Flat shading is enabled by qualifying the vertex output variable (and its corresponding fragment input variable) with the flat qualifier. This qualifier indicates that no interpolation of the value is to be done before it reaches the fragment shader. The value presented to the fragment shader will be the one corresponding to the result of the invocation of the vertex shader for either the first or last vertex of the polygon. This vertex is called the provoking vertex, and can be configured using the OpenGL function glProvokingVertex. For example, the following call:
glProvokingVertex(GL_FIRST_VERTEX_CONVENTION);
Indicates that the first vertex should be used as the value for the flat-shaded variable. The GL_LAST_VERTEX_CONVENTION ...
