Question

I am launching a projectile around a sphere. My code moves it in a counterclockwise direction just fine. However, I would like it to move in a clockwise direction instead.

I’m guessing that it’s a matter of tuning my math.

// these are my stepping and incrementing variables

int goose1_egg1_step = 1;
int &r_goose1_egg1_step = goose1_egg1_step;
float goose1_egg1_divider = 17500;

// the starting theta/phi values are: 5 and 5

int goose1_egg1_theta=5;
int goose1_egg1_phi=5;

// the ending theta/phi values are: 7 and 1
// there is a difference of 2 between the start and end theta values
// there is a difference of 4 between the start and end phi values

float goose1_egg1_theta_increment = 2/goose1_egg1_divider;
float goose1_egg1_phi_increment = 4/goose1_egg1_divider;

This is my function that displays the updated coordinates each frame with a sphere:

if (goose1_egg1_step < goose1_egg1_divider)
{
    float goose1_egg1_theta_math = (goose1_egg1_theta+(goose1_egg1_theta_increment* r_goose1_egg1_step))/10.0*M_PI;
    float goose1_egg1_phi_math = (goose1_egg1_phi-(goose1_egg1_phi_increment* r_goose1_egg1_step))/10.0*2*M_PI;
    r_goose1_egg1_x = Radius * sin(goose1_egg1_theta_math) * cos(goose1_egg1_phi_math);
    r_goose1_egg1_y = Radius * sin(goose1_egg1_theta_math) * sin(goose1_egg1_phi_math);
    r_goose1_egg1_z = Radius * cos(goose1_egg1_theta_math); 

    glPushMatrix();
    glTranslatef(r_goose1_egg1_x,r_goose1_egg1_y,r_goose1_egg1_z);
    glColor3f (1.0, 0.0, 0.0);
    glutSolidSphere (0.075,5,5);
    glEnd();
    glPopMatrix();
}

And here is how I increment the step value:

if (r_goose1_egg1_step < goose1_egg1_divider)       
{
    ++(r_goose1_egg1_step);
}    
else
    r_goose1_egg1_step=1;

Answer 1

Even though you are talking about “clockwise motion” in a sphere, when it only makes sense to me in a plane, it seems to me that what you want could be done just by changing the signals in the two lines where you create goose1_egg1_theta_math and goose1_egg1_phi_math, like this:

    float goose1_egg1_theta_math = (goose1_egg1_theta-(goose1_egg1_theta_increment* r_goose1_egg1_step))/10.0*M_PI;
    float goose1_egg1_phi_math = (goose1_egg1_phi+(goose1_egg1_phi_increment* r_goose1_egg1_step))/10.0*2*M_PI;

This should reverse the way you increment you spherical coordinates, giving you the “counter-clockwise” motion you’re looking for.