How to rotate object in 3D space about its axis

gde23 · 24 Dec 2020, 13:58

Lets say you have your object in its object coordinate system which is somehow arranged in space (rotated translated).
Then to achieve a rotation of the object about its own local object coordinate system you need to first rotate the object (as long as it still in the origin xyz coordinate system) and then apply the rotation that positions it in space afterwards. So the order of the rotations / translations is what makes the difference.

SpyerGame · G gde23 24 Dec 2020, 12:35

@gde23 ur mean is after apply e.g. 'rotX = 25' then rotate the camera in x axis equal 25 degrees?
Can u make example for this prb?

gde23 · wrote on 24 Dec 2020, 15:02

I assume your object is made up of triangles or points or whatever and each of these points is defined in the local coordinate system of the object.

Now as long as you do not translate/rotate your object, the local coordinate system will be the same as the global coordinate system. In this position now when you just rotate about x the object will pitch if you rotate about y it will roll and z yaw. Therefore if you first apply the picht/roll/yaw rotation matrix and afterwards the transformation to the global coordinate system it will be the same as if you first positioned the object in global coordinate system and afterwards rotate about those new rotated axes. (Which however would be more complicated math)

SpyerGame · 24 Dec 2020, 18:09

Yes, exactly at the first time two coordinates are matching but must a way for any time and robust !

kshegunov · 24 Dec 2020, 18:28

The rotation property is a quaternion, which is rather hard to reason about directly. I'd suggest using a QTransform attached to your node instead.

SpyerGame · wrote on 24 Dec 2020, 18:28

@kshegunov Thanks, but I don't figure out completely!
Cause Qtransform is in C++ and there is not in qml and how to use it

gde23 · wrote on 24 Dec 2020, 18:38

It does not matter if you use quaternions or rotation/homgenious matrices for the rotations/translations as long as you keep the order.
The advantage of rotation matrices however is, that you can multiply them to get a single matrix that does all the rotations at once.

SpyerGame · wrote on 24 Dec 2020, 19:11

Are these methods useful for my prb?
@gde23 in Model scope for rotation.x: property, I put this?

SpyerGame · 26 Dec 2020, 17:57

I Can't to solve this, yet!!

kshegunov · wrote on 26 Dec 2020, 17:57

@gde23 said in How to rotate object in 3D space about its axis:

It does not matter if you use quaternions or rotation/homgenious matrices for the rotations/translations as long as you keep the order.

Not in principle, as it is just a matter of representation. Utility and convenience however can vary.

The advantage of rotation matrices however is, that you can multiply them to get a single matrix that does all the rotations at once.

Which is also true for quaternions. They have the same properties, being associative and obeying the distributive property, but do not form a commutative ring. But again, this is just the math, and not that helpful in this case.

@SpyerGame said in How to rotate object in 3D space about its axis:

Cause Qtransform is in C++ and there is not in qml and how to use it

I'm pretty sure there's a Transform QML element that's "related" to the QTransform class ... like this one ...

SpyerGame · wrote on 26 Dec 2020, 19:05

Thanks, yes it is, but I think use fromAxesAndAngles but I don't know how to work each vector3d of axes. In other words if each of a,b & c in Qt.vector3d(a,b,c) !
I know that Qt.vector3d(1,0,0) is X axis and so on but I don't know what is it Qt.vector3d(.54,.1,0) ?
My desired rotation around X,Y & Z of object like this: