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Name
SGIX_fog_factor_to_alpha
Name Strings
GL_SGIX_fog_factor_to_alpha
Version
$Date: 1999/02/19 17:04:27 $ $Revision: 1.4 $
Number
XXX
Dependencies
OpenGL 1.2 is required
Overview
This extension defines a mechanism that allows to replace the
alpha component by the fog blending factor after the input
fragment has been blent with the fog color. This feature is
particularly useful for multipass fog when we want to use the
the framebuffer blend function in conjunction with the fog blend
function.
New Procedures and Functions
None
Issues
- Is that ok as a stand alone extension? Should we include it
somewhere else?
- Should we make additions to the chapter 4, section 4.1.3 Alpha
test and 4.1.6 Blending?
- Does this spec need to reference the earlier fog_blend.spec
that allows for the fragment alpha component to be blended
with the fog color?
New Tokens
Accepted by the <cap> parameter of Enable, Disable, and IsEnabled
GL_FOG_FACTOR_TO_ALPHA_SGIX 0xXXXX
Additions to Chapter 2 of the 1.2 Specification (OpenGL Operation)
None
Additions to Chapter 3 of the 1.2 Specification (Rasterization)
3.10 Fog
... (The rasterized fragment's A value is not changed by fog blending.)...
is removed and replaced by.
The rasterized fragment alpha value is not changed by fog blending,
unless specified otherwise by enabling it with the GL_FACTOR_TO_ALPHA_SGIX
parameter in which case the A value is replaced by the fog
blending factor. This substitution occurs after the alpha test is
performed. If fog is disabled but fog-factor-to-alpha is enabled, the
fragment alpha coming out of the fog-factor-to-alpha in undefined.
Additions to Chapter 4 of the 1.2 Specification (Per-Fragment Operations
and the Framebuffer)
None
Additions to Chapter 5 of the 1.2 Specification (Special Functions)
None
Additions to Chapter 6 of the 1.2 Specification (State and State Requests)
None
Additions to the GLX Specification
None
Errors
None
New State
None
New Implementation Dependent State
Get Value Get Command Type Initial Value Attribute
--------- ----------- ---- ------------- ---------
FOG_FACTOR_TO_ALPHA_SGIX IsEnabled B FALSE fog