View Full Version : OpenGL rotation concatenation

December 22nd, 2013, 11:09 PM
I got an IcoSphere at the center of the canvas, it rotates "down" (-Y) when dragging with the mouse down, "up" when dragging up, and left and right respectively -X and X.
Soon after playing with it (dragging in random directions) it starts rotating in the opposite direction ("up" when I drag it "down") it's supposed to.

Any idea what this issue is called or how to fix it?

I'm using 4x4 matrices for X and Y rotations which I multiply into the final rotation:

about_x += mgl::Rad() * state_->mouse_motion()->delta_y * kAccel;
about_y += mgl::Rad() * state_->mouse_motion()->delta_x * kAccel;

mat_mv.i(); // i() = set to identity
mat_rot.rot_y(about_y); // Rotation about Y
mat_mv *= mat_rot;

mat_rot.rot_x(about_x); // Rotation about X
mat_mv *= mat_rot; // concatenate both rotations

mat_mv3 = mat_mv.to_mat3();

mat_transl.transl(0.0f, 0.0f, frame->zn - 12.0f);
mat_mv *= mat_transl;

mat_mvp = frame->mat_persp * mat_mv;
// send "map_mvp" to the shader

December 25th, 2013, 06:27 PM
May I ask what is the Rad function in your code?

December 26th, 2013, 03:52 AM
It returns a number which when multiplied by degrees yields radians.

inline float
Rad() { return mgl::kPi / 180.0f; }

It looks like making an object rotate only vertically and horizontally no matter how much rotated it already is - is a difficult task.

December 28th, 2013, 05:49 AM
how about
state_->mouse_motion()->delta_y ?

I guess if delta_y is the distance a mouse moved, then it should be radian already.

December 28th, 2013, 04:37 PM
The movement is in points (Qt5 mouse move event) so treating them directly as radians would yield too quick rotations, but the speed of rotation isn't the problem.
The problem is about the direction of the rotation. For example, in Google Earth the Earth is rotated vertically/horizontally (about the x/y axes) compared to you (the viewer), but my code rotates the object vertically/horizontally compared it its own objects space (which is to be expected), that's why in my code after rotating the object about some of its x/y axis the rotations don't correspond any longer to the x/y axes of the world (user/watcher/me) view.

Here's a youtube demonstration, first 3 mouse drags rotate the object properly (around viewer's X/Y axes).
http://www.youtube.com/watch?v=cPYqOgLP4Vc (http://www.youtube.com/watch?v=cPYqOgLP4Vc&feature=youtu.be)