extensions/SGIX/SGIX_spotlight_cutoff.txt

XXX - Not complete yet!!!

Name

    SGIX_spotlight_cutoff

Name Strings

    GL_SGIX_spotlight_cutoff

Version

    $Date: 1997/09/29 21:22:36 $ $Revision: 1.5 $

Number

    131

Dependencies

    OpenGL 1.1 is required.
    SGIX_fragment_light affects the definition of this extension.

Overview

    As specified, the transition to black at the edge of a spotlight cone
    is abrupt and, therefore, can cause aliasing in fragment lighting.
    This extension adds a new parameter to vertex and fragment lights that
    defines an angular region just within the cutoff angle over which the
    spotlight attenuation is transitioned smoothly to zero.  The extension
    is modeled on that from the RenderMan Interface from Pixar.

Issues

    *   Should this single parameter be included in the fragment_light
	spec (and thus removing the requirement of this spec)?

    *	Do we want this to apply to per-vertex lights too?  I would say
	yes at this point because this feature can be used for visual
	effects like those of area light sources.  Vertex lights can
	benefit almost as much as fragment lights in this situation.

    *   Do we want to force a certain falloff function or leave it up to
	the implementation?  I think we should leave it open, but force
	it to be monotonic.

New Procedures and Functions

    None

New Tokens

    Accepted by the <pname> parameter of Lightf, Lighti, Lightfv, and
    Lightiv and by the <pname> parameter of GetLightfv and GetLightiv:

    SPOT_CUTOFF_DELTA_SGIX		0x8193

Additions to Chapter 2 of the 1.1 Specification (OpenGL Operation)

    Insert in Table 2.7 (need to replicate whole table...):

    Param	Type	Default		Description
    ----------------------------------------------------------------
    d_rli	real	0.0		spotlight delta cutoff angle
					(range:  [0.0,90.0])

    Equation 2.5 becomes:

    If d_rli<=c_rli:

    spot_i = (P_pliV.s_dli)^s_rli   c_rli!=180.0 P_pliV.s_dli>=cos(c_rli-d_rli)
             smooth falloff         c_rli!=180.0 cos(c_rli)>=P_pliV.s_dli>cos(c_rli-d_rli)
             0.0                    c_rli!=180.0 P_pliV.s_dli<cos(c_rli)
             1.0                    c_rli==180.0

    If d_rli>c_rli:

    spot_i = (P_pliV.s_dli)^s_rli   c_rli!=180.0 P_pliV.s_dli>=cos(c_rli)
             0.0                    c_rli!=180.0 P_pliV.s_dli<cos(c_rli)
             1.0                    c_rli==180.0


    Before the paragraph beginning "The value of A produced..." insert:

    When d_rli<=c_rli, the spotlight attenuation transitions smoothly to
    0.0 at the cutoff angle over an angle d_rli.  The falloff is
    implementation specific and may be, for instance, linear or cubic,
    however it must monotonically decrease from (cos(c_rli-d_rli))^s_rli
    to 0.0.

    Insert in Table 2.8 (need to replicate whole table...):

    Param	Name			Number of Values
    ----------------------------------------------------
    d_rli	SPOT_CUTOFF_DELTA_SGIX	1

Additions to Chapter 3 of the 1.1 Specification (Rasterization)

    In Section 3.9.2 Lighting Operation insert after lighting equation:

    If d_rli<=c_rli:

    spot_i = (P_pliV.s_dli)^s_rli   c_rli!=180.0 P_pliV.s_dli>=cos(c_rli-d_rli)
             smooth falloff         c_rli!=180.0 cos(c_rli)>=P_pliV.s_dli>cos(c_rli-d_rli)
             0.0                    c_rli!=180.0 P_pliV.s_dli<cos(c_rli)
             1.0                    c_rli==180.0

    If d_rli>c_rli:

    spot_i = (P_pliV.s_dli)^s_rli   c_rli!=180.0 P_pliV.s_dli>=cos(c_rli)
             0.0                    c_rli!=180.0 P_pliV.s_dli<cos(c_rli)
             1.0                    c_rli==180.0

    When d_rli<=c_rli, the spotlight attenuation transitions smoothly to
    0.0 at the cutoff angle over an angle d_rli.  The falloff is
    implementation specific and may be, for instance, linear or cubic,
    however it must monotonically decrease from (cos(c_rli-d_rli))^s_rli
    to 0.0.

Additions to Chapter 4 of the 1.1 Specification (Per-Fragment Operations
and the Frame Buffer)

    None

Additions to Chapter 5 of the 1.1 Specification (Special Functions)

    None

Additions to Chapter 6 of the 1.1 Specification (State and State Requests)

    TBD

Additions to the GLX Specification

    TBD

Dependencies on SGIX_fragment_light

    If SGIX_fragment_light is not implemented, references to fragment
    lighting in this document are invalid and should be ignored.

Errors

    GL_INVALID_VALUE is generated if SPOT_CUTOFF_DELTA_SGIX is outside the
    range [0,90].

New State

    Get Value			Get Command	Type		Initial Value		Attribute
    ---------			-----------	----		-------------		---------
    SPOT_CUTOFF_DELTA_SGIX	GetLightfv	1*xR+		0.0			lighting

New Implementation Dependent State

    None