Smooth shader in OpenGL for OBJ import?

Question

I'm using the OpenGL OBJ loader, which can be downloaded here!

I exported an OBJ model from Blender. The problem is that I want to achieve a smooth shading. As you can see, it is not smooth here. How do I achieve this? Maybe something is wrong at the normal vector calculator?

float* Model_OBJ::calculateNormal( float *coord1, float *coord2, float *coord3 )
{
   /* calculate Vector1 and Vector2 */
   float va[3], vb[3], vr[3], val;
   va[0] = coord1[0] - coord2[0];
   va[1] = coord1[1] - coord2[1];
   va[2] = coord1[2] - coord2[2];

   vb[0] = coord1[0] - coord3[0];
   vb[1] = coord1[1] - coord3[1];
   vb[2] = coord1[2] - coord3[2];

   /* cross product */
   vr[0] = va[1] * vb[2] - vb[1] * va[2];
   vr[1] = vb[0] * va[2] - va[0] * vb[2];
   vr[2] = va[0] * vb[1] - vb[0] * va[1];

   /* normalization factor */
   val = sqrt( vr[0]*vr[0] + vr[1]*vr[1] + vr[2]*vr[2] );

    float norm[3];
    norm[0] = vr[0]/val;
    norm[1] = vr[1]/val;
    norm[2] = vr[2]/val;


    return norm;
}

And glShadeModel( GL_SMOOTH ) is set.

Any ideas?

Answer 1

If you want to do smooth shading, you can't simply calculate the normal for each triangle vertex on a per-triangle basis as you're doing now. That would yield flat shading.

To do smooth shading, you want to sum up the normals you calculate for each triangle to the associated vertices and then normalize the result for each vertex. That will yield a kind of average vector which points in a smooth direction.

Basically take what you're doing and add the resulting triangle normal to all of its vertices. Then normalize the summed vectors for each vertex. It'd look something like this (pseudocode):

for each vertex, vert:
    vert.normal = vec3(0, 0, 0)

for each triangle, tri:
     tri_normal = calculate_normal(tri)
     for each vertex in tri, vert:
          vert.normal += tri_normal

for each vertex, vert:
    normalize(vert.normal)

However, this should normally not be necessary when loading from an OBJ file like this, and a hard surface model like this typically needs its share of creases and sharp corners to look right where the normals are not completely smooth and continuous everywhere. OBJ files typically store the normals for each vertex inside as the artist intended the model to look. So I'd have a look at your OBJ loader and see how to properly fetch the normals contained inside the file out of it.

Answer 2

Thanks pal! I found the option in Blender to export normal vectors, so now I have completely rewritten the data structure to manage normal vectors too. Now it looks smooth! image