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Name
SGIS_texture_edge_clamp
Name Strings
GL_SGIS_texture_edge_clamp
Version
$Date: 1997/02/26 03:36:36 $ $Revision: 1.6 $
Number
35
Dependencies
EXT_texture3D affects the definition of this extension
SGIS_texture_filter4 affects the definition of this extension
Overview
The base OpenGL provides clamping such that the texture coordinates are
limited to exactly the range [0,1]. When a texture coordinate is
clamped using this algorithm, the texture sampling filter straddles the
edge of the texture image, taking 1/2 its sample values from within the
texture image, and the other 1/2 from the texture border. It is
sometimes desirable to clamp a texture without requiring a border, and
without using the constant border color.
This extension defines a new texture clamping algorithm.
CLAMP_TO_EDGE_SGIS clamps texture coordinates at all mipmap levels such
that the texture filter never samples a border texel. When used with a
NEAREST or a LINEAR filter, the color returned when clamping is derived
only from texels at the edge of the texture image. When used with
FILTER4 filters, the filter operations of CLAMP_TO_EDGE_SGIS are defined
but don't result in a nice clamp-to-edge color.
CLAMP_TO_EDGE_SGIS is supported by 1, 2, and 3-dimensional textures
only.
Issues
* Is the arithmetic for FILTER4 filters correct? Is this the right
thing to do?
New Procedures and Functions
None
New Tokens
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_EXT:
CLAMP_TO_EDGE_SGIS 0x812F
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,
CLAMP_TO_EDGE_SGIS
TEXTURE_WRAP_T integer CLAMP, REPEAT,
CLAMP_TO_EDGE_SGIS
TEXTURE_WRAP_R_EXT integer CLAMP, REPEAT,
CLAMP_TO_EDGE_SGIS
TEXTURE_MIN_FILTER integer NEAREST, LINEAR,
NEAREST_MIPMAP_NEAREST,
NEAREST_MIPMAP_LINEAR,
LINEAR_MIPMAP_NEAREST,
LINEAR_MIPMAP_LINEAR,
FILTER4_SGIS,
LINEAR_CLIPMAP_LINEAR_SGIX
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,
LINEAR_LEQUAL_R_SGIS,
LINEAR_GEQUAL_R_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
GENERATE_MIPMAP_SGIS boolean TRUE or FALSE
TEXTURE_CLIPMAP_OFFSET_SGIX 2 floats any 2 values
Table 3.7: Texture parameters and their values.
CLAMP_TO_EDGE_SGIS texture clamping is specified by calling
TexParameteri with <target> set to TEXTURE_1D, TEXTURE_2D, or
TEXTURE_3D_EXT, <pname> set to TEXTURE_WRAP_S, TEXTURE_WRAP_T,
or TEXTURE_WRAP_R_EXT, and <param> set to CLAMP_TO_EDGE_SGIS.
Let [min,max] be the range of a clamped texture coordinate, and let N
be the size of the 1D, 2D, or 3D texture image in the direction of
clamping. Then in all cases
max = 1 - min
because the clamping is always symmetric about the [0,1] mapped range of
a texture coordinate. When used with NEAREST or LINEAR filters,
CLAMP_TO_EDGE_SGIS defines a minimum clamping value of
min = 1 / 2*N
When used with FILTER4 filters, CLAMP_TO_EDGE_SGIS defines a minimum
clamping value of
min = 3 / 2*N, N > 2
min = 1/2 N <= 2
Additions to Chapter 4 of the 1.0 Specification (Per-Fragment Operations
and the Framebuffer)
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_texture3D
If EXT_texture3D is not implemented, then the references clamping of 3D
textures in this file are invalid, and references to TEXTURE_WRAP_R_EXT
should be ignored.
Dependencies on SGIS_texture_filter4
If SGIS_texture_filter4 is not implemented, then discussions about the
interaction of filter4 texture filters and the clamping function
described in this file are invalid, and should be ignored.
Errors
None
New State
Only the type information changes for these parameters:
Get Value Get Command Type Initial Value Attrib
--------- ----------- ---- ------------- ------
TEXTURE_WRAP_S GetTexParameteriv n x Z3 REPEAT texture
TEXTURE_WRAP_T GetTexParameteriv n x Z3 REPEAT texture
TEXTURE_WRAP_R_EXT GetTexParameteriv n x Z3 REPEAT texture
New Implementation Dependent State
None