# why point correspondence to rotation matrix calculation is not my expected?

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cui on 11 May 2022
Commented: cui on 10 Oct 2022
for example, at first ,one point P1(1,0,0) rotate pi/2 angle by y-axis,then get the point P2(0,0,-1), then rotate pi/2 angle by x-axis, then get the point P3(0,1,0). i use eul2rotm function to specify eular angle to get rotation matrix,then get the destination point by premultipy format,but get the result is not my expected P3?
P1 = [1,0,0];
eular = [0,pi/2,pi/2]
eular = 1×3
0 1.5708 1.5708
r = eul2rotm(eular,'ZYX'); % premultipy format
destinationPt = r*P1' % not same as P3 ?
destinationPt = 3×1
0.0000 0 -1.0000
if i take eular decompose into two component, ie rotate pi/2 angle correspondance to y-axis,x-axis, then i can get my expected result P3.
eularY = [0,pi/2,0];
eularX = [0,0,pi/2];
ry = eul2rotm(eularY);
rx = eul2rotm(eularX);
destiPt = rx*ry*P1' % is same as P3, which is my expect result.
destiPt = 3×1
0.0000 1.0000 -0.0000
i don't understand why eular angles vector three components not write together?

Matt J on 11 May 2022
Edited: Matt J on 11 May 2022
r needs to be pre-multiplied by P1, not post-multiplied.
P1 = [1,0,0];
eular = [0,pi/2,pi/2];
r = eul2rotm(eular,'ZYX');
destinationPt = P1*r
destinationPt = 1×3
0.0000 1.0000 0.0000
cui on 10 Oct 2022
It is good to hear that since R2022b, the computer vision toolbox and image process toolbox have started to support the use of the pre-multiply convention as a priority，while the default for other toolkboxs is the pre-multiply convention remains unchanged.

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