OpenGL models show grid-like lines - objective-c

Question

Shading problem solved; grid lines persist. See Update 5
I'm working on an .obj file loader for OpenGL using Objective-C.
I'm trying to get objects to load and render them with shading. I'm not using any textures, materials, etc. to modify the model besides a single light source. When I render any model, the shading is not distributed properly (as seen in the pictures below). I believe this has something to do with the normals, but I'm not sure.
These are my results:
And this is the type of effect I'm trying to achieve:
I thought the problem was that the normals I parsed from the file were incorrect, but after calculating them myself and getting the same results, I found that this wasn't true. I also thought not having GL_SMOOTH enabled was the issue, but I was wrong there too.
So I have no idea what I'm doing wrong here, so sorry if the question seems vague. If any more info is needed, I'll add it.
Update:
Link to larger picture of broken monkey head: http://oi52.tinypic.com/2re5y69.jpg
Update 2: If there's is a mistake in the process of me calculating normals, this is what I'm doing:
Create a triangle for each group of indices.
Calculate the normals for the triangle and store it in a vector.
Ensure the vector is normalized with the following function
:
static inline void normalizeVector(Vector3f *vector) {
GLfloat vecMag = VectorMagnitude(*vector);
if (vecMag == 0.0) {
vector->x /= 1.0;
vector->y /= 0.0;
vector->z /= 0.0;
}
vector->x /= vecMag;
vector->y /= vecMag;
vector->z /= vecMag;
}
Update 3: Here's the code I'm using to create the normals:
- (void)calculateNormals {
for (int i = 0; i < numOfIndices; i += 3) {
Triangle triangle;
triangle.v1.x = modelData.vertices[modelData.indices[i]*3];
triangle.v1.y = modelData.vertices[modelData.indices[i]*3+1];
triangle.v1.z = modelData.vertices[modelData.indices[i]*3+2];
triangle.v2.x = modelData.vertices[modelData.indices[i+1]*3];
triangle.v2.y = modelData.vertices[modelData.indices[i+1]*3+1];
triangle.v2.z = modelData.vertices[modelData.indices[i+1]*3+2];
triangle.v3.x = modelData.vertices[modelData.indices[i+2]*3];
triangle.v3.y = modelData.vertices[modelData.indices[i+2]*3+1];
triangle.v3.z = modelData.vertices[modelData.indices[i+2]*3+2];
Vector3f normals = calculateNormal(triangle);
normalizeVector(&normals);
modelData.normals[modelData.surfaceNormals[i]*3] = normals.x;
modelData.normals[modelData.surfaceNormals[i]*3+1] = normals.y;
modelData.normals[modelData.surfaceNormals[i]*3+2] = normals.z;
modelData.normals[modelData.surfaceNormals[i+1]*3] = normals.x;
modelData.normals[modelData.surfaceNormals[i+1]*3+1] = normals.y;
modelData.normals[modelData.surfaceNormals[i+1]*3+2] = normals.z;
modelData.normals[modelData.surfaceNormals[i+2]*3] = normals.x;
modelData.normals[modelData.surfaceNormals[i+2]*3+1] = normals.y;
modelData.normals[modelData.surfaceNormals[i+2]*3+2] = normals.z;
}
Update 4: Looking further into this, it seems like the .obj file's normals are surface normals, while I need the vertex normals. (Maybe)
If the vertex normals are what I need, if anybody can explain the theory behind calculating them, that'd be great. I tried looking it up but I only found examples, not a theory. (e.g. "get the cross product of each face and normalize it"). If I know what I have to do, I can look it up an individual process if I get stuck and won't have to keep updating this.
Update 5: I re-wrote my whole loader, and got it to work, somehow. Although it shades properly, I still have those grid-like lines that you can see on my original results.

Answer 1

Your normalizeVector function is clearly wrong. Dividing by zero is never a good idea. Should be working when vecMag != 0.0 though. How are you calculating the normals? Using cross-product? When happens if you let OpenGL calculate the normals?

Answer 2

It may be the direction of your normals that's at fault - if they're the wrong way around then you'll only see the polygons that are supposed to be pointing away from you.
Try calling:
glDisable(GL_CULL_FACE);
and see whether your output changes.
Given that we're seeing polygon edges I'd also check for:
glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);