| GL_EXT_texture_mirror_clamp |
EXT_texture_mirror_clamp
GL_EXT_texture_mirror_clamp
Walt Donovan
Dan Ginsburg
Mark J. Kilgard, NVIDIA Corporation (mjk 'at' nvidia.com)
Last Modified Date: $Date: 2003/12/05 $
$Id: //sw/main/docs/OpenGL/specs/GL_EXT_texture_mirror_clamp.txt#3 $
298
How does EXT_texture_mirror_clamp extend ATI_texture_mirror_once?
This EXT extension provides the two wrap modes that
ATI_texture_mirror_once adds but also adds a third new wrap mode
(GL_MIRROR_CLAMP_TO_BORDER_EXT). This extension uses the same
enumerant values for the ATI_texture_mirror_once modes.
Why is the GL_MIRROR_CLAMP_TO_BORDER_EXT mode more interesting than
the two other modes?
Rather than clamp to 100% of the edge of the texture
(GL_MIRROR_CLAMP_TO_EDGE_EXT) or to 50% of the edge and border
color (GL_MIRROR_CLAMP), it is preferable to clamp to 100%
of the border color (GL_MIRROR_CLAMP_TO_BORDER_EXT). This
avoids "bleeding" at smaller mipmap levels.
Consider a texture that encodes a circular fall-off pattern such
as for a projected spotlight. A circular pattern is bi-symmetric
so a "mirror clamp" wrap modes can reduce the memory footprint
of the texture by a fourth. Far outside the spotlight pattern,
you'd like to sample 100% of the border color (typically black
for a spotlight texture). The way to achieve this without any
bleeding of edge texels is with GL_MIRROR_CLAMP_TO_BORDER_EXT.
Does this extension complete the orthogonality of the current five
OpenGL 1.5 wrap modes?
Yes. There are two ways for repetition to operate (repeated
& mirrored) and four ways for texture coordinate clamping to
operate (unclamped, clamp, clamp to edge, & clamp to border).
The complete table of all 8 modes looks like this:
Repeat Mirror
+---------------- ----------------------
Unclamped | REPEAT MIRRORED_REPEAT
Clamp | CLAMP MIRROR_CLAMP
Clamp to edge | CLAMP_TO_EDGE MIRROR_CLAMP_TO_EDGE
Clamp to border | CLAMP_TO_BORDER MIRROR_CLAMP_TO_BORDER
OpenGL 1.0 introduced REPEAT & CLAMP.
OpenGL 1.2 introduced CLAMP_TO_EDGE
OpenGL 1.3 introduced CLAMP_TO_BORDER
OpenGL 1.4 introduced MIRRORED_REPEAT
ATI_texture_mirror_once introduced MIRROR_CLAMP & MIRROR_CLAMP_TO_EDGE
EXT_texture_mirror_clamp introduces MIRROR_CLAMP_TO_BORDER
Do these three new wrap modes work with 1D, 2D, 3D, and cube map
texture targets?
RESOLUTION: Yes.
Do these three new wrap modes work with ARB_texture_non_power_of_two
functionality?
RESOLUTION: Yes.
Do these three new wrap modes interact with NV_texture_rectangle?
RESOLUTION: Mirroring wrap modes are not supported by
GL_TEXTURE_RECTANGLE_NV textures. Conventional mirroring is
already not supported for texture rectangles so supporting
clamped mirroring modes should not be supported either.
Does the specification of MIRROR_CLAMP_EXT & MIRROR_CLAMP_TO_EDGE_EXT
match the ATI_texture_mirror_once specification?
I believe yes. The ATI_texture_mirror_once specification is
somewhat vague what happens to texture coordinates at or very
near (within half a texel of) zero. The presumption is that a
CLAMP_TO_EDGE behavior is used. This specification is quite
explicit that values near zero are clamped to plus or minus
1/(2*N) respectively so that the CLAMP_TO_EDGE behavior is
explicit.
