NAME¶
gl_ClipDistance - provides a forward-compatible mechanism for vertex clipping
DECLARATION¶
gl_ClipDistance is a member of the
gl_PerVertex named block:
out gl_PerVertex {
vec4 gl_Position;
float gl_PointSize;
float gl_ClipDistance[];
};
In fragment shaders, it is intrinsically declared as:
in float gl_ClipDistance[] ;
DESCRIPTION¶
The
gl_ClipDistance variable provides a forward compatible mechanism for
controlling user clipping. The element
gl_ClipDistance[
i]
specifies a clip distance for each user clip plane
i. A distance of 0.0
means that the vertex is on the plane, a positive distance means that the
vertex is insider the clip plane, and a negative distance means that the point
is outside the clip plane. The clip distances will be linearly interpolated
across the primitive and the portion of the primitive with interpolated
distances less than 0.0 will be clipped.
The
gl_ClipDistance array is initially predeclared as unsized and must be
sized by the shader either by redeclaring it with an explicit size, or by
indexing it with only integral constant expressions. The array must be sized
to include all clip planes that are enabled via the OpenGL API; if the size
does not include all enabled planes, results are undefined. The size may be at
most
gl_MaxClipDistances. The number of varying components consumed by
gl_ClipDistance will match the size of the array, no matter how many
planes are enabled. The shader must also set all values in
gl_ClipDistance that have been enabled via the OpenGL API, or results
are undefined. Values written into
gl_ClipDistance planes that are not
enabled have no effect.
In the vertex, tessellation evaluation and geometry languages, a single global
instance of the
gl_PerVertex named block is available and its
gl_ClipDistance member is an output that receives the clip distances
for the current vertex. It may be written at any time during shader execution.
The value written to
gl_ClipDistance 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_ClipDistance
members hold clip distances for each of the control points, which become
available as inputs to the subsequent tessellation evaluation shader.
The value of
gl_ClipDistance (or the
gl_ClipDistance 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_ClipDistance. It is also undefined after the geometry processing stage if
the geometry shader executable calls
EmitVertex() without having
written
gl_ClipDistance 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. Only
elements of the
gl_ClipDistance array that correspond to enabled clip
planes have defined values.
VERSION SUPPORT¶
|
OpenGL Shading Language Version |
|
|
|
|
|
|
|
|
|
|
|
Variable Name |
1.10 |
1.20 |
1.30 |
1.40 |
1.50 |
3.30 |
4.00 |
4.10 |
4.20 |
4.30 |
4.40 |
4.50 |
gl_ClipDistance |
- |
- |
✔ |
✔ |
✔ |
✔ |
✔ |
✔ |
✔ |
✔ |
✔ |
✔ |
Versions 1.30 to 1.40 - vertex shader only.
Versions 1.50 to 3.30 - vertex and geometry shaders only.
SEE ALSO¶
gl_CullDistance(),
gl_PointSize()
COPYRIGHT¶
Copyright © 2011-2014 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/.
COPYRIGHT¶
Copyright © 2011-2014 Khronos Group