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GL_ARB_texture_cube_map
ARB_texture_cube_map
Name Strings
GL_ARB_texture_cube_map
Notice
Copyright OpenGL Architectural Review Board, 1999.
Contact
Michael Gold, NVIDIA (gold 'at' nvidia.com)
Status
Complete. Approved by ARB on 12/8/1999
Version
Last Modified Date: December 14, 1999
Number
ARB Extension #7
Dependencies
None.
Written based on the wording of the OpenGL 1.2.1 specification but
not dependent on it.
Overview
This extension provides a new texture generation scheme for cube
map textures. Instead of the current texture providing a 1D, 2D,
or 3D lookup into a 1D, 2D, or 3D texture image, the texture is a
set of six 2D images representing the faces of a cube. The (s,t,r)
texture coordinates are treated as a direction vector emanating from
the center of a cube. At texture generation time, the interpolated
per-fragment (s,t,r) selects one cube face 2D image based on the
largest magnitude coordinate (the major axis). A new 2D (s,t) is
calculated by dividing the two other coordinates (the minor axes
values) by the major axis value. Then the new (s,t) is used to
lookup into the selected 2D texture image face of the cube map.
Unlike a standard 1D, 2D, or 3D texture that have just one target,
a cube map texture has six targets, one for each of its six 2D texture
image cube faces. All these targets must be consistent, complete,
and have equal width and height (ie, square dimensions).
This extension also provides two new texture coordinate generation modes
for use in conjunction with cube map texturing. The reflection map
mode generates texture coordinates (s,t,r) matching the vertex's
eye-space reflection vector. The reflection map mode
is useful for environment mapping without the singularity inherent
in sphere mapping. The normal map mode generates texture coordinates
(s,t,r) matching the vertex's transformed eye-space
normal. The normal map mode is useful for sophisticated cube
map texturing-based diffuse lighting models.
The intent of the new texgen functionality is that an application using
cube map texturing can use the new texgen modes to automatically
generate the reflection or normal vectors used to look up into the
cube map texture.
An application note: When using cube mapping with dynamic cube
maps (meaning the cube map texture is re-rendered every frame),
by keeping the cube map's orientation pointing at the eye position,
the texgen-computed reflection or normal vector texture coordinates
can be always properly oriented for the cube map. However if the
cube map is static (meaning that when view changes, the cube map
texture is not updated), the texture matrix must be used to rotate
the texgen-computed reflection or normal vector texture coordinates
to match the orientation of the cube map. The rotation can be
computed based on two vectors: 1) the direction vector from the cube
map center to the eye position (both in world coordinates), and 2)
the cube map orientation in world coordinates. The axis of rotation
is the cross product of these two vectors; the angle of rotation is
the arcsin of the dot product of these two vectors.
Issues
Should we place the normal/reflection vector in the (s,t,r) texture
coordinates or (s,t,q) coordinates?
RESOLUTION: (s,t,r). Even if hardware uses "q" for the third
component, the API should claim to support generation of (s,t,r)
and let the texture matrix (through a concatenation with the
user-supplied texture matrix) move "r" into "q".
Should the texture coordinate generation functionality for cube
mapping be specified as a distinct extension from the actual cube
map texturing functionality?
RESOLUTION: NO. Real applications and real implementations of
cube mapping will tie the texgen and texture generation functionality
together. Applications won't have to query two separate
extensions then.
While applications will almost always want to use the texgen
functionality for automatically generating the reflection or normal
vector as texture coordinates (s,t,r), this extension does permit
an application to manually supply the reflection or normal vector
through glTexCoord3f explicitly.
Note that the NV_texgen_reflection extension does "unbundle"
the texgen functionality from cube maps.
Should you be able to have some texture coordinates computing
REFLECTION_MAP_ARB and others not? Same question with NORMAL_MAP_ARB.
RESOLUTION: YES. This is the way that SPHERE_MAP works. It is
not clear that this would ever be useful though.
Should something special be said about the handling of the q
texture coordinate for this spec?
RESOLUTION: NO. But the following paragraph is useful for
implementors concerned about the handling of q.
The REFLECTION_MAP_ARB and NORMAL_MAP_ARB modes are intended to supply
reflection and normal vectors for cube map texturing hardware.
