OpenGL: lines dont get rendered - opengl

Question

Using OpenGL I am trying to render a simple line between the points p1 and p2, but it doesnt work.
However, I am able to render cubes at these positions.
glEnable(GL_LIGHTING);
glEnable(GL_LIGHT0);
GLfloat mat_ambient[] = {0.0, 0.0, 1.0, 1.0};
GLfloat mat_flash[] = {0.0, 0.0, 1.0, 1.0};
GLfloat mat_flash_shiny[] = {50.0};
GLfloat light_position[] = {100.0,-200.0,200.0,0.0};
GLfloat ambi[] = {0.1, 0.1, 0.1, 0.1};
GLfloat lightZeroColor[] = {0.9, 0.9, 0.9, 0.1};
glLightfv(GL_LIGHT0, GL_POSITION, light_position);
glLightfv(GL_LIGHT0, GL_AMBIENT, ambi);
glLightfv(GL_LIGHT0, GL_DIFFUSE, lightZeroColor);
glMaterialfv(GL_FRONT, GL_SPECULAR, mat_flash);
glMaterialfv(GL_FRONT, GL_SHININESS, mat_flash_shiny);
glMaterialfv(GL_FRONT, GL_AMBIENT, mat_ambient);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(p1.x, p1.y, p1.z);
glutSolidCube(20);
glLoadIdentity();
glTranslatef(p2.x, p2.y, p2.z);
glutSolidCube(20);
glDisable( GL_LIGHTING );
glDisable( GL_DEPTH_TEST );
I am using the following code for rendering the line. But I cant find the problem why it doesnt get rendered:
glDisable(GL_LIGHTING);
glEnable(GL_COLOR_MATERIAL);
glLoadIdentity();
glColor3f(1,0,0);
glLineWidth(10.0);
glBegin(GL_LINES); //doesnt work with GL_LINE either
glVertex3f(p1.x, p1.y, p1.z);
glVertex3f(p2.x, p2.y, p2.z);
glEnd();

Answer 1

I assume what happens is that the lines are actually rendered, but there's still a vertex normal set, pointing away from the light source, or being 0. Easy fix: Disable lighting when drawing lines.
While it is possible to "illuminate" lines, the whole operation makes no sense if one assumes the "regular" normal based illumination models. Lines don't have a normal direction, they have a normal plane (or in other words, a line it it's tangent itself).
So the proper way to illuminate a line is by tangent based shading. Fixed function OpenGL can't do it, but it is trivial to implement with a shader.

Answer 2

A couple of suggestions for you, w.r.t your second code snippet:
1) You probably don't need the 'glEnable(GL_COLOR_MATERIAL)', try commenting out that line.
2) You're passing int's to glColor3f(). Try using glColor3f(1.0f, 0.0f, 0.0f); instead.
Also, did you check the line drawing without the cube at the same position? (ie., is it being occluded or not)