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Name Strings
IBM Id: cull_vertex.spec,v 1.4 2000/02/11 15:11:46 brokensh Exp
EXT_cull_vertex affects the definition of this extension
IBM_cull_vertex provides a subset of the vertex culling functionality
found in EXT_cull_vertex without providing a guarantee that faces will
be culled because of it. EXT_cull_vertex is a technically superior
solution, but the vertex culling aspect of IBM_cull_vertex provides
generally useful function cheaply (without imposing the mandated
culling found in EXT_cull_vertex).
* Should the application be allowed to specify eye direction explicitly,
as is the case in EXT_cull_vertex? Not in this specification;
addressed by the full EXT_cull_vertex extension.
* Should a control be provided to reverse the eye direction, which would
be necessary to cull correctly if DepthRange is specified with near >
far? Or if the normals specified were reversed? FrontFace could be
used in combination with two sided lighting to produce realistic
lighting effects if the normals were consistenly reversed. Neither is
addressed in the current specification. Both are addressed by
* Should a control be provided to perform vertex culling with a local eye
position? Not in this specification; addressed by EXT_cull_vertex.
* Should forward compatibility between IBM_cull_vertex and
EXT_cull_vertex be provided (i.e., applications must change if/when we
fully implement EXT_cull_vertex)? Probably not.
New Procedures and Functions
New Tokens
Accepted by the <cap> parameter of Enable, Disable, and IsEnabled, and by
the <pname> parameter of GetBooleanv, GetIntegerv, GetFloatv, and
Additions to Chapter 2 of the GL Specification (OpenGL Operation)
Before the discussion of Clipping, add a description of vertex culling
similar to that found in the EXT_cull_vertex extension:
Vertex Culling
When vertex culling is enabled, vertices are classified as front or
back facing according to the sign of the dot product between the normal
at the vertex and an eye direction vector from the vertex toward the
eye position. When (normal dot eye_direction) <= 0 the vertex is
classified as back facing. When (normal dot eye_direction) > 0 the
vertex is classified as front facing. Vertices are culled when the
face orientation determined by the dot product is the same as the face
specified by CullFace. When all of the vertices of a polygon are
culled, then the polygon may be culled. Unlike EXT_cull_vertex, vertex
culling using IBM_cull_vertex does not necessarily result in polygons
being culled even if all of the vertices of the polygon are culled.
The eye direction is determined by transforming the column vector (0,
0, 1) by the upper leftmost 3x3 matrix taken from the inverse of the
modelview matrix. The eye direction is undefined if the modelview
matrix is singular or nearly singular. This operation in effect
projects the z axis in eye coordinates back into object space. If the
projection matrix or DepthRange settings cause the z axis in window
coordinates to be misaligned with the z axis in eye coordinates, this
extension should not be used.
Vertex culling is performed independently of face culling. Polygons on
the silhouettes of objects may have both front and back facing
vertices. Since polygons are culled only if all of their vertices are
culled and are not necessarily culled by IBM_cull_vertex even in that
case, face culling may have to be used in addition to vertex culling in
order to correctly cull silhouette polygons.
Additions to Chapter 3 of the GL Specification (Rasterization)
Additions to Chapter 4 of the GL Specification (Per-Fragment Operations
and the Framebuffer)
Additions to Chapter 5 of the GL Specification (Special Functions)
Additions to Chapter 6 of the GL Specification (State and State Requests)
Additions to the GLX Specification
GLX Protocol
New State
Get Value Type Get Command Value Sec Attrib
----------------------------- ---- ----------- ----- ---- -------
CULL_VERTEX_IBM B IsEnabled FALSE -- transform/
New Implementation Dependent State