When these modes are used for cube map texturing, the generated
texture coordinates can be thought of as an reflection vector.
The value of the q texture coordinate then simply scales the
vector but does not change its direction. Because only the vector
direction (not the vector magnitude) matters for cube map texturing,
implementations are free to leave q undefined when any of the s,
t, or r texture coordinates are generated using REFLECTION_MAP_ARB
or NORMAL_MAP_ARB.
How should the cube faces be labeled?
RESOLUTION: Match the render man specification's names of "px"
(positive X), "nx" (negative x), "py", "ny", "pz", and "nz".
There does not actually need to be an "ordering for the faces"
(Direct3D 7.0 does number their cube map faces.) For this
extension, the symbolic target names (TEXTURE_CUBE_MAP_POSITIVE_X_ARB,
etc) is sufficient without requiring any specific ordering.
What coordinate system convention should be used? LHS or RHS?
RESOLUTION: The coordinate system is left-handed if you think
of yourself within the cube. The coordinate system is
right-handed if you think of yourself outside the cube.
This matches the convention of the RenderMan interface. If
you look at Figure 12.8 (page 265) in "The RenderMan Companion",
think of the cube being folded up with the observer inside
the cube. Then the coordinate system convention is
left-handed.
The spec just linearly interpolates the reflection vectors computed
per-vertex across polygons. Is there a problem interpolating
reflection vectors in this way?
Probably. The better approach would be to interpolate the eye
vector and normal vector over the polygon and perform the reflection
vector computation on a per-fragment basis. Not doing so is likely
to lead to artifacts because angular changes in the normal vector
result in twice as large a change in the reflection vector as normal
vector changes. The effect is likely to be reflections that become
glancing reflections too fast over the surface of the polygon.
Note that this is an issue for REFLECTION_MAP_ARB, but not
NORMAL_MAP_ARB.
What happens if an (s,t,q) is passed to cube map generation that
is close to (0,0,0), ie. a degenerate direction vector?
RESOLUTION: Leave undefined what happens in this case (but
may not lead to GL interruption or termination).
Note that a vector close to (0,0,0) may be generated as a
result of the per-fragment interpolation of (s,t,r) between
vertices.
Do we need a distinct proxy texture mechanism for cube map
textures?
RESOLUTION: YES. Cube map textures take up six times the
memory as a conventional 2D image texture so proxy 2D texture
determinations won't be of value for a cube map texture.
Cube maps need their own proxy target.
Should we require the 2D texture image width and height to
be identical (ie, square only)?
RESOLUTION: YES. This limitation is quite a reasonable limitation
and DirectX 7 has the same limitation.
This restriction is enforced by generating an INVALID_VALUE
when calling TexImage2D or CopyTexImage2D with a non-equal
width and height.
Some consideration was given to enforcing the "squarness"
constraint as a texture consistency constraint. This is
confusing however since the squareness is known up-front
at texture image specification time so it seems confusing
to silently report the usage error as a texture consistency
issue.
Texture consistency still says that all the level 0 textures
of all six faces must have the same square size.
If some combination of 1D, 2D, 3D, and cube map texturing is
enabled, which really operates?
RESOLUTION: Cube map texturing. In OpenGL 1.2, 3D takes
priority over 2D takes priority over 1D. Cube mapping should
take priority over all conventional n-dimensional texturing
schemes.
Does anything need to be said about combining cube mapping with
multitexture?
RESOLUTION: NO. Cube mapping should be available on all texture
units. The hardware should fully orthogonal in its handling of
cube map textures.
Does it make sense to support borders for cube map textures.
Actually, it does. It would be nice if the texture border pixels
match the appropriate texels from the edges of the other cube map
faces that they junction with. For this reason, we'll leave the
texture border capability implicitly supported.
How does mipmap level-of-detail selection work for cube map
textures?
The existing spec's language about LOD selection is fine.
Should the implementation dependent value for the maximum
texture size for a cube map be the same as MAX_TEXTURE_SIZE?
RESOLUTION: NO. OpenGL 1.2 has a different MAX_3D_TEXTURE_SIZE
for 3D textures, and cube maps should take six times more space
than a 2D texture map of the same width & height. The implementation
dependent MAX_CUBE_MAP_TEXTURE_SIZE_ARB constant should be used for
cube maps then.
