Question

Ok so I’m trying to emulate the behaviour of a flashlight. For the time being I’m just trying to get a spotlight to show up on a sphere. Now the code below works, but it only works when the GL_SPOT_CUTOFF is set to 180 which creates a uniform distribution of light – not what I’m going for. If I try to set it to any other value then it just stops working. Here is the code

GLfloat mat_specular[] = { 1.0, 1.0, 1.0, 1.0 };
GLfloat light_pos [4] = {5, 2, -10, 1.0};

glLightfv (GL_LIGHT0, GL_SPECULAR, mat_specular);
glLightfv (GL_LIGHT0, GL_POSITION, light_pos);

glLightf (GL_LIGHT0, GL_SPOT_CUTOFF, 20.0f);
glLightf (GL_LIGHT0, GL_SPOT_EXPONENT, 100.0);

GLfloat spotDir [] = {0.0, 0.0, 0.0};
glLightfv (GL_LIGHT0, GL_SPOT_DIRECTION, spotDir);

glutSolidSphere (2, 20, 20);

glFlush ();
glutSwapBuffers ();

I’ve enabled lighting and have done all the initialization stuff. All the surface normals are calculated from glutSolidSphere so don’t know what else could be failing…any ideas?

Answer 1

The problem is your spotDir. It is a vector, not a point in your world coordinates.
Try to change it to:

GLfloat spotDir[] = { -5, -2, 10 };

I am lazy, so I will not normalize this vector ;D

This will point your spotlight to the sphere center.

Hope it helps!