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OpenGL OpenGL Lighting problems

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Can somebody tell me how to specify the normal for each vertex? The book I have tells me how to get just the one normal (giving me flat shaded polygons. :<) Also, anybody know how to gourand shade polygons using materials? Please keep in mind that I''m real new to OpenGL... Thanks.

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Specifing a normal for each vertex is easy - just call glNormalxxx() before each glVertexxxx(). Later, when you get better with using OpenGL, you''ll start using vertex arrays, but for learning purposes, you''ll probably eventually wrap every vertex with:

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Uhh, I''m not trying to be ungrateful but...

How do I "get" the normal for each vertex...that OpenGL book I got only shows how to get the one normal and then it cheats on the rest of the examples and uses aux functions and therefor not telling me how to get them...

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Well, you calculate it based on the object you''re trying to render.

If it''s a sphere, the normal is a vector pointing directly away from the center of the sphere.

If it''s a terrain or flatish polygonal model, people generally average the face normals of all the faces which share the vertex.

If you''re drawing some kind of mathematically defined surface, you can figure out the normal based on the derivative of the surface''s equation.

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Oh, and don't forget about glShadeModel();

It accepts one of 2 parameters:
GL_FLAT for flat shading,
GL_SMOOTH of smooth shading.

At initialization, gl has glShadeModel set to GL_SMOOTH, so as long as you didn't call glShadeMode(), specifying a normal per vertex will give you smooth shading, so long as each normal is the normal to the surface, and not the polygon. Here is an example of the difference between a normal to the surface and the normals to the polygons apporximating the surface.

Edited by - succinct on November 2, 2001 3:15:28 PM

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A sphere is definitely the easiest, because, as cheesegrater said, the normal for any point on a sphere is the line from the line from the center of the sphere to the point, normalized.

For an arbitrary mesh of polygons, you keep track of what polygons use what vertices and for each vertex computer the average of all the polygon normals that use that vertex. The necessary data structures for figuring this out are somewhat tricky, because each polygon must reference it's composing vertices, and each vertex must reference all of the polygons that reference it. It becomes a "chicken or the egg" type of problem, but you can solve it.

And, to my knowledge, the only other type of surface is a mathematically defined one. If you can define a surface parametrically, such that it is in terms of two variables, usually u and v, you can find the normal using a little calculus. For a given ( u,v ) on the surface, the surface normal is the unit tangent of the u curve crossed with the unit tangent of the v curve.

Basically, seperate the equation into it's f( u ) and g( v ) parts, expressed as vector functions. Find f'( u ) and g'( v ). Normalize them. Cross them. This is your unit normal vector.

Here is a visual explanation for the normals for a parametric torus.

The torus is just a circle (g(v)) swept around another curve (f(u)). The normal at any given point is the unit gradient of the sweeping curve crossed by the unit gradient of the swept curve.

For a circle defined as
x = r*cos( t )
y = r*sin( t )

the derivitaves (gradients) are
x' = r*-sin( t )
y' = r*cos( t )

just like you learn in highschool (and hopefully you've been to highschool and have had calculus!)

For further explanation, the picture generated used the following function.

void gl::RenderTorus(
int Majors, // number of major subdivisions (along the greater circle)

int Minors, // number of minor subdivisions (along the lesser circle)

float MrX, // major x radius

float MrY, // major y radius

float mrX, // minor x radius

float mrZ, // minor z radius

float Msa, // major start angle

float Mea, // major end angle

float msa, // minor start angle

float mea, // minor end angle

bool InvertNormals
// the vector class uses the operator ^ as a cross product

// the member function .Unit() normalizes the vector.

int NumMajorVertices = Majors + 1;
int NumMinorVertices = Minors + 1;

if( NumMajorVertices <= 2 )
NumMajorVertices = 3;

if( NumMinorVertices <= 2 )
NumMinorVertices = 3;

// calc angle deltas

float MajorAngleDelta = (Mea - Msa)*M_PI/(180*(NumMajorVertices - 1));
float MinorAngleDelta = (mea - msa)*M_PI/(180*(NumMinorVertices - 1));

float MajorAngle1 = Msa*M_PI/180;
float MajorAngle2 = MajorAngle1 + MajorAngleDelta;
for( int i = 0; i < NumMajorVertices - 1; ++i )
// calc major coordinates and normals

float uS1x = cos( MajorAngle1 );
float uS1y = sin( MajorAngle1 );
float uS2x = cos( MajorAngle2 );
float uS2y = sin( MajorAngle2 );