Note that the proxy cube map texture provides a better way to
find out the maximum cube map texture size supported since the
proxy mechanism can take into account the internal format, etc.
In section 3.8.10 when the "largest magnitude coordinate direction"
is choosen, what happens if two or more of the coordinates (rx,ry,rz)
have the identical magnitude?
RESOLUTION: Implementations can define their own rule to choose
the largest magnitude coordinate direction whne two or more of the
coordinates have the identical magnitude. The only restriction is
that the rule must be deterministic and depend only on (rx,ry,rz).
In practice, (s,t,r) is interpolated across polygons so the cases
where |s|==|t|, etc. are pretty arbitary (the equality depends on
interpolation precision). This extension could mandate a particular
rule, but that seems heavy-handed and there is no good reason that
multiple vendors should be forced to implement the same rule.
Should there be limits on the supported border modes for cube maps?
RESOLUTION: NO. The specificiation is written so that cube map
texturing proceeds just like conventional 2D texture mapping once
the face determination is made.
Therefore, all OpenGL texture wrap modes should be supported though
some modes are clearly inappropriate for cube maps. The WRAP mode
is almost certainly incorrect for cube maps. Likewise, the CLAMP
mode without a texture border is almost certainly incorrect for cube
maps. CLAMP when a texture border is present and CLAMP_TO_EDGE are
both reasonably suited for cube maps. Ideally, CLAMP with a texture
border works best if the cube map edges can be replicated in the
approriate texture borders of adjacent cube map faces. In practice,
CLAMP_TO_EDGE works reasonably well in most circumstances.
Perhaps another extension could support a special cube map wrap
mode that automatically wraps individual texel fetches to the
appropriate adjacent cube map face. The benefit from such a mode
is small and the implementation complexity is involved so this wrap
mode should not be required for a basic cube map texture extension.
How is mipmap LOD selection handled for cube map textures?
RESOLUTION: The specification is written so that cube map texturing
proceeds just like conventional 2D texture mapping once the face
determination is made.
Thereforce, the partial differentials in Section 3.8.5 (page
126) should be evaluated for the u and v parameters based on the
post-face determination s and t.
In Section 2.10.3 "Normal Transformation", there are several versions
of the eye-space normal vector to choose from. Which one should
the NORMAL_MAP_ARB texgen mode use?
RESOLUTION: nf. The nf vector is the final normal, post-rescale
normal and post-normalize. In practice, the rescale normal and
normalize operations do not change the direction of the vector
so the choice of which version of transformed normal is used is
not important for cube maps.
None
Accepted by the <param> parameters of TexGend, TexGenf, and TexGeni
when <pname> parameter is TEXTURE_GEN_MODE:
NORMAL_MAP_ARB 0x8511
REFLECTION_MAP_ARB 0x8512
When the <pname> parameter of TexGendv, TexGenfv, and TexGeniv is
TEXTURE_GEN_MODE, then the array <params> may also contain
NORMAL_MAP_ARB or REFLECTION_MAP_ARB.
