blob: 94926cf41a1b150ee0ccda338b9282cf4cbc9382 [file] [log] [blame]
XXX - Not complete yet!!!
Name Strings
$Date: 1996/11/21 00:52:20 $ $Revision: 1.3 $
This extension introduces a method for culling vertexes in object
space based on the value of the dot product between the normal at
the vertex and a culling eye direction.
Culling a polygon by examining its vertexes in object space can be
more efficient than screen space polygon culling since the transformation
to screen space (which may include a division by w) can be avoided for
culled vertexes. Also, vertex culling can be computed before vertexes
are assembled into primitives. This is a useful property when drawing
meshes with shared vertexes, since a vertex can be culled once, and the
resulting state can be used for all primitives which share the vertex.
* Should FrontFace affect the comparison of the dot product?
It may be useful. For non-local eye positions it is easy for
the application to flip the eye direction in order to cull
either front or back faces. This doesn't work as well for
local eye positions. We'll defer this for now; it is easy
to add as an extension later.
* Could determine the eye position/direction in object space
by transforming the vector (0, 0, 1, 0) by the inverse of
the composite of the modelview and projection transformations.
Seems better to have the application provide the eye position/direction
than to have OpenGL pick one arbitrarily.
New Procedures and Functions
void CullParameterfvEXT (enum pname, float *params)
void CullParameterdvEXT (enum pname, double *params)
New Tokens
Accepted by the <cap> parameter of Enable, Disable, and
IsEnabled, and by the <pname> parameter of GetBooleanv,
GetIntegerv, GetFloatv, and GetDoublev:
Accepted by the <pname> parameter of CullParameterfvEXT,
CullParameterdvEXT, GetBooleanv, GetIntegerv, GetFloatv, and
Additions to Chapter 2 of the 1.1 Specification (OpenGL Operation)
Before the discussion of Clipping, add a description of
vertex culling.
Vertex Culling
Vertex culling may be used to eliminate vertexes which are
part of back facing primitives. Vertex culling is enabled
or disabled by using the Enable or Disable commands with
the symbolic constant CULL_VERTEX_EXT.
When vertex culling is enabled, vertexes 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. Vertexes are culled when the
face orientation determined by the dot product is the same
as the face specified by CullFace. When all of the vertexes
of a polygon are culled, then the polygon is culled.
The eye direction used for vertex culling is determined
by the culling eye position. The culling eye position
is homogeneous (like a light position). When the w component
of the position is non-zero, the position is local and the
eye direction at each vertex is computed by subtracting
the vertex position from the eye position. When the w
component of the position is zero, the position is non-local
and the is used as the eye direction for all vertexes.
The culling eye position is specified by the CullParameter
command. Positions specified when <pname> is
CULL_VERTEX_EYE_POSITION_EXT are in eye space and are
transformed by the inverse of the current MODELVIEW
transformation. Positions specified when <pname> is
CULL_VERTEX_OBJECT_POSITION_EXT are in object space and are
used directly.
Vertex culling is performed independently of face culling.
Polygons on the silhouettes of objects may have both front
and back facing vertexes. Since polygons are culled only
when all of their vertexes are culled, 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 1.1 Specification (Rasterization)
Additions to Chapter 4 of the 1.1 Specification (Per-Fragment Operations
and the Frame Buffer)
Additions to Chapter 5 of the 1.1 Specification (Special Functions)
Additions to Chapter 6 of the 1.1 Specification (State and State Requests)
Additions to the GLX Specification
XXX - Not complete yet!!!
GLX Protocol
XXX - Not complete yet!!!
INVALID_ENUM if <pname> parameter to CullParameterfvEXT or
CullParameterdvEXT is not CULL_VERTEX_EYE_POSITION_EXT, or
INVALID_OPERATION if CullParameterfvEXT or CullParameterdvEXT called
between execution of Begin and the corresponding execution of End.
New State
Get Value Get Command Type Value Attrib
--------- ----------- ---- ------- ------
CULL_VERTEX_EXT IsEnabled B False transform/enable
CULL_VERTEX_OBJECT_POSITION_EXT GetFloatv P (0,0,1,0) transform
CULL_VERTEX_EYE_POSITION_EXT GetFloatv P (0,0,1,0) transform
New Implementation Dependent State