use matlab to calculate euler angles from rotation matrix, and then calculate rotation matrix. Why is the input matrix different from the output one?

use matlab to calculate euler angles from rotation matrix, and then calculate rotation matrix. Why is the input matrix different from the output one?

[James Tursa]

Is this for a specific example? Can you post it?

[Walter Roberson]

And post your code.

Answer 1

[Walter Roberson]

http://matlab.wikia.com/wiki/FAQ#Why_is_0.3_-_0.2_-_0.1_.28or_similar.29_not_equal_to_zero.3F

[Yuanqing Chi]

It is for a specific example. The related code is as following. At first, I get a rotation matrix: rcurr; Then use the following code to calculate euler angles.

nPhi=acos(rcurr(3,3));
nphi1=atan2(rcurr(3,1),-rcurr(3,2));
nphi2=atan2(rcurr(1,3),rcurr(2,3));
euler(i,1:3)=[nphi1 nPhi nphi2]/pi*180;
for j=1:3
       if euler(i,j)<0
          euler(i,j)=euler(i,j)+360;
      elseif euler(i,j)>360
            euler(i,j)=euler(i,j)-360;
      end
    end
end

The output euler angles were stored for the next step. At next step, I use the following code to calculate rotation matrix from euler angles.

r=[cos(phi1)*cos(phi2)-sin(phi1)*sin(phi2)*cos(Phi) sin(phi1)*cos(phi2)+cos(phi1)*sin(phi2)*cos(Phi) sin(phi2)*sin(Phi);
    -cos(phi1)*sin(phi2)-sin(phi1)*cos(phi2)*cos(Phi) -sin(phi1)*sin(phi2)+cos(phi1)*cos(phi2)*cos(Phi) cos(phi2)*sin(Phi);
    sin(phi1)*sin(Phi) -cos(phi1)*sin(Phi) cos(Phi)];

while the output "r" is slightly different from the rotation matrix "rcurr" obtained in the previous step.

By the way, the difference only exists in my specific code. If I pick out the above code separately, and test the result, the input "rcurr" and output "r" are the same.

So I don't know what kind of problem can cause this thing.

[Jan]

Without formatting the code is nearly unreadable. Please post one command per line.

Answer 2

[James Tursa]

You can use this FEX submission by John Fuller to double check your code:

http://www.mathworks.com/matlabcentral/fileexchange/20696-function-to-convert-between-dcm--euler-angles--quaternions--and-euler-vectors