extensions/SGIX/SGIX_fog_offset.txt

Name

    SGIX_fog_offset

Name Strings

    GL_SGIX_fog_offset

Version

    $Date: 1999/06/19 00:39:08 $ $Revision: 1.12 $

Number

    65

Dependencies

    OpenGL 1.0 is required.
    The extension is written against the OpenGL 1.2.1 Specification.

Overview

    This extension allows fragments to look brighter in a foggy
    environment, by biasing the fragment eye-coordinate distance prior
    to fog computation. A reference point in eye space (rx ry rz) and an offset
    amount toward the viewpoint (f_o) are specified. When fog offset is
    enabled, the offset amount will be subtracted from the fragment
    distance, making objects appear less foggy.

    If fog computation is done in screen-space coordinates under
    perspective projection, the reference point is used in adjusting the
    fog offset to be correct for fragments whose depth is close to that
    point. The reference point should be redefined when it becomes too
    far away from the primitives being drawn. Under orthographic
    projection, or if fog computation is done in eye-space coordinates,
    the reference point is ignored.

New Procedures and Functions

    None

New Tokens

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

	GL_FOG_OFFSET_SGIX		0x8198

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

	GL_FOG_OFFSET_VALUE_SGIX	0x8199

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_OFFSET_SGIX is enabled, then f_z = z - f_o; otherwise, f_z =
	z. If f_z < 0, then it is clamped to zero prior to fog
	computation.

	"If fog offset is performed in screen space under perspective
	projection, the reference point (rx ry rz) is used to adjust f_o
	appropriately prior to computing f_z. Otherwise, the reference
	point is unused.

	"The equation, along with the parameters d, e, s, (rx ry rz, and
	f_o, is specified with..."


    Change the end of the third paragraph to:

	"...If <pname> is FOG_DENSITY, FOG_START, or FOG_END, then
	<param> is or <params> points to a value that is d, s, or e,
	respectively. If <pname> if FOG_OFFSET_SGIX, then <params>
	points to four values comprising a 3-valued reference point,
	(rx ry rz) followed by f_o. 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, three floating-point values d, e,
	and s, four floating-point values (rx ry rz f_o), 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 offset is enabled. In the initial state, fog is disabled,
	fog offset is disabled, FOG_MODE is EXP, d = 1.0, e = 1.0, s =
	0.0, and (rx ry rz f_o) = (0,0,0,0); 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

Errors

    None

New State

    (table 6.8, p. 198)

    Get Value		    Type    Get Command     Initial Value   Description			    Sec.    Attribute
    ---------		    ----    -----------     -------------   -----------			    ----    ---------
    FOG_OFFSET_SGIX	    B	    IsEnabled	    False	    True if fog offset is enabled   3.10    fog
    FOG_OFFSET_VALUE_SGIX   4 * R   GetFloatv	    (0, 0, 0, 0)    Fog reference point and offset  3.10    fog

New Implementation Dependent State

    None