extensions/SGIX/SGIX_fog_scale.txt

Name

    SGIX_fog_scale

Name Strings

    GL_SGIX_fog_scale

Version

    $Date: 1999/04/02 22:52:37 $ $Revision: 1.6 $

Number

    161

Dependencies

    OpenGL 1.0 is required.
    The extension is written against the OpenGL 1.2.1 Specification.
    SGIX_fog_offset affects the definition of this extension

Overview

    This extension allows fragments to receive more or less fog than the
    amount specified by the fog environment and the distance to the
    fragment center, by scaling the fragment eye-coordinate distance
    prior to fog computation.

    This is particularly interesting for light point objects that punch
    even through thick fog. Fog scale value is specified by setting the
    FOG_SCALE_VALUE_SGIX scale parameter with glFog*. If scale <= 0, the
    resulting operation is clamped to 0. If 0 < scale < 1, the object
    appears brighter. If scale > 1, the object receives more fog. A
    scale of 1 has no effect. Fog scale can be enabled or disabled with
    the FOG_SCALE_SGIX parameter.

Issues

    * Should specifying FOG_SCALE_VALUE_SGIX < 0 generate an error?

    * Should the correct combined equation be (z * scale) - offset, or
      (z - offset) * scale? Performer uses the latter, but the former is
      more consistent with other OpenGL scale and bias operations.


New Procedures and Functions

    None

New Tokens

    Accepted by the <cap> parameter of Enable, Disable, and IsEnabled

	FOG_SCALE_SGIX			0x81FC

    Accepted by the <pname> parameter of GetBooleanv, GetIntegerv,
    GetFloatv, and GetDoublev, and by the <pname> parameter of Fogiv and
    Fogfv:

	FOG_SCALE_VALUE_SGIX		0x81FD

Additions to Chapter 2 of the 1.2.1 Specification (OpenGL Operation)

    None

Additions to Chapter 3 of the 1.2.1 Specification (Rasterization)

  - (3.10, p. 139) Change the second paragraph to:

	"This factor f is computed according to one of three equations:

	    f = exp(-d * f_z),		    (3.24)

	    f = exp(-(d * f_z)^2), or	    (3.25)

		e - f_z
	    f = -------			    (3.26)
		e - s

	f_z is a function of the eye-coordinate distance z from the eye,
	(0,0,0,1) in eye coordinates, to the fragment center. If
	FOG_SCALE_SGIX is enabled, then f_z = z * f_s; otherwise, f_z =
	z. If f_z < 0, then it is clamped to zero prior to fog
	computation.

	"The equation, along with the parameters d, e, s, and f_s, is
	specified with..."

    Change the end of the third paragraph to:

	"...If <pname> is FOG_DENSITY, FOG_START, FOG_END, or
	FOG_SCALE_SGIX, then <param> is or <params> points to a value
	that is d, s, e, or f_s, respectively. If d is specified as less
	than zero, the error INVALID_VALUE is generated."

    Change the final paragraph on page 140 to:

	"The state required for fog consists of a three valued integer
	to select the fog equation, four floating-point values d, e, s,
	and f_s, an RGBA fog color and a fog color index, a single bit
	to indicate whether or not fog is enabled, and a single bit to
	indicate whether or not fog scaling is enabled. In the initial
	state, fog is disabled, fog scaling is disabled, FOG_MODE is
	EXP, d = 1.0, e = 1.0, s = 0.0, and f_s = 1.0; C_f = (0,0,0,0)
	and i_f = 0."

Additions to Chapter 4 of the 1.2.1 Specification (Per-Fragment Operations
and the Framebuffer)

    None

Additions to Chapter 5 of the 1.2.1 Specification (Special Functions)

    None

Additions to Chapter 6 of the 1.2.1 Specification (State and State Requests)

    None

Additions to the GLX Specification

    None

Dependencies on SGIX_fog_offset

    If SGIX_fog_offset is supported, then both scale and offset can be
    applied to z prior to fog computation. Combining the new language of
    the two specifications is straightforward except at the end of the
    second paragraph of section 3.10, which should read:

	"...f_z is a function of the eye-coordinate distance z from the
	eye, (0,0,0,1) in eye coordinates, to the fragment center. If
	both FOG_OFFSET_SGIX and FOG_SCALE_SGIX are enabled, then f_z =
	z * f_s - f_o. If only FOG_OFFSET_SGIX is enabled, then f_z = z
	- f_o. If only FOG_SCALE_SGIX is enabled, then f_z = z * f_s.
	Otherwise, f_z = z. If f_z < 0, then it is clamped to zero prior
	to fog computation."

Errors

    None

New State

    (table 6.8, p. 198)

    Get Value		    Type    Get Command     Initial Value   Description			    Sec.   Attribute
    ---------		    ----    -----------     -------------   -----------			    ----   ---------
    FOG_SCALE_SGIX	    B	    IsEnabled	    False	    True if fog scaling is enabled  3.10   fog
    FOG_SCALE_VALUE_SGIX    R	    GetFloatv	    1.0		    Fog scaling factor		    3.10   fog

New Implementation Dependent State

    None