Accepted by the <cap> parameter of Enable, Disable, IsEnabled, and
by the <pname> parameter of GetBooleanv, GetIntegerv, GetFloatv,
and GetDoublev, and by the <target> parameter of BindTexture,
GetTexParameterfv, GetTexParameteriv, TexParameterf, TexParameteri,
TexParameterfv, and TexParameteriv:
TEXTURE_CUBE_MAP_ARB 0x8513
Accepted by the <pname> parameter of GetBooleanv, GetIntegerv,
GetFloatv, and GetDoublev:
TEXTURE_BINDING_CUBE_MAP_ARB 0x8514
Accepted by the <target> parameter of GetTexImage,
GetTexLevelParameteriv, GetTexLevelParameterfv, TexImage2D,
CopyTexImage2D, TexSubImage2D, and CopySubTexImage2D:
TEXTURE_CUBE_MAP_POSITIVE_X_ARB 0x8515
TEXTURE_CUBE_MAP_NEGATIVE_X_ARB 0x8516
TEXTURE_CUBE_MAP_POSITIVE_Y_ARB 0x8517
TEXTURE_CUBE_MAP_NEGATIVE_Y_ARB 0x8518
TEXTURE_CUBE_MAP_POSITIVE_Z_ARB 0x8519
TEXTURE_CUBE_MAP_NEGATIVE_Z_ARB 0x851A
Accepted by the <target> parameter of GetTexLevelParameteriv,
GetTexLevelParameterfv, GetTexParameteriv, and TexImage2D:
PROXY_TEXTURE_CUBE_MAP_ARB 0x851B
Accepted by the <pname> parameter of GetBooleanv, GetDoublev,
GetIntegerv, and GetFloatv:
MAX_CUBE_MAP_TEXTURE_SIZE_ARB 0x851C
Additions to Chapter 2 of the 1.2 Specification (OpenGL Operation)
-- Section 2.10.4 "Generating Texture Coordinates"
Change the last sentence in the 1st paragraph (page 37) to:
"If <pname> is TEXTURE_GEN_MODE, then either <params> points to
or <param> is an integer that is one of the symbolic constants
OBJECT_LINEAR, EYE_LINEAR, SPHERE_MAP, REFLECTION_MAP_ARB, or
NORMAL_MAP_ARB."
Add these paragraphs after the 4th paragraph (page 38):
"If TEXTURE_GEN_MODE indicates REFLECTION_MAP_ARB, compute the
reflection vector r as described for the SPHERE_MAP mode. Then the
value assigned to an s coordinate (the first TexGen argument value
is S) is s = rx; the value assigned to a t coordinate is t = ry;
and the value assigned to a r coordinate is r = rz. Calling TexGen
with a <coord> of Q when <pname> indicates REFLECTION_MAP_ARB
generates the error INVALID_ENUM.
If TEXTURE_GEN_MODE indicates NORMAL_MAP_ARB, compute the normal
vector nf as described in section 2.10.3. Then the value assigned
to an s coordinate (the first TexGen argument value is S) is s =
nfx; the value assigned to a t coordinate is t = nfy; and the
value assigned to a r coordinate is r = nfz. (The values nfx, nfy,
and nfz are the components of nf.) Calling TexGen with a <coord>
of Q when <pname> indicates NORMAL_MAP_ARB generates the error
INVALID_ENUM.
The last paragraph's first sentence (page 38) should be changed to:
"The state required for texture coordinate generation comprises a
five-valued integer for each coordinate indicating coordinate
generation mode, ..."
Additions to Chapter 3 of the 1.2 Specification (Rasterization)
-- Section 3.6.5 "Pixel Transfer Operations" under "Convolution"
Change this paragraph (page 103) to say:
... "If CONVOLUTION_2D is enabled, the two-dimensional convolution
filter is applied only to the two-dimensional images passed to
DrawPixels, CopyPixels, ReadPixels, TexImage2D, TexSubImage2D,
CopyTexImage2D, CopyTexSubImage2D, and CopyTexSubImage3D, and
returned by GetTexImage with one of the targets TEXTURE_2D,
TEXTURE_CUBE_MAP_POSITIVE_X_ARB, TEXTURE_CUBE_MAP_NEGATIVE_X_ARB,
TEXTURE_CUBE_MAP_POSITIVE_Y_ARB, TEXTURE_CUBE_MAP_NEGATIVE_Y_ARB,
TEXTURE_CUBE_MAP_POSITIVE_Z_ARB, or TEXTURE_CUBE_MAP_NEGATIVE_Z_ARB."
-- Section 3.8.1 "Texture Image Specification"
Change the second and third to last sentences on page 116 to:
"<target> must be one of TEXTURE_2D for a 2D texture, or one of
TEXTURE_CUBE_MAP_POSITIVE_X_ARB, TEXTURE_CUBE_MAP_NEGATIVE_X_ARB,
TEXTURE_CUBE_MAP_POSITIVE_Y_ARB, TEXTURE_CUBE_MAP_NEGATIVE_Y_ARB,
TEXTURE_CUBE_MAP_POSITIVE_Z_ARB, or TEXTURE_CUBE_MAP_NEGATIVE_Z_ARB
for a cube map texture. Additionally, <target> can be either
PROXY_TEXTURE_2D for a 2D proxy texture or PROXY_TEXTURE_CUBE_MAP_ARB
for a cube map proxy texture as discussed in section 3.8.7."
