bool isInside( float x, float y, float x1, float y1, float x2, float y2, float x3, float y3 ) {
float l1 = (x-x1)*(y3-y1) - (x3-x1)*(y-y1),
l2 = (x-x2)*(y1-y2) - (x1-x2)*(y-y2),
l3 = (x-x3)*(y2-y3) - (x2-x3)*(y-y3);
return (l1>0 && l2>0 && l3>0) || (l1<0 && l2<0 && l3<0);
}
It can not be more efficient than this! Each side of a triangle can have independent position and orientation, hence three calculations: l1, l2 and l3 are definitely needed involving 2 multiplications each. Once l1, l2 and l3 are known, result is just a few basic comparisons and boolean operations away.
