openGL - how to rotate an object without rotating lights - opengl

Question

i want to draw a rotating cube in the middle of the screen, and i want it to be lit by a light above it (i want it to look as if the cube was being lit from a fixed screen position). my problem is that i don't know how to prevent the light from rotating with the cube.
here's the code:
(SUMMARY: initGL, paintGL, and resizeGl are the functions that you always have to implement. in paintGL i use makeCube(). in makeCube() i use glBegin(GL_QUADS) to make a cube,and i use calcNormals() to calculate the normals of the cube )
-------------initGL--------------------------
angle=0.0;
glEnable (GL_DEPTH_TEST);
glEnable (GL_LIGHTING);
GLfloat LightDiffuse[]= { 1.0f, 1.0f, 1.0f, 1.0f };
GLfloat LightPosition[]= { 0.0f, 1.5f,1.5f, 1.0f };
glLightfv(GL_LIGHT0, GL_DIFFUSE, LightDiffuse);
glLightfv(GL_LIGHT0, GL_POSITION,LightPosition);
glEnable (GL_LIGHT0);
--------------paintGL------------------
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(0.0, 0.0, -13.0);
glRotatef(angle,0.0f,1.0f,0.0f);
makeCube();
angle+=0.3;
--------------void makeCube()-------------------
float P[8][3]={ {-1,-1, 1},{1,-1, 1},{1,1, 1},{-1,1, 1},
{-1,-1,-1},{1,-1,-1},{1,1,-1},{-1,1,-1}};
float * planes[6][4] ={ {P[0],P[1],P[2],P[3]},
{P[1],P[5],P[6],P[2]},
{P[4],P[7],P[6],P[5]},
{P[0],P[3],P[7],P[4]},
{P[3],P[2],P[6],P[7]},
{P[0],P[4],P[5],P[1]}};
int i;
for(i=0;i<6;i++){
float *normal;
normal = calcNormal(planes[i][0],planes[i][1],planes[i][2]);
glBegin(GL_QUADS);
glNormal3f(normal[0], normal[1], normal[2]);
glVertex3f(planes[i][0][0],planes[i][0][1],planes[i][0][2]);
glVertex3f(planes[i][1][0],planes[i][1][1],planes[i][1][2]);
glVertex3f(planes[i][2][0],planes[i][2][1],planes[i][2][2]);
glVertex3f(planes[i][3][0],planes[i][3][1],planes[i][3][2]);
glEnd();
}
----------------float* calcNormal()----------------------
float vec1[3] = {P2[0]-P1[0],P2[1]-P1[1],P2[2]-P1[2]};
float vec2[3] = {P3[0]-P2[0],P3[1]-P2[1],P3[2]-P2[2]};
float cross[3] = {vec1[1]*vec2[2]-vec2[1]*vec1[2],
vec1[2]*vec2[0]-vec2[2]*vec1[0],
vec1[0]*vec2[1]-vec2[0]*vec1[1]};
float modCross = sqrt(cross[0]*cross[0]+cross[1]*cross[1]+cross[2]*cross[2]);
cross[0]/=modCross;
cross[1]/=modCross;
cross[2]/=modCross;
return cross;
-------------resizeGL--------------------------
glViewport(0, 0, width, height);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
GLfloat x = GLfloat(width) / height;
glFrustum(-x, +x, -1.0, +1.0, 4.0, 15.0);
glMatrixMode(GL_MODELVIEW);

Answer 1

It seems that you're transforming the position of the light in your paintGL section.
Looking over old code, I found an app in my code directory that loads and rotates .OBJ meshes, while allowing the light to be moved.
I think that the solution is to set the position of the light each frame. (Can't remember it's been over 18 months since I touched the project)
void idleFunc()
{
light(); /// *** I think you need to replicate this functionality ****
glPushMatrix();
myGluLookAt(0.0, -.50, -6.0, /* eye is at (0,0,5) */
0.0, 0.0, 0.0, /* center is at (0,0,0) */
0.0, 1.0, 0.); /* up is in positive Y direction */
transformFunc();
displayFunc();
glPopMatrix();
}
void displayFunc()
{
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
if (useDrawList)
glCallList(DLid);
else
drawObj(loadedObj);
drawLight0(); // *** just displays an unlit sphere at the position of the light **
glutSwapBuffers();
frameCount++;
}
/* set the poition of each of the lights */
void light()
{
glLightfv(GL_LIGHT0, GL_POSITION, lightPos1);
glLightfv(GL_LIGHT1, GL_POSITION, lightPos2);
}

Answer 2

i solved this problem drawing the cube with VERTEX ARRAYS rather than DIRECT MODE, it seems that rotations or lights affect the object in a different way with each method, which is quite weird