Add the following paragraphs after the first paragraph on page 117:
"A 2D texture consists of a single 2D texture image. A cube
map texture is a set of six 2D texture images. The six cube map
texture targets form a single cube map texture though each target
names a distinct face of the cube map. The TEXTURE_CUBE_MAP_*_ARB
targets listed above update their appropriate cube map face 2D
texture image. Note that the six cube map 2D image tokens such as
TEXTURE_CUBE_MAP_POSITIVE_X_ARB are used when specifying, updating,
or querying one of a cube map's six 2D image, but when enabling
cube map texturing or binding to a cube map texture object (that is
when the cube map is accessed as a whole as opposed to a particular
2D image), the TEXTURE_CUBE_MAP_ARB target is specified.
When the target parameter to TexImage2D is one of the six cube map
2D image targets, the error INVALID_VALUE is generated if the width
and height parameters are not equal.
If cube map texturing is enabled at the time a primitive is
rasterized and if the set of six targets are not "cube complete",
then it is as if texture mapping were disabled. The targets of
a cube map texture are "cube complete" if the array 0 of all six
targets have identical, positive, and square dimensions, the array
0 of all six targets were specified with the same internalformat,
and the array 0 of all six targets have the same border width."
After the 14th paragraph (page 116) add:
"In a similiar fashion, the maximum allowable width and height
(they must be the same) of a cube map texture must be at least
2^(k-lod)+2bt for image arrays level 0 through k, where k is the
log base 2 of MAX_CUBE_MAP_TEXTURE_SIZE_ARB."
-- Section 3.8.2 "Alternate Texture Image Specification Commands"
Update the second paragraph (page 120) to say:
... "Currently, <target> must be
TEXTURE_2D, TEXTURE_CUBE_MAP_POSITIVE_X_ARB,
TEXTURE_CUBE_MAP_NEGATIVE_X_ARB, TEXTURE_CUBE_MAP_POSITIVE_Y_ARB,
TEXTURE_CUBE_MAP_NEGATIVE_Y_ARB, TEXTURE_CUBE_MAP_POSITIVE_Z_ARB,
or TEXTURE_CUBE_MAP_NEGATIVE_Z_ARB." ...
Add after the second paragraph (page 120), the following:
"When the target parameter to CopyTexImage2D is one of the six cube
map 2D image targets, the error INVALID_VALUE is generated if the
width and height parameters are not equal."
Update the fourth paragraph (page 121) to say:
... "Currently the target arguments of TexSubImage1D and
CopyTexSubImage1D must be TEXTURE_1D, the <target> arguments of
TexSubImage2D and CopyTexSubImage2D must be one of TEXTURE_2D,
TEXTURE_CUBE_MAP_POSITIVE_X_ARB, TEXTURE_CUBE_MAP_NEGATIVE_X_ARB,
TEXTURE_CUBE_MAP_POSITIVE_Y_ARB, TEXTURE_CUBE_MAP_NEGATIVE_Y_ARB,
TEXTURE_CUBE_MAP_POSITIVE_Z_ARB, or TEXTURE_CUBE_MAP_NEGATIVE_Z_ARB,
and the <target> arguments of TexSubImage3D and CopyTexSubImage3D
must be TEXTURE_3D." ...
-- Section 3.8.3 "Texture Parameters"
Change paragraph one (page 124) to say:
... "<target> is the target, either TEXTURE_1D,
TEXTURE_2D, TEXTURE_3D, or TEXTURE_CUBE_MAP_ARB." ...
Add a final paragraph saying:
"Texture parameters for a cube map texture apply to cube map
as a whole; the six distinct 2D texture images use the
texture parameters of the cube map itself.
-- Section 3.8.5 "Texture Minification" under "Mipmapping"
Change the first full paragraph on page 130 to:
... "If texturing is enabled for one-, two-, or three-dimensional
texturing but not cube map texturing (and TEXTURE_MIN_FILTER
is one that requires a mipmap) at the time a primitive is
rasterized and if the set of arrays TEXTURE_BASE_LEVEL through q =
min{p,TEXTURE_MAX_LEVEL} is incomplete, based on the dimensions of
array 0, then it is as if texture mapping were disabled."
