```Date: Jan 17, 2013 1:34 AM
Author: Roger Stafford
Subject: Re: Incorrect rotation calculations

"Eugene" wrote in message <kd7vof\$gcu\$1@newscl01ah.mathworks.com>...> % rotation angle> a = degtorad(45.0);> %rotation matrix (around X axis)> MX = [1.0 0.0    0.0;>       0.0 cos(a) -sin(a);>       0.0 sin(a) cos(a)];> %source point coords> x = 0.5> y = 0.5> z = 0.7071> > % plot source point> plot3(x,y,z,':go');grid on;> hold on;> %plot radius-vector to source pt> line([0 x],[0 y],[0 z],'color','g');> > % rotation> new_pt = MX * [x; y; z]> > % plot destination point> x = new_pt(1,1); y = new_pt(2,1); z = new_pt(3,1);> plot3(x, y, z,':ro');> %plot radius-vector to destination pt> line([0 x],[0 y],[0 z],'color','r');> > I have source point [0.5 0.5 0.7071], after rotation- [0.5 -0.1464 0.8535]> But it is incorrect answer! > It seems to me y value can't be negative and z also has invalid value.> What's wrong in my solution?- - - - - - - - -  That rotation looks valid to me.  You have rotated the point counterclockwise from the y-axis toward the z-axis by 45 degrees.  The point (.5,.5,.7071) should end up at (.5,-.1464,.8535) just where you have it.  Why don't you think that is right?  The original point was more than 45 degrees from the y-axis, so after 45 degrees more it would rotate past the z-axis and produce a negative y.  Were you expecting a rotation in the opposite direction?  If so, your MX would be incorrect.Roger Stafford
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