GL_INTEL_texture_scissor

Name

 INTEL_texture_scissor

Name Strings

 GL_INTEL_texture_scissor

Version

   $Date: 1997/05/09 02:42:17 $ $Revision: 1.1 $

Number

   135

Dependencies

    EXT_texture3D is affected by the definition of this extension
    EXT_texture is required

Overview

This extension enables one to render with clipping based on texture 
coordinates. The only way to clip a primitive while rendering in GL 1.1 is
to define a Clipping Plane. Clipping of the primitive is then  effected
based on the spatial coordinates of the primitive with reference to the user
defined clipping plane. This extension adds a new dimension to the clipping
of primitives, namely-clipping based on texture coordinates as opposed
to spatial coordinates.

One of the advantages of clipping planes, is that for a given viewing volume
all the primitives that make up the models in the current scene may be
clipped against a set of clipping planes that are defined once at the start
for the current viewing volume. Two issues that are worth  mentioning in
this case are :

1. The cost of defining clipping planes is amortized over all primitives in
the scene
2. The job of determining the equation of clipping planes that  enclose a
convex region defining the  current viewing volume is fairly straightforward.

Both the above factors that make clip planes so profitable in 3D graphics
rendering, do not necessarily hold true when dealing with 3D texture mapped
based volume rendering of large volume datasets. This    is specifically
true when one uses 3D texture mapping hardware to do volumetric morphing of
large volumes. Three factors require the ability to clip based on texture
coordinates

First, most texture memory in machines are never large enough to fit the
entire volumetric dataset. Hence, to use 3D texture mapping to render
volumes, one has to partition the volume into blocks large enough to fit in
the available texture memory. This requires one to clip the primitives the
straddle texture block boundaries, for oblique viewing angles. In
straightforward volume rendering, this clipping may be easily done through
the definition of clip planes in GL, because there is a linear correspondence
between the texture coordinate 'w' and the spatial coordinate 'z' of each
point in the volume.

Second, when one morphs a source volume to obtain the target volume, the
linear correspondence between the texture coordinate 'w' and the spatial
coordinate 'z' of each point in the volume is lost. Because    morphs are
inherently non-linear different points in the original volume (identified
by a unique 'z' and an unique 'w') are mapped to different locations in the
target and in the process lose their linear relationship. Thus no one set
of clipping planes can be defined for all the primitives that result when
rendering morphed volumes. 

Third, even if one were to try and define clip planes for each of the
resulting primitives when rendering morphed volumes, the equations are not
easy to determine - both in terms of speed and ease of     implementation!
One has to first determine the intersection points where the texture
coordinate goes out of bounds, i.e. below 0.0 or above 1.0, and then define
a planes that would encapsulte those parts of the primitive that have
texture coordinates in the correct range. 

Finally, the fact that all this has to be done in software slows down the
process tremendously. 

An easier solution would be to enable, a simple test in hardware that checks
to see if a fragments texel coordinates are within a  user defined range. If
it passes this test the fragment is rendered, if not it is discarded. 

Besides the immediate advantage in performance, this solution would also
enable one to simplify application code considerably.

An added benefit of this extension is that it provides one with the facility
of tiling 2D textures in 1D, 2D or 3D.

New Procedures and Functions

void    TexScissorINTEL(GLenum    target, 
                        GLclampf   tlow,
                        GLclampf   thigh);
void    TexScissorFuncINTEL(GLenum    target, Glenum lfunc, Glenum hfunc)

New Tokens

Accepted by the <cap> parameter of  Enable, Disable, IsEnabled :

TEXTURE_SCISSOR_INTEL

Accepted by the <pname> parameter of parameter of GetBooleanv, GetIntegerv,
GetFloatv, and GetDoublev 

TEXTURE_SCISSOR_S_INTEL [ returns two values : low and high ]
TEXTURE_SCISSOR_T_INTEL [ returns two values : low and high ]
TEXTURE_SCISSOR_R_INTEL [ returns two values : low and high ]

by the <target> parameter of glTexScissorINTEL and glTexScissorFuncINTEL

	GL_S
	GL_T
	GL_R

Additions to Chapter 2 of the GL 1.1 Specification

None

Additions to Chapter 3 of the GL 1.1 Specification (Rasterization)

None

Additions to Chapter 4 of the GL 1.1 Specification (Per Fragment Operations and the Framebuffer)

The INTEL_texturescissor conditionally discards a fragment based on the
outcome of a comparison between the texture coordinate value of the incoming
fragment and the texture bounds set by the TexScissorINTEL() function. The
test is controlled with 

void TexScissorINTEL(GLenum target, GLclampf tlow, GLclampf thigh)

The texture scissor test is controlled by the texture function set by
TexScissorFuncINTEL() :

 void TexScissorFuncINTEL(GLenum target, GLenum lfunc, Glenum hfunc)

lfunc can be any of GL_GREATER, GL_GEQUAL, or GL_EQUAL while lfunc can be
any of GL_LESS, GL_LEQUAL or GL_EQUAL. These values enable one to handle
boundary cases that involve adjacent regions overlapping.

Thus, for lfunc set to GL_GEQUAL and hfunc set to GL_LEQUAL, if the target
texture coordinate (s, t, or r) is greater than or equal to tlow and less
than or equal to thigh (i.e. tlow <= s, t, r <= thigh), the texture test
passes. Otherwise, the test fails and the fragment is discarded. The test
is enabled or disabled with the Enable and Disable commands, using the
symbolic constant TEXTURE_SCISSOR_INTEL. When disabled, the texture scissor
test is not performed and it is as if the texture scissor test always passes.

'tlow' and 'thigh' are two floating point values in the range <0.0, 1.0>
(tlow is always less than or equal to thigh). The default values are 0.0 for
tlow and 1.0 for thigh. Initially, the texture scissor test is disabled by
default.

Additions to Chapter 5 of the GL 1.1 Specification (Special Functions)

None

Additions to Chapter 6 of the GL 1.1 Specification (State and State Requests)

None

Additions to GLX Specification

None

GLX Protocol

None

Dependencies on EXT_texture3D

If EXT_texture3D is not supported, references to texture target GL_R are
invalid and should be ignored.

Errors

INVALID_VALUE is generated if glTexScissorINTEL() is called with  tlow > thigh.

INVALID_VALUE is generated if glTexScissorINTEL() is called with  tlow < 0.0
or  tlow > 1.0

INVALID_VALUE is generated if glTexScissorINTEL() is called with  thigh < 0.0
or thigh > 1.0

INVALID_VALUE is generated if glTexScissorFuncINTEL() is called with lfunc or
hfunc set to any other value other than [GL_GREATER, GL_GEQUAL, or GL_EQUAL]
and [GL_LESS, GL_LEQUAL or GL_EQUAL] respectively.

New State

None

New Implementation Dependent State

Get Value                  Get Command Type   Initial Value        Attribute
TEXTURE_SCISSOR_INTEL      IsEnabled    B        False         texture / enable
TEXTURE_SCISSOR_S_INTEL    GetFloatv    R      0.0, 1.0        texture / enable
TEXTURE_SCISSOR_T_INTEL    GetFloatv    R      0.0, 1.0        texture / enable
TEXTURE_SCISSOR_R_INTEL    GetFloatv    R      0.0, 1.0        texture / enable
TEXTURE_SCISSOR_FUNC_INTEL GetIntegerv  Z6 GL_GEQUAL, GL_LESS  texture / enable

Implementation Support

   List of OpenGL implementations supporting the GL_INTEL_texture_scissor extension

Original File

   Original text file for the GL_INTEL_texture_scissor extension