I have implemented a Phong and Gourand shader for a triangle mesh that is being imported from an STL file. It appears to be working fairly allright but some triangles seem to be what I can only describe as upside down due to lack of better wording. This effect is visible in both shading models albeit a bit smoother in Phong. Here is an example:
On something basic like a cube the rendering is perfect. I am no expert but from what I have read, I am lead to suspect that perhaps the winding of the vertices in the mesh might be causing normals that are somewhat incorrect. Could this be possible and if so, how does one correct for that?
The effect of interpolating the normals by either division or by area weight does not really make too much of a difference. I can also confirm that the normals that I am calculating are basically the same as those that come with the STL as calculated by FreeCAD. The manually calculated normals are slightly better though because they are area weighed. They are calculated as follows:
glm::vec3 e1 = vecs[1] - vecs[0];
glm::vec3 e2 = vecs[2] - vecs[0];
faceNorms[i] = glm::cross(e1, e2);
with "vecs" being the 3 points of my vertices. I then sum up all these area-weighed normals for each face that uses a normal and finally normalize that.
PS. FreeCAD renders the mesh in a "flat" fashion and I guess thereby avoids this issue but unfortunately rendering "flat" with GLES 2.0 is a bit of a performance waste from what I have read.
