SGIS_texture_lod
GL_SGIS_texture_lod
$Date: 1997/05/30 01:34:44 $ $Revision: 1.8 $
24
EXT_texture is required
EXT_texture3D affects the definition of this extension
EXT_texture_object affects the definition of this extension
SGI_detail_texture affects the definition of this extension
SGI_sharpen_texture affects the definition of this extension
This extension imposes two constraints related to the texture level of
detail parameter LOD, which is represented by the Greek character lambda
in the GL Specification. One constraint clamps LOD to a specified
floating point range. The other limits the selection of mipmap image
arrays to a subset of the arrays that would otherwise be considered.
Together these constraints allow a large texture to be loaded and
used initially at low resolution, and to have its resolution raised
gradually as more resolution is desired or available. Image array
specification is necessarily integral, rather than continuous. By
providing separate, continuous clamping of the LOD parameter, it is
possible to avoid "popping" artifacts when higher resolution images
are provided.
Note: because the shape of the mipmap array is always determined by
the dimensions of the level 0 array, this array must be loaded for
mipmapping to be active. If the level 0 array is specified with a
null image pointer, however, no actual data transfer will take
place. And a sufficiently tuned implementation might not even
allocate space for a level 0 array so specified until true image
data were presented.
* Should detail and sharpen texture operate when the level 0 image
is not being used?
A: Sharpen yes, detail no.
* Should the shape of the mipmap array be determined by the
dimensions of the level 0 array, regardless of the base level?
A: Yes, this is the better solution. Driving everything from
the base level breaks the proxy query process, and allows
mipmap arrays to be placed arbitrarily. The issues of
requiring a level 0 array are partially overcome by the use
of null-point loads, which avoid data transfer and,
potentially, data storage allocation.
* With the arithmetic as it is, a linear filter might access an
array past the limit specified by MAX_LEVEL or p. But the
results of this access are not significant, because the blend
will weight them as zero.
   New Procedures and Functions |
None
Accepted by the <pname> parameter of TexParameteri, TexParameterf,
TexParameteriv, TexParameterfv, GetTexParameteriv, and GetTexParameterfv:
TEXTURE_MIN_LOD_SGIS 0x813A
TEXTURE_MAX_LOD_SGIS 0x813B
TEXTURE_BASE_LEVEL_SGIS 0x813C
TEXTURE_MAX_LEVEL_SGIS 0x813D
| Additions to Chapter 2 of the 1.0 Specification (OpenGL Operation) |
None
| Additions to Chapter 3 of the 1.0 Specification (Rasterization) |
GL Specification Table 3.7 is updated as follows:
Name Type Legal Values
---- ---- ------------
TEXTURE_WRAP_S integer CLAMP, REPEAT
TEXTURE_WRAP_T integer CLAMP, REPEAT
TEXTURE_WRAP_R_EXT integer CLAMP, REPEAT
TEXTURE_MIN_FILTER integer NEAREST, LINEAR,
NEAREST_MIPMAP_NEAREST,
NEAREST_MIPMAP_LINEAR,
LINEAR_MIPMAP_NEAREST,
LINEAR_MIPMAP_LINEAR,
FILTER4_SGIS
TEXTURE_MAG_FILTER integer NEAREST, LINEAR,
FILTER4_SGIS,
LINEAR_DETAIL_SGIS,
LINEAR_DETAIL_ALPHA_SGIS,
LINEAR_DETAIL_COLOR_SGIS,
LINEAR_SHARPEN_SGIS,
LINEAR_SHARPEN_ALPHA_SGIS,
LINEAR_SHARPEN_COLOR_SGIS
TEXTURE_BORDER_COLOR 4 floats any 4 values in [0,1]
DETAIL_TEXTURE_LEVEL_SGIS integer any non-negative integer
DETAIL_TEXTURE_MODE_SGIS integer ADD, MODULATE
TEXTURE_MIN_LOD_SGIS float any value
TEXTURE_MAX_LOD_SGIS float any value
TEXTURE_BASE_LEVEL_SGIS integer any non-negative integer
TEXTURE_MAX_LEVEL_SGIS integer any non-negative integer
Table 3.7: Texture parameters and their values.
