GL_SGIS_fog_function

Name

    SGIS_fog_function

Name Strings

    GL_SGIS_fog_function

Version

    Last Modified Date: March 30, 1999
    Author Revision: $Header: //depot/main/doc/registry/extensions/SGI/fog_func.spec#13 $

Number

    64	

Dependencies

    OpenGL 1.1 is required. Written against the OpenGL 1.2 Specification.

Overview

    This extension allows to define application-specific fog blend-factor
    function.  Function is defined by the set of the "control" points and
    should be monotonic. Each control point represented as a pair of the
    eye-space distance value and corresponding value of the fog blending 
    factor. The minimum number of control points is one. The maximum 
    number is implementation dependent.
    

New Procedures and Functions

    void FogFuncSGIS(sizei n, const float* points);

    void GetFogFuncSGIS(float *points);

Issues

   - The Bali team introduced 2 new features in this spec. 1) the
   GetFogFuncSGIS procedure that allows to querry automatically
   the current number of contol points. It is much nicer than
   the previous spec that was using gets. 2) If the user give
   fog blending factor ouside the [0.0, 1.0] range it should not
   generate an error but the valuse should be calmed instead.

   However, this extension has already been shipped and it is a
   problem. We would like to see this extension to be an EXT to
   be able to do the right thing.

   - If the above changes are not acceptable, the sentence "All the points
   of the current fog factor function are queried by calling 
   GetFogFuncSGIS." in section "Additions to Chapter 6 of the 1.2 
   Specification (State and State Requests)" should be replaced by:
   "All the points of the current fog factor function are queried by 
   calling GetFloatv or GetDoublev with <pname> set to FOG_FUNC_SGIS."
   Also, the sentence "INVALID_OPERATION is generated if GetFogFuncSGIS
   is executed between execution of Begin and the corresponding
   execution to End. " in the "Errors" section should be ignored.

   The sentences "The first value of each value pair in <points>
   specifies a value of eye-space distance (should be not negative),
   and the second value of each value pair specifies the corresponding
   value of the fog blend factor. The fog blend factor values are
   clamped to [0.0, 1.0]." should be replced by "The first 
   value of each value pair in <points> specifies a value of eye-space 
   distance (should be not negative), and the second value of each value pair 
   specifies the corresponding value of the fog blend factor (should be in 
   [0.0, 1.0] range)." Also "INVALID_VALUE is generated if fog factor
   value out of [0.0, 1.0] range." should be added to the error section.

New Tokens

    Accepted by the <param> parameter of Fogf and Fogi, and by the <params> 
    parameter of Fogiv and Fogfv, when their <pname> parameter is FOG_MODE,
    and by the <pname> parameter of GetDoublev and GetFloatv:

        FOG_FUNC_SGIS                           = 0x812A

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

        FOG_FUNC_POINTS_SGIS                    = 0x812B
        MAX_FOG_FUNC_POINTS_SGIS                = 0x812C

    May be returned by GetBooleanv, GetDoublev, GetIntegerv, and GetFloatv
    when their <pname> parameter is FOG_MODE:

        FOG_FUNC_SGIS                           = 0x812A

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 function for computing blending factor may be specified by calling 
    FogFuncSGIS with <points> pointing at an array of pairs of floating point 
    values, and <n> set to the number of value pairs in <points>. The first 
    value of each value pair in <points> specifies a value of eye-space 
    distance (should be not negative), and the second value of each value pair 
    specifies the corresponding value of the fog blend factor. The fog
    blend factor values are clamped to [0.0, 1.0]. The order in which 
    the points are specified is significant (if there is more then one
    point) based on the following requirements: 1. distance value of
    each point is not smaller then the distance value of its
    predecessor; 2. fog factor value of each point is not bigger then
    the fog factor value of its predecessor. The <n> value 
    pairs in <points> completely specify the function, replacing any previous 
    specification that may have existed. At least one control point should be 
    specified. The maximum number of control points is implementation dependent
    and may be inquired by Get commands. Initially fog function defined by a 
    single point (0.0, 1.0).
   
    Fog factor function is evaluated by fitting a curve through the points
    specified by FogFuncSGIS. This curve may be linear between adjacent points,
    or it may be smoothed, but it will pass exactly through the points, limited
    only by the resolution of the implementation. The value pair with the 
    lowest distance value specifies the fog function value for all values of 
    distance less than or equal to that pair's distance. Likewise, the value
    pair with the greatest distance value specifies the function value for 
    all values of distance greater than or equal to that pair's distance.

    If <pname> is FOG_MODE, and <param> is, or <params> point to an 
    integer FOG_FUNC_SGIS then application specified fog factor function is
    selected for the fog calculation. 

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)

    The max allowed number of points in the fog factor function specification 
    of a fog is queried by calling GetBooleanv, GetIntegerv, GetFloatv, 
    and GetDoublev with <pname> set to MAX_FOG_FUNC_POINTS_SGIS.
    The number of points in the current fog factor function is queried by 
    calling GetBooleanv, GetIntegerv, GetFloatv, and GetDoublev with <pname> 
    set to FOG_FUNC_POINTS_SGIS. All the points of the current fog
    factor function are queried by calling GetFogFuncSGIS. Points are
    returned  in the same format as they are specified in the
    glFogFuncSGIS  function.

Additions to the GLX Specification

    None

Errors

    INVALID_VALUE is generated if FogFuncSGIS parameter <n> is
    negative.

    INVALID_VALUE is generated if eye spce-distance is less then zero.

    INVALID_VALUE is generated if specified control points don't confirm
    with order requirements.

    INVALID_OPERATION is generated if FogFuncSGIS is executed between 
    execution of Begin and the corresponding execution to End.

    INVALID_OPERATION is generated if GetFogFuncSGIS is executed
    between execution of Begin and the corresponding execution to
    End. 

New State

    Get Value			Get Command		Type		Initial Value	Attribute
    ---------			-----------		----		-------------	---------
    MAX_FOG_FUNC_POINTS_SGIS	GetIntegerv		Z+		XXX
    FOG_FUNC_POINTS_SGIS	GetIntegerv		Z+		1		fog
    FOG_FUNC_SGIS		GetFloatv		2 x n x R+	(0.0, 1.0)

New Implementation Dependent State

    None

Implementation Support

   List of OpenGL implementations supporting the GL_SGIS_fog_function extension

Original File

   Original text file for the GL_SGIS_fog_function extension