What should this extension be called?
Calling the extension EXT_texture_mirror_once might cause
confusion since this extension has additional functionality.
Also, "once" never appears in the specification.
EXT_texture_mirror_clamp is a good name because it implies
support for all the clamped versions of mirroring.
There is GL_MIRRORED_REPEAT and then GL_MIRROR_CLAMP_EXT,
GL_MIRROR_CLAMP_TO_EDGE_EXT, and GL_MIRROR_CLAMP_TO_BORDER_EXT.
Why does the first enumerant name say "MIRRORED" while the other
three say "MIRROR"?
This extension follows the naming precedent set by the
ATI_texture_mirror_once specification.
Moreover, MIRRORED_REPEAT uses "mirrored" to help that the
mirroring repeats infinitely. For the other three modes,
there is just one mirror that occurs and then a clamp.
Dependencies
Written based on the wording of the OpenGL 1.4.
Extends ATI_texture_mirror_once by adding
GL_MIRROR_CLAMP_TO_BORDER_EXT.
NV_texture_rectangle trivially affects the definition of this
extension.
Overview
EXT_texture_mirror_clamp extends the set of texture wrap modes to
include three modes (GL_MIRROR_CLAMP_EXT, GL_MIRROR_CLAMP_TO_EDGE_EXT,
GL_MIRROR_CLAMP_TO_BORDER_EXT) that effectively use a texture map
twice as large as the original image in which the additional half
of the new image is a mirror image of the original image.
This new mode relaxes the need to generate images whose opposite
edges match by using the original image to generate a matching
"mirror image". This mode allows the texture to be mirrored only
once in the negative s, t, and r directions.
New Procedure and Functions
None
Accepted by the <param> parameter of TexParameteri and TexParameterf,
and by the <params> parameter of TexParameteriv and TexParameterfv,
when their <pname> parameter is TEXTURE_WRAP_S, TEXTURE_WRAP_T,
or TEXTURE_WRAP_R:
MIRROR_CLAMP_EXT 0x8742 (same value as MIRROR_CLAMP_ATI)
MIRROR_CLAMP_TO_EDGE_EXT 0x8743 (same value as MIRROR_CLAMP_TO_EDGE_ATI)
MIRROR_CLAMP_TO_BORDER_EXT 0x8912
   Additions to Chapter 2 of the OpenGL 1.2.1 Specification (Operation) |
None
| Additions to Chapter 3 if the OpenGL 1.2.1 Specification (Rasterization): |
- (3.8.4, page 136, as amended by the NV_texture_rectangle extension)
Add the 3 new wrap modes to the list of wrap modes unsupported for
the TEXTURE_RECTANGLE_NV texture target.
"Certain texture parameter values may not be specified for textures
with a target of TEXTURE_RECTANGLE_NV. The error INVALID_ENUM
is generated if the target is TEXTURE_RECTANGLE_NV and the
TEXTURE_WRAP_S, TEXTURE_WRAP_T, or TEXTURE_WRAP_R parameter is set to
REPEAT, MIRRORED_REPEAT_IBM, MIRROR_CLAMP_EXT, MIRROR_CLAMP_TO_EDGE_EXT, and
MIRROR_CLAMP_TO_BORDER_EXT. The error INVALID_ENUM is generated
if the target is TEXTURE_RECTANGLE_NV and the TEXTURE_MIN_FILTER is
set to a value other than NEAREST or LINEAR (no mipmap filtering
is permitted). The error INVALID_ENUM is generated if the target
is TEXTURE_RECTANGLE_NV and TEXTURE_BASE_LEVEL is set to any value
other than zero."
