[whynotme]

Hi,

I want to covert between global and local coordinates when rotating 3D object because some program only allow to rotate by global or local coordinate.





Example in this image, if I rotate Y by global coordinate (black coordinate) then the red object will rotate around black y axis. But I want to rotate it around y' axis, the local axis of the object. How can I rotate object around local axis using global rotation?

And how can I rotate around global axis using local axis?

Thanks

This post has been edited by whynotme: 28 January 2012 - 03:34 PM

[Xittenn]

. . . . edited out, DrRocket's answer(s) is/are much more comprehensible . . . . .

This post has been edited by Xittenn: 29 January 2012 - 12:27 AM

[DrRocket]

whynotme, on 28 January 2012 - 03:24 PM, said:

Hi,

I want to covert between global and local coordinates when rotating 3D object because some program only allow to rotate by global or local coordinate.





Example in this image, if I rotate Y by global coordinate (black coordinate) then the red object will rotate around black y axis. But I want to rotate it around y' axis, the local axis of the object. How can I rotate object around local axis using global rotation?

First apply a rotation transformation to rotate your black coordinates to the red coordinates. Then perform your desired rotation around the red coordinates -- this is just the composition of two transformations and hence can be viewed as a single transformation with respect to the black coordinates.

whynotme said:

And how can I rotate around global axis using local axis?

Thanks

Same idea as above but in reverse order.

[whynotme]

Thanks, but I still don't fully understand

I am not good at maths, especially at 3D transform. I am working with Flash, it is a 3D environment but has only 3 rotation tools: around fixed x, y, and z axes.
Now I have a cube at a random position and inclined at random angle. And I want to rotate the cube around its edge, but I only have rotation around global x, y, z axes, how can I rotate the cube around its edge - a random axis?

And there is another problem, I don't know why and how to solve:
When I apply the rotation, ie rotationX(30) then rotationY(50), rotationZ(70) - the angle is just for example.
Then I reverse the rotation, rotationX(-30), rotationY(-50), rotationZ(-70), the angles are exactly like the previous step, just negative, but the cube didn't come back to the beginning, it stopped at a random position. What is this problem and how to solve it?

Regards.

This post has been edited by whynotme: 30 January 2012 - 04:34 PM

[DrRocket]

whynotme, on 30 January 2012 - 04:31 PM, said:

Thanks, but I still don't fully understand

I am not good at maths, especially at 3D transform. I am working with Flash, it is a 3D environment but has only 3 rotation tools: around fixed x, y, and z axes.
Now I have a cube at a random position and inclined at random angle. And I want to rotate the cube around its edge, but I only have rotation around global x, y, z axes, how can I rotate the cube around its edge - a random axis?

And there is another problem, I don't know why and how to solve:
When I apply the rotation, ie rotationX(30) then rotationY(50), rotationZ(70) - the angle is just for example.
Then I reverse the rotation, rotationX(-30), rotationY(-50), rotationZ(-70), the angles are exactly like the previous step, just negative, but the cube didn't come back to the beginning, it stopped at a random position. What is this problem and how to solve it?

Regards.

Rotation is not commutative. To get back where you started you need to apply the inverse rotations in the reverse order.