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GL_ARB_texture_env_crossbar
ARB_texture_env_crossbar
Name Strings
GL_ARB_texture_env_crossbar
Contact
Bimal Poddar, Intel, bimal.poddar@intel.com
Jack Middleton, Sun, Jack.Middleton@Eng.Sun.COM
Rick Hammerstone, ATI Technologies, Inc. (rhammers 'at' ati.com)
Status
Complete. Approved by ARB on February 16, 2001.
Version
Last modified date: 2001/02/02
Number
ARB Extension #18
Dependencies
This extension is written against the OpenGL 1.2.1 Specification.
OpenGL 1.1, ARB_multitexture and ARB_texture_env_combine are required
by this extension.
Overview
This extension adds the capability to use the texture color from
other texture units as sources to the COMBINE_ARB enviornment
function. The ARB_texture_env_combine extension defined texture
enviornment functions which could use the color from the
current texture unit as a source. This extension adds
the ability to use the color from any texture unit as a source.
Issues
1. Should only "upstream" textures be allowed as inputs to a
given texture unit? (Texture inputs to unit <m> are restricted to
the range TEXTURE0_ARB to TEXTURE<m>_ARB).
- RESOLVED: No. Most vendors support this functionality.
2. Do we still need TEXTURE as a source if we allow TEXTURE<n>_ARB?
- RESOLVED: Yes, since this allows backwards compatibility.
3. What should be the behavior when the texture environment
references a texture unit that is not enabled or does not have a
valid texture object bound to it?
- RESOLVED: Texture blending should be disabled on the texture
unit that is referencing the invalid or disabled texture. This
seems to be a reasonable compromise between disabling all
texturing and generating "default" texel values. Also, this
behavior is more easily forward compatible to an extension
which separates the texture lookup and texture blend units.
None
Accepted by the <params> parameter of TexEnvf, TexEnvi, TexEnvfv,
and TexEnviv when the <pname> parameter value is SOURCE0_RGB_ARB,
SOURCE1_RGB_ARB, SOURCE2_RGB_ARB, SOURCE0_ALPHA_ARB,
SOURCE1_ALPHA_ARB, or SOURCE2_ALPHA_ARB
TEXTURE<n>_ARB 0x84C0+<n>
where <n> is in the range 0 to MAX_TEXTURE_UNITS_ARB.
Additions to Chapter 2 of the GL Specification (OpenGL Operation)
None
Additions to Chapter 3 of the GL Specification (Rasterization)
Modify paragraph describing tables 3.21 and 3.22 in the
ARB_texture_env_combine spec to read:
The arguments Arg0, Arg1 and Arg2 are determined by the values of
SOURCE<n>_RGB_ARB, SOURCE<n>_ALPHA_ARB, OPERAND<n>_RGB_ARB and
OPERAND<n>_ALPHA_ARB. In the following two tables, Ct and At are
the filtered texture RGB and alpha values; Ct<n> and At<n> are the
filtered texture RGB and alpha values from the texture bound to
texture unit <n>; Cc and Ac are the texture environment RGB and
alpha values; Cf and Af are the RGB and alpha of the primary color
of the incoming fragment; and Cp and Ap are the RGB and alpha
values resulting from the previous texture environment. On texture
environment 0, Cp and Ap are identical to Cf and Af, respectively.
The relationship is described in tables 3.21 and 3.22.
Added to table 3.21 of the ARB_texture_env_combine spec:
SOURCE<n>_RGB_ARB OPERAND<n>_RGB_ARB Argument
----------------- -------------- --------
TEXTURE<n>_ARB SRC_COLOR Ct<n>
ONE_MINUS_SRC_COLOR (1-Ct<n>)
SRC_ALPHA At<n>
ONE_MINUS_SRC_ALPHA (1-At<n>)
Table 3.21: Arguments for COMBINE_RGB_ARB functions
Added to table 3.22 of the ARB_texture_env_combine spec:
SOURCE<n>_ALPHA_ARB OPERAND<n>_ALPHA_ARB Argument
----------------- -------------- --------
TEXTURE<n>_ARB SRC_ALPHA At<n>
ONE_MINUS_SRC_ALPHA (1-At<n>)
Table 3.22: Arguments for COMBINE_ALPHA_ARB functions
Additions to Chapter 4 of the GL Specification (Per-Fragment Operations
and the Framebuffer)
None
Additions to Chapter 5 of the GL Specification (Special Functions)
None
Additions to Chapter 6 of the GL Specification (State and State Requests)
None
Additions to Appendix F of the GL Specification (ARB Extensions)
Inserted after the second paragraph of F.2.12:
If the value of TEXTURE_ENV_MODE is COMBINE_ARB, the texture
function associated with a given texture unit is computed using
the values specified by SOURCE<n>_RGB_ARB, SOURCE<n>_ALPHA_ARB,
OPERAND<n>_RGB_ARB and OPERAND<n>_ALPHA_ARB. If TEXTURE<n>_ARB is
specified as SOURCE<n>_RGB_ARB or SOURCE<n>_ALPHA_ARB, the texture
value from texture unit <n> will be used in computing the texture
function for this texture unit.
Inserted after the third paragraph of F.2.12:
If a texture environment for a given texture unit references a
texture unit that is disabled or does not have a valid texture
object bound to it, then it is as if texture blending is disabled
for the given texture unit. Every texture unit implicitly
references the texture object that is bound to it, regardless
of the texture function specified by COMBINE_RGB_ARB or COMBINE_ALPHA_ARB.
Additions to the GLX Specification
None
GLX Protocol
None
Errors
INVALID_ENUM is generated if <params> value for SOURCE0_RGB_ARB,
SOURCE1_RGB_ARB, SOURCE2_RGB_ARB, SOURCE0_ALPHA_ARB,
SOURCE1_ALPHA_ARB or SOURCE2_ALPHA_ARB is not one of TEXTURE,
CONSTANT_ARB, PRIMARY_COLOR_ARB, PREVIOUS_ARB, or TEXTURE<n>_ARB,
where <n> is in the range 0 to MAX_TEXTURE_UNITS_ARB.
New State
None
Revision History
01/02/02 bpoddar Added original EXT/ARB contributors to the contact
list
00/12/06 bpoddar Added comments from the ARB meeting.
Changed texturing disable to texture blending
disable. Added description of table 3.21
and 3.22 to define Ct<n> and At<n>.
00/12/01 bpoddar Added clarification regarding disabling of
texture blend unit when it references a
disabled texture.
Removed some changes which were already
specified in ARB_texture_env_combine spec.
00/10/31 jmiddleton First version of the spec
Implementation Support
List of OpenGL implementations supporting the GL_ARB_texture_env_crossbar extension
Original File
Original text file for the GL_ARB_texture_env_crossbar extension
Page generated on Sun Nov 20 18:36:02 2005