So I was reading "The Official OpenGL Guide" and in a section where they taught material lighting, they suddenly used the "flat" qualifier for an input variable in the fragment shader. I google'd the matter and all I came up with was "flat shading" and "smooth shading" and the differences between them, which I cannot understand how it is related to a simple "MatIndex" variable. here's the example code from the book:
struct MaterialProperties {
vec3 emission;
vec3 ambient;
vec3 diffuse;
vec3 specular;
float shininess;
};
// a set of materials to select between, per shader invocation
const int NumMaterials = 14;
uniform MaterialProperties Material[NumMaterials];
flat in int MatIndex; // input material index from vertex shader
What is this all about?
Answer
In the general case, there is not a 1:1 mapping between a vertex and a fragment. By default, the associated data per vertex is interpolated across the primitive to generate the corresponding associated data per fragment, which is what smooth shading does.
Using the flat keyword, no interpolation is done, so every fragment generated during the rasterization of that particular primitive will get the same data. Since primitives are usually defined by more than one vertex, this means that the data from only one vertex is used in that case. This is called the provoking vertex in OpenGL.
Also note that integer types are never interpolated. You must declare them as flat in any case.
In your specific example, the code means that each primitive can only have one material, which is defined by the material ID of the provoking vertex of each primitive.