Irrlicht Engine

How do Opengl coordinate become irrlicht coordinate?

Opengl matrix is rotation, and Opengl rotation matrix is 4*4.

How do opengl matrix become irrlicht rotation "vector"?

"openGL matrix -> irrlicht matrix -> rotation vecter"

Is it right? How do opengl matrix become irrlicht matrix?

Irrlicht also has a 4x4 matrix. Check matrix4.h, method getRotationDegrees on how to get the values back into Euler angles.