Vector uS1 = Vector( MrX*uS1x,MrY*uS1y,0 );
Vector uN1 = Vector( -MrX*uS1y,MrY*uS1x,0 ).Unit() ^ Vector( 0,0,1 );

Vector uS2 = Vector( MrX*uS2x,MrY*uS2y,0 );
Vector uN2 = Vector( -MrX*uS2y,MrY*uS2x,0 ).Unit() ^ Vector( 0,0,1 );

// calc rotation angles for minor ellipse (so it lines up with major normal)

float RotationAngle1 = acos( uN1.x );
if( ((-M_PI < MajorAngle1) && (MajorAngle1 < 0 ))
|| ((2*M_PI > MajorAngle1) && (MajorAngle1 > M_PI)) )
RotationAngle1 = -RotationAngle1;

float RotationAngle2 = acos( uN2.x );
if( ((-M_PI < MajorAngle2) && (MajorAngle2 < 0 ))
|| ((2*M_PI > MajorAngle2) && (MajorAngle2 > M_PI)) )
RotationAngle2 = -RotationAngle2;

// cache rotation coefficients

float ct1 = cos( RotationAngle1 );
float st1 = sin( RotationAngle1 );
float ct2 = cos( RotationAngle2 );
float st2 = sin( RotationAngle2 );

glBegin( GL_QUAD_STRIP );
float MinorAngle = msa*M_PI/180;
for( int j = 0; j < NumMinorVertices; ++j )
// v == minor param

float vSx = cos( MinorAngle );
float vSz = sin( MinorAngle );

// rotate minor ellipse so it is aligned with the normal of the major ellipse at this point

Vector vS = Vector( mrX* vSx,0,mrZ*vSz );
Vector vT = Vector( mrX*-vSz,0,mrZ*vSx );

Vector vS1r = Vector( ct1*vS.x,st1*vS.x,vS.z );
Vector vT1r = Vector( ct1*vT.x,st1*vT.x,vT.z );
Vector vS2r = Vector( ct2*vS.x,st2*vS.x,vS.z );
Vector vT2r = Vector( ct2*vT.x,st2*vT.x,vT.z );

Vector vN1 = ((vT1r ^ vS1r).Unit() ^ vT1r).Unit();
Vector vN2 = ((vT2r ^ vS2r).Unit() ^ vT2r).Unit();

if( InvertNormals )
vN1 = -vN1;
vN2 = -vN2;

glNormal3fv( vN1 );
glVertex3fv( uS1 + vS1r );
glNormal3fv( vN2 );
glVertex3fv( uS2 + vS2r );

MinorAngle += MinorAngleDelta;

MajorAngle1 += MajorAngleDelta;
MajorAngle2 += MajorAngleDelta;

I hope that helps,
-- Succinct

Edited by - succinct on November 2, 2001 5:14:11 PM

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Uhh, hmm.

Ok, you guys are really being nice and putting lots of time into this but, all this info is really do I "smooth shade" a cube? I thought you had to give a "normal" for each vertex...

>just like you learn in highschool (and hopefully you''ve been to >highschool and have had calculus!)
Hmm, uhh... In highschool.

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Guest Anonymous Poster
If you have the object in a .3ds file you can use
a software named 3DExploration (search with
and this software give you a file (see "Save as..") with
all information about texturing normals vertex position...
as .cpp file for OpenGL.
That''s all

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When you have already the normal for a polygon it is
not difficult to find the vertex-normals.
take a vertex, go through your polygons, take the Face(polygon)-Normal from all the polygons who share that vertex,
add them and normalize it, so can get the vertex-normals.

Try a Cube, becaue a cube only has eight corners, vertices

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After you figure out the vertex issues, to gourand shade a polygon you can do the following:


//define light and material properties
float ambientLight[] = {0.5f,0.5f,0.5f,1.0f}; // ambient light
float specularLight[]= {1.0f,1.0f,1.0f,1.0f }; //specular light

//material reacting to ambient light
float matAmbient[] = {1.0f, 1.0f, 1.0f, 1.0f};

then add the following to your scene initialization

glEnable (GL_LIGHTING); // Enable lighting
glEnable (GL_COLOR_MATERIAL); //enable materials

