How to do rotation?

[AlexAzazel]

Suppose I have two points represented by 3D vectors

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core::vector3df vecCentral;
core::vector3df vecOrbital;

I want to rotate vecOrbital around vecCentral. How do I use rotation matrix

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core::matrix4 matix;

or Euler angles to do that?

[mongoose7]

You can't do it with a rotation matrix. You need a translation matrix to move vecCentral to the origin, then a rotation matrix to rotate the world, then the inverse of the translation matrix to move the origin to vecCentral. I'd be trying to find m.translate() and m.rotate() in the matrix class. Perhaps look in matrix4.h or check the documentation.

[AlexAzazel]

You can't do it with a rotation matrix. You need a translation matrix to move vecCentral to the origin, then a rotation matrix to rotate the world, then the inverse of the translation matrix to move the origin to vecCentral. I'd be trying to find m.translate() and m.rotate() in the matrix class. Perhaps look in matrix4.h or check the documentation.

So rotation matrix can only rotate points around origin of the coordinate system? Then I will have to do rotation myself.... WTF irrlicht?

[Kojack]

You can't do it with a rotation matrix. You need a translation matrix to move vecCentral to the origin, then a rotation matrix to rotate the world, then the inverse of the translation matrix to move the origin to vecCentral. I'd be trying to find m.translate() and m.rotate() in the matrix class. Perhaps look in matrix4.h or check the documentation.

So rotation matrix can only rotate points around origin of the coordinate system? Then I will have to do rotation myself.... WTF irrlicht?

It's not Irrlicht's fault, blame mathematicians from 160+ years ago. That's just how it works. A rotation matrix rotates around the origin, a scale matrix scales around the origin. But a translation matrix effectively moves the origin, and all of these can be combined together in a single matrix4.

Is vecOrbital a world position or an offset from vecCentral? If it's an offset then a pure rotation matrix is fine since. You rotate vecOrbital with the rotation matrix then add the result to vecCentral when setting the position of whatever you are rotation.
If vecOrbital is a world position, then you can use mongoose7's suggestion, build a matrix that combines a translate (to move the origin to vecCentral), a rotation, then a translate (to move the origin back to 0,0,0).

I'd probably do it using quaternions instead. For example (if vecOrbital is a world position):

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irr::core::quaternion q;
q.fromAngleAxis(rotationSpeed * frameDeltaTime, rotationAxis);
vecOrbital = vecCentral + q * (vecOrbital - vecCentral);
 

[AlexAzazel]

I'd probably do it using quaternions instead. For example (if vecOrbital is a world position):

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irr::core::quaternion q;
q.fromAngleAxis(rotationSpeed * frameDeltaTime, rotationAxis);
vecOrbital = vecCentral + q * (vecOrbital - vecCentral);
 

Thanks a lot! I'll try that too, for now I used complex number since I wanted to rotate only in 2 dimensions but I'll try that when I get to 3D rotation.