectorPerp(Vector2 a) { Vector2 p = new Vector2(-a.Y, a.X); return p; } Vector2 vectorSub(Vector2 a, Vector2 b) { Vector2 c = new Vector2(a.X - b.X, a.Y - b.Y); return c; } This is where i think the problem is // project sphere along an axis, and find it's dimensions. void sphereInterval(Vector2 axis, Vector2 c, float r, ref float min, ref float max) { float length = VectorLength(axis); float cn = Vector2.Dot(axis, c); min = cn - (r + length); max = cn + (r + length); } Another possible spot could be here more likely the one above bool collidePolygonSphereAxis(Vector2 axis, Vector2[] a, Vector2 c, float r) { float mina = 0, maxa = 0; float minb = 0, maxb = 0; polygonInterval(axis, a, ref mina, ref maxa); sphereInterval(axis, c, r, ref minb, ref maxb); return (mina <= maxb && minb <= maxa); } The sphereInterval function is where i think the problem lies as for some reason regardless of the Circle location on the screen i always get a collision. I thank you for your time and for reading this rather boring and ranty post. Best Regards Jouei. [Edited by - Jouei on September 10, 2009 7:29:01 PM]

Math and Physics Programming

Started by Jouei September 09, 2009 03:53 PM

10 comments, last by Jouei 14 years, 7 months ago

oliii

2,202

September 10, 2009 06:03 PM

once yo have the closest point on the polygon, you can work out the MTD quite easily, and also the collision test.

If the distance squared is greater than the radius of the sphere squared, no collision, else....

bool collide(const Sphere& sphere, const Polygon& polygon, Vector& mtd){    Vector closestPoint;    float distance_squared = closestPointOnPolygon(sphere.position, sphere.radius, polygon.vertices, polygon.numvertices, closestPoint);    if(distance_squared > (sphere.radius * sphere.radius))        return false;    float distance = sqrt(distance_squared);    mtd = (sphere.position - closest) * ((sphere.radius - distance) / distance);    return true;}

a small demo.

I use Dave Eberly's triangle distance routine. It's more optimised than the code above, and returns a 's' and 't' values, the parametric representation of the point (that you can use to say, interpolate for texture coordinates and so on).

Everything is better with Metal.

Jouei

102

Author

September 10, 2009 07:28 PM

Well i got that working nicely now. I would like to thank you all for your input and taking the time to help this math gibbled me.

It seems everything is working as expected i still got a bit to go to improve my collision system but the hard part i believe is done and i am truly grateful for that :)

So thanks again and i will now marked this Post as solved when i get it all done i will probably post all the code and make a doc for it as it is for C# and XNA witch i seem to find is most commonly used by newer game programmers.

Best regards Jouei.

Circle to Polygon Collision Detection (Solved)

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Circle to Polygon Collision Detection (Solved)

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