Base Array
----------
Although it is not explicitly stated, it is the clear intention
of the OpenGL specification that texture minification filters
NEAREST and LINEAR, and all texture magnification filters, be
applied to image array zero. This extension introduces a
parameter, BASE_LEVEL, that explicitly specifies which array
level is used for these filter operations. Base level is specified
for a specific texture by calling TexParameteri, TexParameterf,
TexParameteriv, or TexParameterfv with <target> set to TEXTURE_1D,
TEXTURE_2D, or TEXTURE_3D_EXT, <pname> set to TEXTURE_BASE_LEVEL_SGIS,
and <param> set to (or <params> pointing to) the desired value. The
error INVALID_VALUE is generated if the specified BASE_LEVEL is
negative.
Level of Detail Clamping
------------------------
The level of detail parameter LOD is defined in the first paragraph
of Section 3.8.1 (Texture Minification) of the GL Specification, where
it is represented by the Greek character lambda. This extension
redefines the definition of LOD as follows:
LOD'(x,y) = log_base_2 (Q(x,y))
/ MAX_LOD LOD' > MAX_LOD
LOD = ( LOD' LOD' >= MIN_LOD and LOD' <= MAX_LOD
\ MIN_LOD LOD' < MIN_LOD
\ undefined MIN_LOD > MAX_LOD
The variable Q in this definition represents the Greek character rho,
as it is used in the OpenGL Specification. (Recall that Q is computed
based on the dimensions of the BASE_LEVEL image array.) MIN_LOD is the
value of the per-texture variable TEXTURE_MIN_LOD_SGIS, and MAX_LOD is
the value of the per-texture variable TEXTURE_MAX_LOD_SGIS.
Initially TEXTURE_MIN_LOD_SGIS and TEXTURE_MAX_LOD_SGIS are -1000 and
1000 respectively, so they do not interfere with the normal operation of
texture mapping. These values are respecified for a specific texture
by calling TexParameteri, TexParemeterf, TexParameteriv, or
TexParameterfv with <target> set to TEXTURE_1D, TEXTURE_2D, or
TEXTURE_3D_EXT, <pname> set to TEXTURE_MIN_LOD_SGIS or
TEXTURE_MAX_LOD_SGIS, and <param> set to (or <params> pointing to) the
new value. It is not an error to specify a maximum LOD value that is
less than the minimum LOD value, but the resulting LOD values are
not defined.
LOD is clamped to the specified range prior to any use. Specifically,
the mipmap image array selection described in the Mipmapping Subsection
of the GL Specification is based on the clamped LOD value. Also, the
determination of whether the minification or magnification filter is
used is based on the clamped LOD.
Mipmap Completeness
-------------------
The GL Specification describes a "complete" set of mipmap image arrays
as array levels 0 through p, where p is a well defined function of the
dimensions of the level 0 image. This extension modifies the notion
of completeness: instead of requiring that all arrays 0 through p
meet the requirements, only arrays 0 and arrays BASE_LEVEL through
MAX_LEVEL (or p, whichever is smaller) must meet these requirements.
The specification of BASE_LEVEL was described above. MAX_LEVEL is
specified by calling TexParameteri, TexParemeterf, TexParameteriv, or
TexParameterfv with <target> set to TEXTURE_1D, TEXTURE_2D, or
TEXTURE_3D_EXT, <pname> set to TEXTURE_MAX_LEVEL_SGIS, and <param> set
to (or <params> pointing to) the desired value. The error
INVALID_VALUE is generated if the specified MAX_LEVEL is negative.
If MAX_LEVEL is smaller than BASE_LEVEL, or if BASE_LEVEL is greater
than p, the set of arrays is incomplete.
Array Selection
---------------
Magnification and non-mipmapped minification are always performed
using only the BASE_LEVEL image array. If the minification filter
is one that requires mipmapping, one or two array levels are
selected using the equations in the table below, and the LOD value
is clamped to a maximum value that insures that no array beyond
the limits specified by MAX_LEVEL and p is accessed.