- Table 3.19, page 137: Change first three entries in table:
"TEXTURE_WRAP_S integer CLAMP, CLAMP_TO_BORDER, CLAMP_TO_EDGE,
MIRRORED_REPEAT, MIRROR_CLAMP_EXT,
MIRROR_CLAMP_TO_BORDER_EXT,
MIRROR_CLAMP_TO_EDGE_EXT, REPEAT
TEXTURE_WRAP_T integer CLAMP, CLAMP_TO_BORDER, CLAMP_TO_EDGE,
MIRRORED_REPEAT, MIRROR_CLAMP_EXT,
MIRROR_CLAMP_TO_BORDER_EXT,
MIRROR_CLAMP_TO_EDGE_EXT, REPEAT
TEXTURE_WRAP_R integer CLAMP, CLAMP_TO_BORDER, CLAMP_TO_EDGE,
MIRRORED_REPEAT, MIRROR_CLAMP_EXT,
MIRROR_CLAMP_TO_BORDER_EXT,
MIRROR_CLAMP_TO_EDGE_EXT, REPEAT"
- (3.8.7, page 140) After the last paragraph of the section add:
"Wrap Mode MIRROR_CLAMP_EXT
Wrap mode MIRROR_CLAMP_EXT mirrors and clamps the texture coordinate,
where mirroring and clamping a value f computes
mirrorClamp(f) = min(1, max(1/(2*N), abs(f)))
where N is the size of the one-, two-, or three-dimensional texture
image in the direction of wrapping.
Wrap Mode MIRROR_CLAMP_TO_EDGE_EXT
Wrap mode MIRROR_CLAMP_TO_EDGE_EXT mirrors and clamps to edge the
texture coordinate, where mirroring and clamping to edge a value f
computes
mirrorClampToEdge(f) = min(1-1/(2*N), max(1/(2*N), abs(f)))
where N is the size of the one-, two-, or three-dimensional texture
image in the direction of wrapping.
Wrap Mode MIRROR_CLAMP_TO_BORDER_EXT
Wrap mode MIRROR_CLAMP_TO_BORDER_EXT mirrors and clamps to border the
texture coordinate, where mirroring and clamping to border a value
f computes
mirrorClampToBorder(f) = min(1+1/(2*N), max(1/(2*N), abs(f)))
where N is the size of the one-, two-, or three-dimensional texture
image in the direction of wrapping."
- (3.8.8, page 142) Delete this phrase because it is out of date and
unnecessary given the current way section 3.8.7 is written:
"(if the wrap mode for a coordinate is CLAMP or CLAMP_TO_EDGE)"
None
None
None
| Additions to the GLX Specification |
None
| Dependencies on NV_texture_rectangle |
If NV_texture_rectangle is not supported, ignore the statement that
the initial value for the S, T, and R wrap modes is CLAMP_TO_EDGE
for rectangular textures.
Ignore the error for a texture target of TEXTURE_RECTANGLE_NV.
None
INVALID_ENUM is generated when TexParameter is called with
a target of TEXTURE_RECTANGLE_NV and the TEXTURE_WRAP_S,
TEXTURE_WRAP_T, or TEXTURE_WRAP_R parameter is set to REPEAT,
MIRRORED_REPEAT_IBM, MIRROR_CLAMP_EXT, MIRROR_CLAMP_TO_EDGE_EXT,
or MIRROR_CLAMP_TO_BORDER_EXT.
(table 6.15, p230) amend the following entries [Z5 changed to Z8]:
Get Value Type Get Command Initial Value Description Sec Attribute
-------------- ---- --------------- --------------- ------------------- ----- ---------
TEXTURE_WRAP_S n*Z8 GetTexParameter REPEAT except Texture wrap mode S 3.8.7 texture
for rectangular
which is
CLAMP_TO_EDGE
TEXTURE_WRAP_T n*Z8 GetTexParameter REPEAT except Texture wrap mode T 3.8.7 texture
for rectangular
which is
CLAMP_TO_EDGE
TEXTURE_WRAP_R n*Z8 GetTexParameter REPEAT except Texture wrap mode R 3.8.7 texture
for rectangular
which is
CLAMP_TO_EDGE
| New Implementation Dependent State |
None
None
 |  | Last update: November 14, 2006.
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