Follow the first full paragraph on page 130 with:
"If cube map texturing is enabled and TEXTURE_MIN_FILTER is one that
requires mipmap levels at the time a primitive is rasterized and
if the set of six targets are not "mipmap cube complete", then it
is as if texture mapping were disabled. The targets of a cube map
texture are "mipmap cube complete" if the six cube map targets are
"cube complete" and the set of arrays TEXTURE_BASE_LEVEL through
q are not incomplete (as described above)."
-- Section 3.8.7 "Texture State and Proxy State"
Change the first sentence of the first paragraph (page 131) to say:
"The state necessary for texture can be divided into two categories.
First, there are the nine sets of mipmap arrays (one each for the
one-, two-, and three-dimensional texture targets and six for the
cube map texture targets) and their number." ...
Change the second paragraph (page 132) to say:
"In addition to the one-, two-, three-dimensional, and the six cube
map sets of image arrays, the partially instantiated one-, two-,
and three-dimensional and one cube map sets of proxy image arrays
are maintained." ...
After the third paragraph (page 132) add:
"The cube map proxy arrays are operated on in the same manner
when TexImage2D is executed with the <target> field specified as
PROXY_TEXTURE_CUBE_MAP_ARB with the addition that determining that a
given cube map texture is supported with PROXY_TEXTURE_CUBE_MAP_ARB
indicates that all six of the cube map 2D images are supported.
Likewise, if the specified PROXY_TEXTURE_CUBE_MAP_ARB is not
supported, none of the six cube map 2D images are supported."
Change the second sentence of the fourth paragraph (page 132) to:
"Therefore PROXY_TEXTURE_1D, PROXY_TEXTURE_2D, PROXY_TEXTURE_3D,
and PROXY_TEXTURE_CUBE_MAP_ARB cannot be used as textures, and their
images must never be queried using GetTexImage." ...
-- Section 3.8.8 "Texture Objects"
Change the first sentence of the first paragraph (page 132) to say:
"In addition to the default textures TEXTURE_1D, TEXTURE_2D,
TEXTURE_3D, and TEXTURE_CUBE_MAP_ARB, named one-, two-,
and three-dimensional texture objects and cube map texture objects
can be created and operated on." ...
Change the second paragraph (page 132) to say:
"A texture object is created by binding an unused name to
TEXTURE_1D, TEXTURE_2D, TEXTURE_3D, or TEXTURE_CUBE_MAP_ARB." ...
"If the new texture object is bound to TEXTURE_1D, TEXTURE_2D,
TEXTURE_3D, or TEXTURE_CUBE_MAP_ARB, it remains a one-, two-,
three-dimensional, or cube map texture until it is deleted."
Change the third paragraph (page 133) to say:
"BindTexture may also be used to bind an existing texture object to
either TEXTURE_1D, TEXTURE_2D, TEXTURE_3D, or TEXTURE_CUBE_MAP_ARB."
Change paragraph five (page 133) to say:
"In the initial state, TEXTURE_1D, TEXTURE_2D, TEXTURE_3D,
and TEXTURE_CUBE_MAP have one-dimensional, two-dimensional,
three-dimensional, and cube map state vectors associated
with them respectively." ... "The initial, one-dimensional,
two-dimensional, three-dimensional, and cube map texture is therefore
operated upon, queried, and applied as TEXTURE_1D, TEXTUER_2D,
TEXTURE_3D, and TEXTURE_CUBE_MAP_ARB respectively while 0 is bound
to the corresponding targets."
Change paragraph six (page 133) to say:
... "If a texture that is currently bound to one of the targets
TEXTURE_1D, TEXTURE_2D, TEXTURE_3D, or TEXTURE_CUBE_MAP_ARB is
deleted, it is as though BindTexture has been executed with the
same <target> and <texture> zero." ...
