# Qt 3D Camera projection matrix

• How is the projection matrix property of Qt 3D `(Q)Camera` defined? I am a complete newbie and have been trying to do some background reading as the Qt 3D docs are pretty sparse, but the only references to camera projection matrices that I have found have defined them as 3x4 matrices. This makes sense, as the mapping is from 3D homogeneous coordinates to a projection on the image plane in 2D homogeneous coordinates. However, the Qt property is a 4x4 matrix.

• @Bob64

Got confused by Vector4D and Matrix4x4. Sure, a quaternion is `a+ib+jc+kd`

Here is probably one of the best explanations you will find out there :)
It's Unity, but the 3D stuff is the same, but better check what your values look like.

• @Bob64

I think it's a quaternion in its matrix form (X,Y,Z + Angle).
https://doc.qt.io/qt-5/qquaternion.html

https://en.wikipedia.org/wiki/Quaternion

• @Pl45m4 thanks for the suggestion. A quaternion is effectively a 4D vector, so if there is any connection with the 4X4 projection matrix I suppose the columns might be quaternions. As I say, I am quite new to this stuff, but my impression is that quaternions tend to be used to represent rotations, and I am struggling to see how that might relate to a projection.

To rephrase my original question, I guess I am looking for some description of what the matrix in Qt means geometrically, and when one might need to modify its value, etc.

• @Bob64

Got confused by Vector4D and Matrix4x4. Sure, a quaternion is `a+ib+jc+kd`

Here is probably one of the best explanations you will find out there :)
It's Unity, but the 3D stuff is the same, but better check what your values look like.