Question

I’m trying to find the proper vector for a direction of a spot light I have (I am trying to make it flashlight-like). I want it to face the same direction as the camera is always facing, like you’re holding a flashlight out in front of you.
I can’t seem to find the right directional vector, however.

At the moment, I have the light sort of following the front/back movement of the camera, but not the turn angle. In addition, the actual spot is pointed upwards, instead of directly ahead, like I want.

I know all the normals of the individual objects are working; I tested just general lighting already.

I’ll just post code that’s relevant to the question. If you really need my whole code, please say so.
Light code:

float sco=20;         //  Spot cutoff angle
float Exp=0;          //  Spot exponent

float Ambient[]   = {0.01*ambient ,0.01*ambient ,0.01*ambient ,1.0};
float Diffuse[]   = {0.01*diffuse ,0.01*diffuse ,0.01*diffuse ,1.0};
float Specular[]  = {0.01*specular,0.01*specular,0.01*specular,1.0};
//  Light direction
float Position[]  = {Ox+3, 2, Oz,1};
float Direction[] = {Ox+lx, here, Oz+lz, 0};

glLightfv(GL_LIGHT0,GL_AMBIENT ,Ambient);
glLightfv(GL_LIGHT0,GL_DIFFUSE ,Diffuse);
glLightfv(GL_LIGHT0,GL_SPECULAR,Specular);
glLightfv(GL_LIGHT0,GL_POSITION,Position);

glLightfv(GL_LIGHT0,GL_SPOT_DIRECTION,Direction);
glLightf(GL_LIGHT0,GL_SPOT_CUTOFF,sco);
glLightf(GL_LIGHT0,GL_SPOT_EXPONENT,Exp);

Camera code:

//  Camera Values
double Ox=0, Oz=0;
float Oy=1.0;
float angle = 0.0;
float lx=0.0,lz=-1.0;
float deltaAngle = 0.0;
float deltaMove = 0;
double here;

void computePos(float deltaMove) {
    Ox += deltaMove * lx * 0.1f;
    Oz += deltaMove * lz * 0.1f;
}
void computeDir(float deltaAngle) {
    angle += deltaAngle;
    lx = sin(angle);
    lz = -cos(angle);
}

void display() {
    here = 2.0f*Oy;
    if (deltaMove)
        computePos(deltaMove);
    if (deltaAngle)
        computeDir(deltaAngle);

    gluLookAt(Ox,2,Oz, Ox+lx, here, Oz+lz, 0,1,0);
}


void key(unsigned char ch,int x,int y) {
    //  Exit on ESC
    if (ch == 27)
        exit(0);
    // WASD controls
    else if (ch == 'a' || ch == 'A')
        deltaAngle = -0.01;
    else if (ch == 'd' || ch == 'D')
        deltaAngle = 0.01;

    else if (ch == 'w' || ch == 'W') {
        collidefront=collision(1);
        collidextra=collision(3);
        if (collideback == 2 && collidextra == 3) { deltaMove = 0; collideback = 0; }
        else if (collideback == 2) { deltaMove = 0.1; collidefront = 0;}
        else if (collidefront == 1) deltaMove = 0;
        else 
            deltaMove = 0.1; 
        }

    else if (ch == 's' || ch == 'S') {
        collideback=collision(2);
        if (collidefront == 1) { deltaMove = -0.3; collidefront = 0; }
        else if (collideback == 2) deltaMove = 0;
        else 
            deltaMove = -0.1; 
        }

    else if ((ch == 'e' || ch == 'E') && here < 4)
        Oy += 0.01;
    else if ((ch == 'c' || ch == 'C') && here > .5)
        Oy -= 0.01;

    Project(fov,asp,dim);
    glutPostRedisplay();
}

Thank you for your help.

Answer 1

Since you are using gluLookAt, you can easily compute the vector subtracting the center and the eye:

(...)
gluLookAt(eyeX, eyeY, eyeZ, centerX, centerY, centerZ, upX, upY, upZ);
(...)
spotlightVecX = centerX - eyeX;
spotlightVecY = centerY - eyeY;
spotlightVecZ = centerZ - eyeZ;

You may normalize it after you calculate the vector.

Hope it helps.