-- Section 3.8.10 "Texture Application"
Replace the beginning sentences of the first paragraph (page 138)
with:
"Texturing is enabled or disabled using the generic Enable
and Disable commands, respectively, with the symbolic constants
TEXTURE_1D, TEXTURE_2D, TEXTURE_3D, or TEXTURE_CUBE_MAP_ARB to enable
the one-dimensional, two-dimensional, three-dimensional, or cube
map texturing respectively. If both two- and one-dimensional
textures are enabled, the two-dimensional texture is used. If the
three-dimensional and either of the two- or one-dimensional textures
is enabled, the three-dimensional texture is used. If the cube map
texture and any of the three-, two-, or one-dimensional textures is
enabled, then cube map texturing is used. If texturing is disabled,
a rasterized fragment is passed on unaltered to the next stage of the
GL (although its texture coordinates may be discarded). Otherwise,
a texture value is found according to the parameter values of the
currently bound texture image of the appropriate dimensionality.
However, when cube map texturing is enabled, the rules are
more complicated. For cube map texturing, the (s,t,r) texture
coordinates are treated as a direction vector (rx,ry,rz) emanating
from the center of a cube. (The q coordinate can be ignored since
it merely scales the vector without affecting the direction.) At
texture application time, the interpolated per-fragment (s,t,r)
selects one of the cube map face's 2D image based on the largest
magnitude coordinate direction (the major axis direction). If two
or more coordinates have the identical magnitude, the implementation
may define the rule to disambiguate this situation. The rule must
be deterministic and depend only on (rx,ry,rz). The target column
in the table below explains how the major axis direction maps to
the 2D image of a particular cube map target.
major axis
direction target sc tc ma
---------- ------------------------------- --- --- ---
+rx TEXTURE_CUBE_MAP_POSITIVE_X_ARB -rz -ry rx
-rx TEXTURE_CUBE_MAP_NEGATIVE_X_ARB +rz -ry rx
+ry TEXTURE_CUBE_MAP_POSITIVE_Y_ARB +rx +rz ry
-ry TEXTURE_CUBE_MAP_NEGATIVE_Y_ARB +rx -rz ry
+rz TEXTURE_CUBE_MAP_POSITIVE_Z_ARB +rx -ry rz
-rz TEXTURE_CUBE_MAP_NEGATIVE_Z_ARB -rx -ry rz
Using the sc, tc, and ma determined by the major axis direction as
specified in the table above, an updated (s,t) is calculated as
follows
s = ( sc/|ma| + 1 ) / 2
t = ( tc/|ma| + 1 ) / 2
This new (s,t) is used to find a texture value in the determined
face's 2D texture image using the rules given in sections 3.8.5
and 3.8.6." ...
Additions to Chapter 4 of the 1.2 Specification (Per-Fragment Operations
and the Frame Buffer)
None
Additions to Chapter 5 of the 1.2 Specification (Special Functions)
-- Section 5.4 "Display Lists"
In the first paragraph (page 179), add PROXY_TEXTURE_CUBE_MAP_ARB
to the list of PROXY_* tokens.
Additions to Chapter 6 of the 1.2 Specification (State and State Requests)
-- Section 6.1.3 "Enumerated Queries"
Change the fourth paragraph (page 183) to say:
"The GetTexParameter parameter <target> may be one of TEXTURE_1D,
TEXTURE_2D, TEXTURE_3D, or TEXTURE_CUBE_MAP_ARB, indicating the
currently bound one-dimensional, two-dimensional, three-dimensional,
or cube map texture object. For GetTexLevelParameter,
<target> may be one of TEXTURE_1D, TEXTURE_2D, TEXTURE_3D,
TEXTURE_CUBE_MAP_POSITIVE_X_ARB, TEXTURE_CUBE_MAP_NEGATIVE_X_ARB,
TEXTURE_CUBE_MAP_POSITIVE_Y_ARB, TEXTURE_CUBE_MAP_NEGATIVE_Y_ARB,
TEXTURE_CUBE_MAP_POSITIVE_Z_ARB, TEXTURE_CUBE_MAP_NEGATIVE_Z_ARB,
PROXY_TEXTURE_1D, PROXY_TEXTURE_2D, PROXY_TEXTURE_3D, or
PROXY_TEXTURE_CUBE_MAP_ARB, indicating the one-dimensional
texture object, two-dimensional texture object, three-dimensional
texture object, or one of the six distinct 2D images making up
the cube map texture object or one-dimensional, two-dimensional,
three-dimensional, or cube map proxy state vector. Note that
TEXTURE_CUBE_MAP_ARB is not a valid <target> parameter for
GetTexLevelParameter because it does not specify a particular cube
map face."
