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