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Aumnayan

OpenGL Normal Calculations

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I'm working on various aspects of shading and am experiencing some results that I didn't expect. Right now, I just want to eliminate the normals as the potential cause, however since I'm fairly new to openGL I'm having a hard time doing this with any amount of certanty.

If someone is willing to take a look at my normal generation code and give me their oppinion/point out problems, I would appreciate it.

The verts are stored in one large VBO at the moment, then drawn through a glDrawRangeElements call using GL_TRIANGLES. I'm unsure if/how the order matters, however the triangles and their verts are arranged in the following order:

triangle_1 = v1, v2, v0
triangle_2 = v2, v3, v0
triangle_3 = v3, v4, v0
triangle_4 = v4, v1, v0

[source lang="cpp"]void build_TriNormals (SVN_TERRAIN_VERTEX *svnVerts, int svnStride)
{
int i, j, k;
SVN_TERRAIN_VERTEX *v[6];

for (i = 0; i < svnStride; i++) {
for (j = 0; j < svnStride; j++) {
// v2
// |
// v1 - v0 - v3
// |
// v4

v[0] = &svnVerts[(i * svnStride) + j];

if (v[0]) {
v[1] = (!j) ? NULL: v[0] - 1;
v[2] = (i == svnStride - 1) ? NULL: v[0] + svnStride;
v[3] = (j == svnStride - 1) ? NULL: v[0] + 1;
v[4] = (!i) ? NULL: v[0] - svnStride;
v[5] = v[1];

SVN_VEC3 normal;
memset((char*) &normal, 0, sizeof(SVN_VEC3));

for (k = 1; k < 5; k++) {
if (v[k] && v[k + 1]) {
SVN_VEC3 obj1, obj2;

obj1 = subVec3((SVN_VEC3*) v[k], (SVN_VEC3*) v[0]);
obj2 = subVec3((SVN_VEC3*) v[k + 1], (SVN_VEC3*) v[0]);

obj1 = crossVec3(&obj1, &obj2);
normal = addVec3(&normal, &obj1);
}
}

normal = normalizeVec3(&normal);
v[0]->Xnorm = normal.x;
v[0]->Ynorm = normal.y;
v[0]->Znorm = normal.z;
}
}
}
}

SVN_VEC3 normalizeVec3(SVN_VEC3* vec)
{
SVN_VEC3 retVal;
float length;

length = sqrt(pow(vec->x, 2) + pow(vec->y, 2) + pow(vec->z, 2));
retVal.x = vec->x / length;
retVal.y = vec->y / length;
retVal.z = vec->z / length;

return retVal;
}

SVN_VEC3 crossVec3(SVN_VEC3 *left, SVN_VEC3 *right)
{
SVN_VEC3 retVal;

retVal.x = (left->y * right->z) - (left->z * right->y);
retVal.y = (left->z * right->x) - (left->x * right->z);
retVal.z = (left->x * right->y) - (left->y * right->x);

return retVal;
}

SVN_VEC3 crossVec3(SVN_VEC3 *left, SVN_VEC3 *right)
{
SVN_VEC3 retVal;

retVal.x = (left->y * right->z) - (left->z * right->y);
retVal.y = (left->z * right->x) - (left->x * right->z);
retVal.z = (left->x * right->y) - (left->y * right->x);

return retVal;
}[/source]

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Aside from major performance issues, there is nothing wrong with the vector math itself.
However your normal calculation is wrong.
[i]obj1[/i] should be normalized before being added to [i]normal[/i].


L. Spiro

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Thanks L. Spiro, that is what I was looking for. This is primarily a tool to help me learn the in's and outs of opengl, so I'm not horrible concerned with performance, which I can come back to if it ever envolves into something more then a science experiment.

To be clear, that particular bit should read:

[source lang="cpp"].
.
.

obj1 = crossVec3(&obj1, &obj2);
obj1 = normalizeVec3(&obj1);
normal = addVec3(&normal, &obj1);
}
}

normal = normalizeVec3(&normal);
.
.
.[/source]

correct?

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[quote name='Aumnayan' timestamp='1343923634' post='4965574']
correct?
[/quote]
Does it give correct results?
You never explained or provided screenshots of the problem you were experiencing.


L. Spiro

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I apologize. After I made the changes you suggested, my scene now looks like:

[img]http://www.dumpyourphoto.com/files4/128812/UoGzCAd.jpg[/img]

There are still several areas to work on, but I don't believe what's remaining has to do with the normal calculations. I appreciate you're help.

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