How to rotate a lightsource around a fixed object (OpenGL)? - opengl

Question

I'm trying to make a light source rotate around my character model in my OpenGL project, but as I try it, all I got so far is my model rotating like crazy (or the floor).
My rendering code looks like this:
void mainRender() {
updateState();
renderScene();
glFlush();
glutPostRedisplay();
//spin = (spin + 30) % 360;
Sleep(30);
}
void renderScene() {
glClearColor(backgrundColor[0],backgrundColor[1],backgrundColor[2],backgrundColor[3]);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); // limpar o depth buffer
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
updateCam();
renderFloor();
modelAL.Translate(0.0f,1.0f,0.0f);
modelAL.Draw();
}
void renderFloor() {
// set things up to render the floor with the texture
glShadeModel(GL_SMOOTH);
glEnable(type);
glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);
glPushMatrix();
glTranslatef(-(float)planeSize/2.0f, 0.0f, -(float)planeSize/2.0f);
float textureScaleX = 10.0;
float textureScaleY = 10.0;
glColor4f(1.0f,1.0f,1.0f,1.0f);
int xQuads = 40;
int zQuads = 40;
for (int i = 0; i < xQuads; i++) {
for (int j = 0; j < zQuads; j++) {
glBegin(GL_QUADS);
glTexCoord2f(1.0f, 0.0f); // coords for the texture
glNormal3f(0.0f,1.0f,0.0f);
glVertex3f(i * (float)planeSize/xQuads, 0.0f, (j+1) * (float)planeSize/zQuads);
glTexCoord2f(0.0f, 0.0f); // coords for the texture
glNormal3f(0.0f,1.0f,0.0f);
glVertex3f((i+1) * (float)planeSize/xQuads, 0.0f, (j+1) * (float)planeSize/zQuads);
glTexCoord2f(0.0f, 1.0f); // coords for the texture
glNormal3f(0.0f,1.0f,0.0f);
glVertex3f((i+1) * (float)planeSize/xQuads, 0.0f, j * (float)planeSize/zQuads);
glTexCoord2f(1.0f, 1.0f); // coords for the texture
glNormal3f(0.0f,1.0f,0.0f);
glVertex3f(i * (float)planeSize/xQuads, 0.0f, j * (float)planeSize/zQuads);
glEnd();
}
}
glDisable(type);
glPopMatrix();
}
How could I make this new lightsource rotate around my "modelAL" object?

Answer 1

For the fixed pipeline, light source position assigned with glLight() are transformed with the model-view matrix, just as normal objects are. So you can use the transformation functions to position and rotate your light source as you would normal objects.
To rotate a light source (or other object) around a point, you need to follow these steps. Let L be where the light source will be when the rotation is 0 degrees, and O be the subject - the object around which you want to rotate the light source.
Position the light source at L-O (the position of the light source relative to the subject)
Rotate it about the required axis (probably the Y axis)
Translate it by O to move it into position.
Because of the way OpenGL works, you essentially do these in backwards order. Basically it would go like this:
glPushMatrix();
glTranslatef(O.x,O.y,O.z);
glRotate(angle,0,1,0);
GLfloat lightpos[4] = {L.x-O.x,L.y-O.y,L.z-O.z,1};
glLightfv(GL_LIGHT0,GL_POSITION,lightpos);
glPopMatrix();
Note, this only applies to positioned light sources, not directional ones i.e. with w=0.