Opengl Model Matrix doesnt rotate around the origin after one rotation

Question

I want to rotate my model Matix in x, y, and z direction, but it rotates in an unexpected way. I use Qt.

QMatrix4x4 mMatrix;
mMatrix.setToIdentity();
mMatrix.rotate(yAngle, QVector3D(0, 1, 0));
mMatrix.rotate(zAngle, QVector3D(0, 0, 1));
mMatrix.translate(cube->getPosition());

After the first rotation the followed rotations rotate around the base of the new model Matrix, while I want the followed rotations to rotate around the origin.

I drew a little sketch so my problem might be clearer (black shows how it is right now, green is how i want it to be): The green arrow shows how i want it to be.

Answer 1

I think you will get the desired result if you simply reverse the order of calls you make on the QMatrix4x4 class. To combine the two rotations:

QMatrix4x4 mMatrix;
mMatrix.setToIdentity();
mMatrix.rotate(zAngle, QVector3D(0, 0, 1));
mMatrix.rotate(yAngle, QVector3D(0, 1, 0));

The documentation is kind of lacking, but from looking at the QMatrix4x4 source code, I'm getting the impression that it applies matrix operations the way it's more or less standard with matrix libraries that are used for OpenGL, and the way the OpenGL fixed pipeline used to work.

This means that when you combine matrices, the new matrix is multiplied from the right. As a result, when the combined matrix is applied to vectors, the last specified sub-transformation is applied to the vectors first. Or putting it differently, you specify transformations in the reverse of the order you want them applied.

In your example, it you want to rotate around the y-axis first, then around the z-axis, you need to specify the z-rotation first, then the y-rotation.

Answer 2

I got figured it out by myself, each object needs to have its own rotation Matrix / Quaterunion. A new rotation around the world origin ist done by creating a rotation matrix with the wanted rotation and right multiply the exiting rotation matrix of the object.

Answer 3

Well, if you want the green arrow thing to happen, then it means that you want a rotation around the x-axis, 90 degrees in the negative direction (or 270 degrees in the positive direction).

To make things simple, think of it like this:

• Which of the axes there remains the same? x , right? Since x doesn't change, it looks like you're rotating around the x, and you are.
• Now, point your thumb towards the direction that positive x is directed at and casually close the rest of your fingers like a cylinder. The direction that those rest of your fingers curling at is the direction of the rotation.
• Since you want a rotation of 90 degrees in the opposite direction that those 4 fingers of yours are curling at, or 270 in the same direction... yeah.

(1, 0, 0) would be the direction vector pointing towards the positive x . While I don't know so much about the functions you're using, my guess is that the second call should be:

mMatrix.rotate(270, QVector3D(1, 0, 0));

Instead, or optionally -90 if it supports negative values.