Minification Filter Maximum LOD Array level(s)
------------------- ----------- --------------
NEAREST_MIPMAP_NEAREST M + 0.4999 floor(B + 0.5)
LINEAR_MIPMAP_NEAREST M + 0.4999 floor(B + 0.5)
NEAREST_MIPMAP_LINEAR M floor(B), floor(B)+1
LINEAR_MIPMAP_LINEAR M floor(B), floor(B)+1
where:
M = min(MAX_LEVEL,p) - BASE_LEVEL
B = BASE_LEVEL + LOD
For NEAREST_MIPMAP_NEAREST and LINEAR_MIPMAP_NEAREST the specified
image array is filtered according to the rules for NEAREST or
LINEAR respectively. For NEAREST_MIPMAP_LINEAR and
LINEAR_MIPMAP_LINEAR both selected arrays are filtered according to
the rules for NEAREST or LINEAR, respectively. The resulting values
are then blended as described in the Mipmapping section of the
OpenGL specification.
Additional Filters
------------------
Sharpen filters (described in SGIS_sharpen_texture) operate on array
levels BASE_LEVEL and BASE_LEVEL+1. If the minimum of MAX_LEVEL and p
is not greater than BASE_LEVEL, then sharpen texture reverts to a
LINEAR magnification filter. Detail filters (described in
SGIS_detail_texture) operate only when BASE_LEVEL is zero.
Texture Capacity
----------------
In Section 3.8 the OpenGL specification states:
"In order to allow the client to meaningfully query the maximum
image array sizes that are supported, an implementation must not
allow an image array of level one or greater to be created if a
`complete' set of image arrays consistent with the requested
array could not be supported."
Given this extension's redefinition of completeness, the above
paragraph should be rewritten to indicate that all levels of the
`complete' set of arrays must be supportable. E.g.
"In order to allow the client to meaningfully query the maximum
image array sizes that are supported, an implementation must not
allow an image array of level one or greater to be created if a
`complete' set of image arrays (all levels 0 through p) consistent
with the requested array could not be supported."
| Additions to Chapter 4 of the 1.0 Specification (Per-Fragment Operations and the Frame Buffer) |
None
| Additions to Chapter 5 of the 1.0 Specification (Special Functions) |
None
| Additions to Chapter 6 of the 1.0 Specification (State and State Requests) |
None
| Additions to the GLX Specification |
None
| Dependencies on EXT_texture |
EXT_texture is required.
| Dependencies on EXT_texture3D |
If EXT_texture3D is not supported, references to 3D texture mapping and
to TEXTURE_3D_EXT in this document are invalid and should be ignored.
| Dependencies on EXT_texture_object |
If EXT_texture_object is implemented, the state values named
TEXTURE_MIN_LOD_SGIS
TEXTURE_MAX_LOD_SGIS
TEXTURE_BASE_LEVEL_SGIS
TEXTURE_MAX_LEVEL_SGIS
are added to the state vector of each texture object. When an attribute
set that includes texture information is popped, the bindings and
enables are first restored to their pushed values, then the bound
textures have their LOD and LEVEL parameters restored to their pushed
values.
| Dependencies on SGIS_detail_texture |
If SGIS_detail_texture is not supported, references to detail texture
mapping in this document are invalid and should be ignored.
| Dependencies on SGIS_sharpen_texture |
If SGIS_sharpen_texture is not supported, references to sharpen texture
mapping in this document are invalid and should be ignored.
INVALID_VALUE is generated if an attempt is made to set
TEXTURE_BASE_LEVEL_SGIS or TEXTURE_MAX_LEVEL_SGIS to a negative value.
Initial
Get Value Get Command Type Value Attrib
--------- ----------- ---- ------- ------
TEXTURE_MIN_LOD_SGIS GetTexParameterfv n x R -1000 texture
TEXTURE_MAX_LOD_SGIS GetTexParameterfv n x R 1000 texture
TEXTURE_BASE_LEVEL_SGIS GetTexParameteriv n x R 0 texture
TEXTURE_MAX_LEVEL_SGIS GetTexParameteriv n x R 1000 texture
| New Implementation Dependent State |
None
None
 |  | Last update: November 14, 2006.
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