//Set the lighting materials
glMaterialfv(GL_FRONT, GL_AMBIENT, matAmbient);
glMaterialfv(GL_FRONT, GL_SPECULAR, specularLight);
glMateriali (GL_FRONT, GL_SHININESS, 128); //strong shiny effect

//Setup LIGHT0
glLightfv(GL_LIGHT0, GL_AMBIENT, ambientLight); //ambient light
glLightfv(GL_LIGHT0, GL_SPECULAR, specularLight);//specular light
glLightfv(GL_LIGHT0, GL_POSITION, lightPosition);//light position

glEnable (GL_LIGHT0); //enable light0

glShadeModel (GL_SMOOTH);//enable gourand shading


Hope it helps

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Guest Anonymous Poster
Original post by Etwinox
Uhh, hmm.

Ok, you guys are really being nice and putting lots of time into this but, all this info is really do I "smooth shade" a cube? I thought you had to give a "normal" for each vertex...

Smooth shading works fine with face normals. With a cube, specify the same normal for each vertex in a face. The ''smoothness'' of smooth shading will come from the changing angle to the light source at the different vertexes (which is computed by openGL for you) rather than from differing normals.

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Why would you smooth shade a cube? All of the normals on a particular face share the same normal. Even worse, if you''re only using 8 vertices for the cube, because each vertex is used more than once (4 vertices per face, each vertex shares 3 faces), there is not a particular "vertex normal" or "surface normal" that relates to each vertex in a 1 to 1 manner. Each vertex will use 3 different normals, one for each face it''s rendering. This is because each vertex lies on a sharp edge.

If you were to use a single "surface" normal for each vertex, the cube would not look like a cube, but like a sphere that is very under tesselated, like it doesn''t use enough polygons to approximate the surface of the sphere.

Now, if you were talking about using more than 8 vertices, like having vertices internal to a particular plane (face), then each vertex on a particular face shares the same normal.

If you''re looking to explore smooth shading, try a different primitive that doesn''t have sharp edges, such as a sphere, cone, or cylinder. I learned how to describe these shapes in 11th grade trig class - it''s all sines and cosines.

If you''re interested in some code for them post here about it and I can send you some or post some here. My advice, though, is to "borrow" some of the math books you use in high school trig and calculus, and get your hands on some college level texts, if you can. I still have my high school trig book, two different high school calc books, and all of my college math books. Every once in a while, I still have to go and double check something from that high school trig book!

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I didn''t think of it that way. I thought you "had" to smooth shade an do games get a "spotlight" effect then if
the surface consists of only one normal? I didn''t think it would
work considering the lighting is only a brightness level when its one normal (or so it seems).

Course, now I know why my attempts at smooth shading a cube were
not exactly correct looking.

Thank god, I thought I would of been stuck on chapter five of that book forever. ^_^

Oh, almost forgot, I would be happy to see that source code you were talking about if you have the free time, but its upto you.

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By a ''spotlight'' effect I''m going to assume that you''re talking about a specular reflection. (The white patch you see when looking at an apple or a shiny ball.)

The specular reflection occurs when the vector of light ''mirroring'' off of the object happens to line up well with the vector to the eye. Think of the light bouncing off of the surface at the same angle it hit. If the bounce points right at your eye or camera, you get a specular hightlight. This happens at some points of a flat surface but not others, since the reflected light rays won''t match up when reflected from various points, even if they are reflected at nearly the same angle.

OpenGL''s shading model (gourand shading) won''t do specular highlighting within a face. To get this you need to tesselate your flat surfaces into larger groups of triangles. However, with a flat surface these tesselated faces will still all have the same normal.

If you want this to happen on one big quad, you need phong shading, which isn''t built into OpenGL.

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If you want a normal for a triangle, this is the code:

float x,y,z;

GLVECTOR triangle[3];
glVECTOR normal;
GLVECTOR temp[2];

//move the triangle temporarily to the origin.
temp[0] = triangle[1] - triangle[0];
temp[1] = triangle[2] - triangle[0];
//No real code, i know, but you know how to do that.

normal.x = triangle[1].y*triangle[2].z - triangle[2].y*triangle[1].z;
normal.y = triangle[1].z*triangle[2].x - triangle[2].z*triangle[1].x;
normal.z = triangle[1].x*triangle[2].y - triangle[2].x*triangle[1].y;

Now you got a good normal. (Needs to be normalized, though, but you can use glEnable(GL_NORMALIZE), i think..)

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