main/src/georegression/metric/Area2D_F64.java
public static double quadrilateral( Quadrilateral_F64 quad ) {
		double bx = quad.b.x-quad.a.x;
		double by = quad.b.y-quad.a.y;
		double cx = quad.c.x-quad.a.x;
		double cy = quad.c.y-quad.a.y;
		double dx = quad.d.x-quad.a.x;
		double dy = quad.d.y-quad.a.y;

		if( (bx * cy - by * cx >= 0) == (cx * dy - cy * dx >= 0)) {
			return triangle(quad.a,quad.b,quad.c) + triangle(quad.a,quad.c,quad.d);
		} else {
			return triangle(quad.a,quad.b,quad.d) + triangle(quad.b,quad.c,quad.d);
		}
	}
public static double quadrilateral( Quadrilateral_F64 quad ) {
		double bx = quad.b.x-quad.a.x;
		double by = quad.b.y-quad.a.y;
		double cx = quad.c.x-quad.a.x;
		double cy = quad.c.y-quad.a.y;
		double dx = quad.d.x-quad.a.x;
		double dy = quad.d.y-quad.a.y;

		if( (bx * cy - by * cx >= 0) == (cx * dy - cy * dx >= 0)) {
			return triangle(quad.a,quad.b,quad.c) + triangle(quad.a,quad.c,quad.d);
		} else {
			return triangle(quad.a,quad.b,quad.d) + triangle(quad.b,quad.c,quad.d);
		}
	}
Area of a quadrilateral computed from two triangles.

@param quad quadrilateral
@return area