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# Confuse in OpenGL's Lighting when rendering a triangle mesh

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3 replies to this topic

### #1gomen  Members

Posted 04 January 2014 - 11:18 AM

Hello, Everyone,

I'am a beginner in OpenGL. Recently I encounter a confuse problem about OpenGL's lighting.

I write a small program in C++ that rendering a triangle mesh, the program is just simple, get

every triangle's vertex position, compute its normal vector, then render it. Following is my code:

void OpenGLTests::Rendering()
{
GLfloat mat_specular[] = { 1.0, 1.0, 1.0, 1.0 };
GLfloat mat_shininess[] = { 50.0 };
GLfloat light_position[] = { 0.0, 0.0, -1.0, 0.0 };

glClearColor (0.0, 0.0, 0.0, 0.0);

glMaterialfv(GL_FRONT, GL_SPECULAR, mat_specular);
glMaterialfv(GL_FRONT, GL_SHININESS, mat_shininess);
glLightfv(GL_LIGHT0, GL_POSITION, light_position);

glEnable(GL_LIGHTING);
glEnable(GL_LIGHT0);

//Draw the mesh
glEnableClientState(GL_VERTEX_ARRAY);
glVertexPointer(3, GL_DOUBLE, 0, m_pPoints);
glBegin(GL_TRIANGLES);
for ( unsigned int i = 0; i < m_nTriangleCount; ++i)
{
unsigned int vs[3] = {m_pTriangleArray[3*i], m_pTriangleArray[3*i+1], m_pTriangleArray[3*i+2]};

//Get this trangle's points
CPoint pt1(m_pPoints[vs[0]].xyz[0],m_pPoints[vs[0]].xyz[1],m_pPoints[vs[0]].xyz[2]) ;
CPoint pt2(m_pPoints[vs[1]].xyz[0],m_pPoints[vs[1]].xyz[1],m_pPoints[vs[1]].xyz[2]);
CPoint pt3(m_pPoints[vs[2]].xyz[0],m_pPoints[vs[2]].xyz[1],m_pPoints[vs[2]].xyz[2]);
//if ( pt1.x <= 0 || pt2.x <= 0 || pt3.x <= 0 )
//	continue;

//Compute this triangle's normal
CVector v1(pt2.x-pt1.x, pt2.y-pt1.y, pt2.z-pt1.z);
CVector v2(pt3.x-pt1.x, pt3.y-pt1.y, pt3.z-pt1.z);
CVector vn = VectorCrossProduct1(v2, v1);
VectorNormalize(vn);

glNormal3d(vn.x, vn.y, vn.z);
//double dx = abs(pt1.x)+abs(pt2.x)+abs(pt3.x)/3;
//double dy = abs(pt1.y)+abs(pt2.y)+abs(pt3.y)/3;
//double dz = abs(pt1.z)+abs(pt2.z)+abs(pt3.z)/3;
//double d = dx+dy+dz;
//glColor4d(dx/d,dy/d,dz/d, 0);

glArrayElement(vs[0]);
glArrayElement(vs[1]);
glArrayElement(vs[2]);
}
glEnd();
} 

When I render the whole mesh, It looks like this picture:

It seems that have many black areas.

When I render the front part(triangles whose Z coordinate is greater than 0, just uncomment the red code ) of the mesh only,

It seems that everything is OK, But once I rotate the mesh, those black areas will emerge

again, see this picture:

My problem is why those black areas emerge? I'm sure the normal vector of every triangle

is computed correctly. Anyone can help me? thanks .

### #2Vortez  Members

Posted 04 January 2014 - 12:20 PM

Seem to me like the near and far plane are not setup correctly, i might be wrong tho.

### #3gomen  Members

Posted 04 January 2014 - 07:52 PM

Seem to me like the near and far plane are not setup correctly, i might be wrong tho.

Thanks Vortez, I check my near plane setting, it was set to 0, but the mesh's some vertices Z coordinate are greater than 0,

Now I set near plane to 10, it works correctly, thanks your help very much

### #4Vortez  Members

Posted 04 January 2014 - 08:16 PM

no problem

Edited by Vortez, 04 January 2014 - 08:16 PM.

Old topic!

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