GLAPI/glProvokingVertex
Core in version  4.6  

Core since version  3.2  
Core ARB extension  ARB_provoking_vertex 
glProvokingVertex: specifiy the vertex to be used as the source of data for flat shaded varyings
Function Definition
void glProvokingVertex(GLenum provokeMode);
 provokeMode
 Specifies the vertex to be used as the source of data for flat shaded varyings.
Description
Flatshading a vertex shader varying output means to assign all vetices of the primitive the same value for that output. The vertex from which these values is derived is known as the provoking vertex and glProvokingVertex specifies which vertex is to be used as the source of data for flat shaded varyings.
provokeMode must be either GL_FIRST_VERTEX_CONVENTION or GL_LAST_VERTEX_CONVENTION, and controls the selection of the vertex whose values are assigned to flatshaded varying outputs. The interpretation of these values for the supported primitive types is:
Primitive Type of Polygon i  First Vertex Convention  Last Vertex Convention 

point  i  i 
independent line  2i  1  2i 
line loop  i  i + 1, if i < n
1, if i = n

line strip  i  i + 1 
independent triangle  3i  2  3i 
triangle strip  i  i + 2 
triangle fan  i + 1  i + 2 
line adjacency  4i  2  4i  1 
line strip adjacency  i + 1  i + 2 
triangle adjacency  6i  5  6i  1 
triangle strip adjacency  2i  1  2i + 3 
If a vertex or geometry shader is active, userdefined varying outputs may be flatshaded by using the flat qualifier when declaring the output.
Notes
glProvokingVertex is available only if the GL version is 3.2 or greater.
Errors
GL_INVALID_ENUM is generated if provokeMode is not an accepted value.
Copyright
Copyright © 2010 Khronos Group. This material may be distributed subject to the terms and conditions set forth in the Open Publication License, v 1.0, 8 June 1999. http://opencontent.org/openpub/.