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After a long time, I try to use JAVA again. I'm using the vectmath package, with which I'd like to rotate a 3d vector with a rotation matrix. So I wrote that:

   double x=2, y=0.12;
   Matrix3d rotMat = new Matrix3d(1,0,0, 0,1,0, 0,0,1); //identity matrix
   rotMat.rotX(x); //rotation on X axis
   rotMat.rotY(y); // rotation on Y axis
   System.out.println("rot :\n" + rotMat); // diagonal shouldn't have 1 value on it

result:

rot :
0.9928086358538663, 0.0, 0.11971220728891936
0.0, 1.0, 0.0
-0.11971220728891936, 0.0, 0.9928086358538663

Unfortunately, it doesn't give me what I expected. It is like he ignored the first rotation (around X) and only take the second one (around Y). If I comment the rotMat.rotX(x); it gives me the same result.

I suspect either a mistake with the print, or with variable management.

Thanks

Rigel
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1 Answers1

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Methods rotX and rotY set/overwrite the matrix elements, so your subsequent call to rotY(y) cancels the call to rotX(x). Something like this should work:

Vector3d vector = ... //a vector to be transformed
double x=2, y=0.12;
Matrix3d rotMat = new Matrix3d(1,0,0, 0,1,0, 0,0,1); //identity matrix
rotMat.rotX(x); //rotation on X axis
rotMat.transform(vector);
rotMat.rotY(y); // rotation on Y axis
rotMat.transform(vector);
// the vector should now have both x and y rotation
// transformations applied
System.out.println("Rotated vector :\n" + vector);
JanneK
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  • Thanks! Now, I understand my error : I thought that the rotX will rotate the existing matrix. Instead it will replace it by the rotation matrix X(angle). Whatever, the ratMat need to be initialized before using this method. – Rigel Aug 05 '15 at 11:07