Normals

dFdx/dFdy

You can calculate normals in the fragment shader without passing any vertex normals either to or from the vertex shader. However, that these normals will not interpolate across faces.

in vec3 vertex_coord;

// https://community.khronos.org/t/getting-the-normal-with-dfdx-and-dfdy/70177
// http://web.archive.org/web/20120302100158/http://athile.net/library/blog/?tag=dfdx
vec3 normal()
{
    vec3 x = dFdx(vertex_coord);
    vec3 y = dFdy(vertex_coord);
    return normalize(cross(x, y));
}