NeHe OpenGL Lesson48-ArcBall Rotation (Mouse Rotation)

This sample provides us an ArcBall module that we cocould use to rotate object along the screen with mouse. the underneath idea is mapping a 2d screen position into a 3d sphere coordinate. A rotation Quat cocould be calculated out by two sphere coordinates (the start, end position ). then a rotation matrix created based on the quat.

This sample also provide some other useful technologies:

1) convert quat to a rotation matrix;

2) use OpenGL fucntion"GlMultMatrixf (Transform. M );"To set up the model view matrix directly;

 

Mapping 2D Mouse Position into 3D Sphere Coordinate

At first we simply scale down the mouse coordinates from the range of [0... width), [0... height) to [-1... 1], [1... -1] (Y flipped ).

X  =  (MousePt.X / ((Width  - 1) / 2));Y  = -(MousePt.Y / ((Height - 1) / 2));

Calculate the 3D Sphere coordinates:

length      = (X * X) + (Y * Y);//If the point is mapped outside of the sphere... (length > radius squared)if (length > 1.0f){    GLfloat norm;    //Compute a normalizing factor (radius / sqrt(length))    norm    = 1.0f / FuncSqrt(length);    //Return the "normalized" vector, a point on the sphere    NewVec->s.X = TempPt.s.X * norm;    NewVec->s.Y = TempPt.s.Y * norm;    NewVec->s.Z = 0.0f;}else    //Else it's on the inside{    //Return a vector to a point mapped inside the sphere sqrt(radius squared - length)    NewVec->s.X = TempPt.s.X;    NewVec->s.Y = TempPt.s.Y;    NewVec->s.Z = FuncSqrt(1.0f - length);}

 

The full source code cocould be found here.