openGL: Losing details from a loaded obj file - c++

Question

I'm loading a .obj file into my program (without a .mtl file).
In the vertex shader, I have this:
#version 330
layout(location = 0) in vec3 in_position;
layout(location = 1) in vec3 in_color;
and my vertex structure looks like this:
struct VertexFormat {
glm::vec3 position;
glm::vec3 color;
glm::vec3 normal;
glm::vec2 texcoord;
VertexFormat() { every atribute is glm::vec3(0, 0, 0); }
VertexFormat(glm::vec3 _position, glm::vec3 _normal, glm::vec2 _texcoord, glm::vec3 _color) {
position = _position;
normal = _normal;
texcoord = _texcoord;
// color = glm::vec3(texcoord, cos(texcoord.x + texcoord.y));
color = normal;
}
}
Because I don't have a .mtl file, the color attribute depends on the other vertex attributes.
If I let color = glm::vec3(texcoord, cos(texcoord.x + texcoord.y));, the object loses some of the details (like a human face is just an ellipsoid).
This does not happen when I let color = normal;.
I want the color to not depend only on the normal attribute because then every object is colored as a rainbow.
Any idea why and how can I make it work?
EDIT:
This is an object with color = normal:
And this is with color = glm::vec3(texcoord, cos(texcoord.x + texcoord.y));:
The only things changed between the two pictures are the fact that I commented color = normal; and decommented the other.

Answer 1

In your comment you wrote
I would prefer to not use lighting at all. I don't understand why without lighting the first works (shows the details), while the other one doesn't
Perceived details depend on the color contrast in the final picture. The stronger the contrast, the stronger the detail (there's a strong relation to so called spatial frequencies as well).
Anyway, creased, edges, bulges, etc. in the mesh create a strong local position depending variation of the surface normal, which is what you see. In mathematical terms you could write this as
|| ∂/∂r n(r) ||
where n denotes the normal and r denotes the position, which becomes very large for creases and such.
The variation of a color depending position c(r) however would be
|| ∂/∂r c(r) ||
But since c(r) depends on only r and no local features c acts just like a constant and the local spatial variation in color is constant as well, i.e. has no strong features.
Essentially it means that you can make details visible only based on derivatives of surface features such as the normals.
The easiest way to do this is to use illumination. But you can use other methods as well, for example you can calculate the local variations of the normals (giving you the curvature of the surface) and make stronger curves areas brighter. Or you perform post processing on the screen space geometry, applying something like a first or second order gradient filter.
But you will not get around to apply math to it. There's no such thing as a free meal. Also don't expect people to write code for you without being clear what you actually want.