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Get the directional rotation angle between 2 vectors

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  1. struct Vector
  2. {
  3.     double x, y;
  4. };
  5.  
  6.  
  7. double getRotateAngle(Vector vec1, Vector vec2)
  8. {
  9. const double epsilon = 1.0e-6;
  10. const double PI = acos(-1.0); // 180 degree
  11. double angle = 0;
  12.  
  13. // normalize
  14. Vector norVec1, norVec2;
  15. norVec1.x = vec1.x / sqrt(pow(vec1.x, 2) + pow(vec1.y, 2));
  16. norVec1.y = vec1.y / sqrt(pow(vec1.x, 2) + pow(vec1.y, 2));
  17. norVec2.x = vec2.x / sqrt(pow(vec2.x, 2) + pow(vec2.y, 2));
  18. norVec2.y = vec2.y / sqrt(pow(vec2.x, 2) + pow(vec2.y, 2));
  19.  
  20. // dot product
  21. double dotProd = (norVec1.x * norVec2.x) + (norVec1.y * norVec2.y);
  22. if ( abs(dotProd - 1.0) <= epsilon )
  23. 	angle = 0;
  24. else if ( abs(dotProd + 1.0) <= epsilon )
  25. 	angle = PI;
  26. else {
  27. 	double cross = 0;
  28. 	angle = acos(dotProd);
  29. 	//cross product (clockwise or counter-clockwise)
  30. 	cross = (norVec1.x * norVec2.y) - (norVec2.x * norVec1.y);
  31.  
  32. 	if (cross < 0) // vec1 rotate clockwise to vec2
  33. 			angle = 2 * PI - angle;
  34. 	}
  35.  
  36. 	return angle*(180 / PI);
  37.  
  38. }

URL: http://www.cnblogs.com/lsmdiao0812/archive/2010/05/05/1728122.html

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