cocos2d for iPhone

A fast, easy to use, free, and community supported 2D game engine

1. 

   Solitude
   Member

   Hi everyone,

   I have a problem which requires me to detect whether a certain points is inside a triangle. Does cocos2d have certain method to help with this?

   I've been thinking how to do a workaround with point in a rectangle detection function, but I can't since the 4 points registered will most likely won't have a shape of a parallelogram (It'll more likely form a shape of striped triangles with 2 of the points is squeezed together).

   If you are interested to know what my problem is. It's something like this:
   - I have a certain point at the center of the screen.
   - The player will draw random curve on the screen.
   - I'll have to detect whether there's any sprite is inside the shape formed by the certain point, the point at the tip of the curve, and the point at the end of the curve.

   So far my idea is to subdivide the points on the curve into a few line segments. From these line segments I can form triangles with the certain point, and I could just do 4 times checks on each sprite's corners whether any of these 4 points is inside the triangles.

   So, to reiterate my question, is there any method in cocos2d to do something like this?

   Posted 1 year ago #
2. 

   Benihana
   Member

   Did you look at something like:
   http://www.blackpawn.com/texts/pointinpoly/default.html

   And in case you need a refresher on cross products (code at the bottom even):
   http://www.lighthouse3d.com/opengl/maths/

   Posted 1 year ago #
3. 

   GameBit
   Member

   CGFloat GBDot(const CGPoint v1, const CGPoint v2) {
   	return v1.x*v2.x + v1.y*v2.y;
   }
   
   CGPoint GBSub(const CGPoint v1, const CGPoint v2) {
   	return CGPointMake(v1.x - v2.x, v1.y - v2.y);
   }
   
   BOOL GBPointInTriangle( CGPoint point, CGPoint pointA, CGPoint pointB, CGPoint pointC ) {
   	//http://www.blackpawn.com/texts/pointinpoly/default.html
   
   	CGPoint v0 = GBSub(pointC, pointA);
   	CGPoint v1 = GBSub(pointB, pointA);
   	CGPoint v2 = GBSub(point, pointA);
   
   	// Compute dot products
   	CGFloat dot00 = GBDot(v0, v0);
   	CGFloat dot01 = GBDot(v0, v1);
   	CGFloat dot02 = GBDot(v0, v2);
   	CGFloat dot11 = GBDot(v1, v1);
   	CGFloat dot12 = GBDot(v1, v2);
   
   	// Compute barycentric coordinates
   	CGFloat invDenom = 1 / (dot00 * dot11 - dot01 * dot01);
   	CGFloat u = (dot11 * dot02 - dot01 * dot12) * invDenom;
   	CGFloat v = (dot00 * dot12 - dot01 * dot02) * invDenom;
   
   	// Check if point is in triangle
   	return (u > 0) && (v > 0) && (u + v < 1);
   }

   P.S. i believe you can replace GBDot with ccDot and GBSub with ccSub

   Posted 1 year ago #
4. 

   Solitude
   Member

   Thanks for the input, both of you.

   @benihana: I actually was thinking something like that, but may that method may incur some inaccuracy and probably more costly than the method given by GameBit

   @GameBit: Thanks. I'll look at the source.

   Posted 1 year ago #

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