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# Matrix rotation by 90 degree about Y.

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Hi, I have a weird (for me) problem, i am trying to transform vector (1,0,0) using a matrix that was rotated 90 degree (which was converted to randians) at the Y axis, supostly the resulted vector should be (0,0,-1) but i am geting (2.6301,0,-1), When i try to rotate the same vector by 90.1 or 89.9 degree then i get correct transformation, something like (-0.001, 0, -0.099999), but with exact 90 degree i am getting too much difference. -------------------------------- Im am converting 90 degree to radians using pi/180 = 0.017453 then i do 90*0.017453 = 1.57077 radians; in my matrix rotation procedure i am calculating matrix_11 = cosine(1.57077) = 2.6301 which is X value that later i will get in my vector transformation when i do: new_x:=(x*matrix._11)+(y*matrix._21)+(z*matrix._31)+(matrix._41); The weird thing is that i have tryed in excel the follow formula values: cos(1.5703)=0.0004 cos(1.5704)=0.0003 cos(1.5705)=0.0002 cos(1.5706)=0.0001 cos(1.5707)=9.6327 (!!) //this is 90 degree, cos(1.5708)=-3.067E (!!)//this is 90.001 degree, cos(1.5709)=0.0001 So am i wrong thinking vector (1,0,0) rotated 90 deg at Y is (0,0,-1)? thanks

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Quote:
 supostly the resulted vector should be (0,0,-1) but i am geting (2.6301,0,-1)
Can you post the exact output of the program? It looks like you may be misreading numbers expressed in scientific notation.

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The numbers you're getting from Excel are clipped off. What is really there is some value followed by an 'E', then a negative number. These expressions represent numbers that are actually very close to zero: [google] "scientific notation".

Depending on how exactly you're investigating the supposedly erroneous numbers in your code, it may be the same thing there.

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in answer to the question of the full excel output (from my excel)

=cos(1.5707) = 9.63268E-05 = .0000963268
=cos(1.5708) = -3.67321E-06 = -.00000367321

=cos(1.5703) 0.000496327
=cos(1.5704) 0.000396327
=cos(1.5705) 0.000296327
=cos(1.5706) 0.000196327
=cos(1.5707) 9.63268E-05
=cos(1.5708) -3.67321E-06
=cos(1.5709) -0.000103673

to his specific value of 1.57077

=cos(1.57077) = 2.63268E-05 = .0000263268

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