-- Section 6.1.4 "Texture Queries"
Change the first paragraph (page 184) to read:
... "It is somewhat different from the other get commands; <tex>
is a symbolic value indicating which texture (or texture face in the
case of a cube map texture target name) is to be obtained.
TEXTURE_1D indicates a one-dimensional texture, TEXTURE_2D
indicates a two-dimensional texture, TEXTURE_3D indicates a
three-dimensional texture, and TEXTURE_CUBE_MAP_POSITIVE_X_ARB,
TEXTURE_CUBE_MAP_NEGATIVE_X_ARB, TEXTURE_CUBE_MAP_POSITIVE_Y_ARB,
TEXTURE_CUBE_MAP_NEGATIVE_Y_ARB, TEXTURE_CUBE_MAP_POSITIVE_Z_ARB,
and TEXTURE_CUBE_MAP_NEGATIVE_Z_ARB indicate the respective face of
a cube map texture.
Additions to the GLX Specification
None
Errors
INVALID_ENUM is generated when TexGen is called with a <coord> of Q
when <pname> indicates REFLECTION_MAP_ARB or NORMAL_MAP_ARB.
INVALID_VALUE is generated when the target parameter to TexImage2D
or CopyTexImage2D is one of the six cube map 2D image targets and
the width and height parameters are not equal.
New State
(table 6.12, p202) add the following entries:
Get Value Type Get Command Initial Value Description Sec Attribute
--------- ---- ----------- ------------- ----------- ------ --------------
TEXTURE_CUBE_MAP_ARB B IsEnabled False True if cube map 3.8.10 texture/enable
texturing is enabled
TEXTURE_BINDING_CUBE_MAP_ARB Z+ GetIntegerv 0 Texture object 3.8.8 texture
for TEXTURE_CUBE_MAP
TEXTURE_CUBE_MAP_POSITIVE_X_ARB nxI GetTexImage see 3.8 positive x face 3.8 -
cube map texture
image at lod i
TEXTURE_CUBE_MAP_NEGATIVE_X_ARB nxI GetTexImage see 3.8 negative x face 3.8 -
cube map texture
image at lod i
TEXTURE_CUBE_MAP_POSITIVE_Y_ARB nxI GetTexImage see 3.8 positive y face 3.8 -
cube map texture
image at lod i
TEXTURE_CUBE_MAP_NEGATIVE_Y_ARB nxI GetTexImage see 3.8 negative y face 3.8 -
cube map texture
image at lod i
TEXTURE_CUBE_MAP_POSITIVE_Z_ARB nxI GetTexImage see 3.8 positive z face 3.8 -
cube map texture
image at lod i
TEXTURE_CUBE_MAP_NEGATIVE_Z_ARB nxI GetTexImage see 3.8 negative z face 3.8 -
cube map texture
image at lod i
(table 6.14, p204) change the entry for TEXTURE_GEN_MODE to:
Get Value Type Get Command Initial Value Description Sec Attribute
--------- ---- ----------- ------------- ----------- ------ ---------
TEXTURE_GEN_MODE 4xZ5 GetTexGeniv EYE_LINEAR Function used for 2.10.4 texture
texgen (for s,t,r,
and q)
(the type changes from 4xZ3 to 4xZ5)
New Implementation Dependent State
(table 6.24, p214) add the following entry:
Get Value Type Get Command Minimum Value Description Sec Attribute
--------- ---- ----------- ------------- ----------- ------ --------------
MAX_CUBE_MAP_TEXTURE_SIZE_ARB Z+ GetIntegerv 16 Maximum cube map 3.8.1 -
texture image
dimension
Backwards Compatibility
This extension replaces EXT_texture_cube_map. The tokens and
name strings now refer to ARB instead of EXT. Enumerant values
are unchanged.
Implementation Support
List of OpenGL implementations supporting the GL_ARB_texture_cube_map extension
Original File
Original text file for the GL_ARB_texture_cube_map extension
Page generated on Sun Nov 20 18:35:52 2005