man glProvokingVertex (3): specifiy the vertex to be used as the source of data for flat shaded varyings

C SPECIFICATION

void glProvokingVertex(GLenum provokeMode);

PARAMETERS

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 .PP 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, user-defined 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.

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