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Lighting problem

Started by juxie Mar 9, 2007 at 3:52 AM 8 replies 800+ views

Original Post

Mar 9, 2007 at 3:52 AM

Hi I encountered a problem with lights. I am not sure why it behaves this way.


	gluLookAt(0, 50, 0,
			  0, 0, 0,
			  0, 0, -1);

	glEnable(GL_LIGHTING);

	float light_pos[4] = {0, 20, 0, 1};
	float light_am[4] = {0, 0, 0, 1};
	float light_di[4] = {1, 1, 1, 1};
	float light_spe[4] = {1, 1, 1, 1};
	float light_dir[] = {0, -1, 0};

	glLightfv(GL_LIGHT0, GL_AMBIENT, light_am);
	glLightfv(GL_LIGHT0, GL_DIFFUSE, light_di);
	glLightfv(GL_LIGHT0, GL_SPECULAR, light_spe);

	glLightfv(GL_LIGHT0, GL_POSITION, light_pos);

	glLightf(GL_LIGHT0, GL_SPOT_CUTOFF, 10);
	glLightf(GL_LIGHT0, GL_SPOT_EXPONENT, 1);
	glLightfv(GL_LIGHT0, GL_SPOT_DIRECTION, light_dir);

	glEnable(GL_COLOR_MATERIAL);
	glColorMaterial(GL_FRONT, GL_AMBIENT_AND_DIFFUSE);

	float mat_am[4] = {0, 0, 0, 1};
	float mat_di[4] = {1, 1, 1, 1};
	float mat_spe[4] = {1, 1, 1, 1};
	float mat_em[4] = {0, 0, 0, 1};

        glMaterialfv(GL_FRONT, GL_AMBIENT, mat_am);
	glMaterialfv(GL_FRONT, GL_DIFFUSE, mat_di);
        glMaterialfv(GL_FRONT, GL_SPECULAR, mat_spe);
        //glMaterialfv(GL_FRONT, GL_EMISSION, mat_em);
        glMaterialf(GL_FRONT, GL_SHININESS, 70);

	glEnable(GL_LIGHT0);

	glPushMatrix();	
	glColor3f(1, 0, 0);
	glTranslatef(0, -20, 0);;
	glutSolidCube(10);
	glPopMatrix();

	glPushMatrix();	
	glColor3f(1, 0, 0);
	glTranslatef(5, -5, 0);
	auxSolidSphere(2.0);
	glPopMatrix();

	glPushMatrix();	
	glColor3f(1, 0, 0);
	glTranslatef(-5, -5, 0);
	auxSolidSphere(2.0);
	glPopMatrix();

	glPushMatrix();	
	glColor3f(1, 0, 0);
	glTranslatef(0, -5, -5);
	auxSolidSphere(2.0);
	glPopMatrix();

	glPushMatrix();	
	glColor3f(1, 0, 0);
	glTranslatef(0, -5, 5);
	auxSolidSphere(2.0);
	glPopMatrix();

I get the sphere lit correctly. but I don't get the cube lit correctly. I thought the cube supposed to be lit by the spotlight created too. But it didn't. Any idea why?

Lord_Evil

Mar 9, 2007 at 4:08 AM

Could you elaborate a bit on what is wrong with the rendering? What is wrong with tne lighting of the cube?

juxie

Mar 9, 2007 at 4:37 AM

I have 4 spheres on top of a cube.

---O---
| |
O X O
| |
---O---

O = sphere
X = spotlight from (0, 20, 0) in direction (0, -1, 0)

With the spotlight cut off is 10, I can see each of the spheres has half of its face lit.
But the cube is just black.

Todo

Mar 9, 2007 at 4:41 AM

Yes, and bare in mind that what you're seeing is probably correct, but that you are mistaking per-vertex lighting with per-pixel lighting. On a regular, non-tesselated cube, per-vertex lighting will look "off" (especially specular highlights). One option is to tesselate each face of the cube into smaller elements. Also, depending on your setup, vertices shouldn't be shared by adjacent faces, because of conflicting normals.

EDIT: although it shouldn't be completely black I guess. I'm not sure about this, but do GLUT primitives generate normals?

rfterdarc

Mar 9, 2007 at 4:43 AM

Post a screenshot(s) to confirm Todo answer :)

juxie

Mar 9, 2007 at 4:50 AM

how do I post screen shot?

Lord_Evil

Mar 9, 2007 at 6:31 AM

Check the FAQ. Basically you get some webspace to host your image then insert a link here.

Here it is said that glut primitives provide vertex positions and normals, so that shouldn't be the problem. However, enable GL_NORMALIZE to make sure the normals are normalized ([looksaround]) before rendering (not sure if glutXXX does this already, might be so but I didn't find a clue).

juxie

Mar 9, 2007 at 8:03 AM

I tried with GL_NORMALIZE, however, it doesn't work.
I wanted to model some kind of table.

I wonder if texturing will solve the problem?

Lord_Evil

Mar 9, 2007 at 8:36 AM

Texturing wouldn't solve lighting problems just make the cube look different. Post a screenshot, please.

Kalidor

Mar 9, 2007 at 1:11 PM

Quote:
Original post by juxie
I have 4 spheres on top of a cube.
       ---O---       |     |       O  X  O       |     |       ---O---
O = sphere
X = spotlight from (0, 20, 0) in direction (0, -1, 0)

With the spotlight cut off is 10, I can see each of the spheres has half of its face lit.
But the cube is just black.

Is the spotlight cutoff large enough to include the vertices of the cube? As Todo said, OpenGL lighting is per-vertex so if no vertices receive light, the cube doesn't receive light.

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