gl_Position — contains the position of the current vertex
gl_Position is a member of the gl_PerVertex
named block:
out gl_PerVertex {
vec4 gl_Position;
float gl_PointSize;
float gl_ClipDistance[];
};
In the vertex, tessellation evaluation and geoemtry languages, a single
global instance of the gl_PerVertex named block is available and
its gl_Position member is an output that receives the
homogeneous vertex position. It may be written at any time during shader execution.
The value written to gl_Position will be used by primitive assembly,
clipping, culling and other fixed functionality operations, if present, that operate on
primitives after vertex processing has occurred.
In the tessellation control language, the gl_PerVertex named block
is used to construct an array, gl_out[], whose gl_Position
members hold the homogeneous control point position, which become available as inputs to the subsequent
tessellation evaluation shader.
The value of gl_Position (or the gl_Position member of the gl_out[]
array, in the case of the tessellation control shader)
is undefined after the vertex, tessellation control, and tessellation evaluation
shading stages if the corresponding shader executable does
not write to gl_Position. It is also undefined after the geometry processing stage if the geometry shader executable calls
EmitVertex without having
written gl_Position since the last call to EmitVertex
(or hasn't written it at all).
In the tessellation control, tessellation evaluation and geoemetry languages,
the gl_PerVertex named block is used to construct an array, gl_in[]
of per-vertex or per-control point inputs whose content represents the corresponding
outputs written by the previous stage.
Copyright © 2011-2012 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/.