ax_deimos

11-18-2001, 10:41 PM

Why is it that when I use glRotation I fail to get a rotation around a specified axis?

I include the following example code

//A structure that represents either a point

//or a vector in x,y,z

struct xyz { double p[3];}

//A structure that represents a set of 4

//vectors

struct xyzVB

{ xyz vect[4]; //vect[0]=Vector from parent

//vect[1]-vect[0] = unit x vector

//vect[2]-vect[0] = unit y vector

//vect[3]-vect[0] = unit z vector

};

xyzVB CoordSys1 = {0,0,0,

1,0,0,

0,1,0,

0,0,1};

const int SELECT_X_AXIS = 1;

const int SELECT_Y_AXIS = 2;

const int SELECT_Z_AXIS = 3;

//This is used to select the current axis

int AXIS = SELECT_Y_AXIS;

xyzVB CoordSys2 = {0,0,0,

(Arbitrary set of

Orthogonal unit vectors)};

xyz a = CoordSys1.vect[AXIS];

// A unit vector

xyz b = CoordSys2.vect[AXIS];

// A unit vector

//Note that DotProduct(a, b) is not zero

//'c' is the unit normal of 'a x b'

//It serves as the axis of rotation

xyz c = CrossProduct(a,b);

//The angle between vectors 'a' and 'b' is

//the arc_cos (in degrees) of the dot

//product of 'a' & 'b'

//I know it will only get CCW angles ranging

//from 0 to 180 degrees

//* note: DotProduct(a,b) != 0)

double ang = ArcCos_degrees(DotProduct(a,b));

glRotate(angle, c.p[0], c.p[1], c.p[2]);

//I use a screen 480,480 in size

glDrawWireSphere(22,8,8);

Using this code I can rotate about any axis other than the one specified by the variable AXIS. Why is this so?

email me a ax_deimos@yahoo.com

I include the following example code

//A structure that represents either a point

//or a vector in x,y,z

struct xyz { double p[3];}

//A structure that represents a set of 4

//vectors

struct xyzVB

{ xyz vect[4]; //vect[0]=Vector from parent

//vect[1]-vect[0] = unit x vector

//vect[2]-vect[0] = unit y vector

//vect[3]-vect[0] = unit z vector

};

xyzVB CoordSys1 = {0,0,0,

1,0,0,

0,1,0,

0,0,1};

const int SELECT_X_AXIS = 1;

const int SELECT_Y_AXIS = 2;

const int SELECT_Z_AXIS = 3;

//This is used to select the current axis

int AXIS = SELECT_Y_AXIS;

xyzVB CoordSys2 = {0,0,0,

(Arbitrary set of

Orthogonal unit vectors)};

xyz a = CoordSys1.vect[AXIS];

// A unit vector

xyz b = CoordSys2.vect[AXIS];

// A unit vector

//Note that DotProduct(a, b) is not zero

//'c' is the unit normal of 'a x b'

//It serves as the axis of rotation

xyz c = CrossProduct(a,b);

//The angle between vectors 'a' and 'b' is

//the arc_cos (in degrees) of the dot

//product of 'a' & 'b'

//I know it will only get CCW angles ranging

//from 0 to 180 degrees

//* note: DotProduct(a,b) != 0)

double ang = ArcCos_degrees(DotProduct(a,b));

glRotate(angle, c.p[0], c.p[1], c.p[2]);

//I use a screen 480,480 in size

glDrawWireSphere(22,8,8);

Using this code I can rotate about any axis other than the one specified by the variable AXIS. Why is this so?

email me a ax_deimos@yahoo.com