fog_coord.txt

Name

    EXT_fog_coord

Name Strings

    GL_EXT_fog_coord

Contact

    Jon Leech, Silicon Graphics (ljp 'at' sgi.com)

Status

    Shipping (version 1.6)

Version

    $Date: 1999/06/21 19:57:19 $ $Revision: 1.11 $

Number

    149

Dependencies

    OpenGL 1.1 is required.
    The extension is written against the OpenGL 1.2 Specification.

Overview

    This extension allows specifying an explicit per-vertex fog
    coordinate to be used in fog computations, rather than using a
    fragment depth-based fog equation.

Issues

  * Should the specified value be used directly as the fog weighting
    factor, or in place of the z input to the fog equations?

        As the z input; more flexible and meets ISV requests.

  * Do we want vertex array entry points? Interleaved array formats?

        Yes for entry points, no for interleaved formats, following the
        argument for secondary_color.

  * Which scalar types should FogCoord accept? The full range, or just
    the unsigned and float versions? At the moment it follows Index(),
    which takes unsigned byte, signed short, signed int, float, and
    double.

        Since we're now specifying a number which behaves like an
        eye-space distance, rather than a [0,1] quantity, integer types
        are less useful. However, restricting the commands to floating
        point forms only introduces some nonorthogonality.

        Restrict to only float and double, for now.

  * Interpolation of the fog coordinate may be perspective-correct or
    not. Should this be affected by PERSPECTIVE_CORRECTION_HINT,
    FOG_HINT, or another to-be-defined hint?

        PERSPECTIVE_CORRECTION_HINT; this is already defined to affect
        all interpolated parameters. Admittedly this is a loss of
        orthogonality.

  * Should the current fog coordinate be queryable?

        Yes, but it's not returned by feedback.

  * Control the fog coordinate source via an Enable instead of a fog
    parameter?

        No. We might want to add more sources later.

  * Should the fog coordinate be restricted to non-negative values?

        Perhaps. Eye-coordinate distance of fragments will be
        non-negative due to clipping. Specifying explicit negative
        coordinates may result in very large computed f values, although
        they are defined to be clipped after computation.

  * Use existing DEPTH enum instead of FRAGMENT_DEPTH? Change name of
    FRAGMENT_DEPTH_EXT to FOG_FRAGMENT_DEPTH_EXT?

        Use FRAGMENT_DEPTH_EXT; FOG_FRAGMENT_DEPTH_EXT is somewhat
        misleading, since fragment depth itself has no dependence on
        fog.

New Procedures and Functions

    void FogCoord[fd]EXT(T coord)
    void FogCoord[fd]vEXT(T coord)
    void FogCoordPointerEXT(enum type, sizei stride, void *pointer)

New Tokens

    Accepted by the <pname> parameter of Fogi and Fogf:

        FOG_COORDINATE_SOURCE_EXT           0x8450

    Accepted by the <param> parameter of Fogi and Fogf:

        FOG_COORDINATE_EXT                  0x8451
        FRAGMENT_DEPTH_EXT                  0x8452

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

        CURRENT_FOG_COORDINATE_EXT          0x8453
        FOG_COORDINATE_ARRAY_TYPE_EXT       0x8454
        FOG_COORDINATE_ARRAY_STRIDE_EXT     0x8455

    Accepted by the <pname> parameter of GetPointerv:

        FOG_COORDINATE_ARRAY_POINTER_EXT    0x8456

    Accepted by the <array> parameter of EnableClientState and
    DisableClientState:

        FOG_COORDINATE_ARRAY_EXT            0x8457

Additions to Chapter 2 of the OpenGL 1.2 Specification (OpenGL Operation)

  These changes describe a new current state type, the fog coordinate,
  and the commands to specify it:

  - (2.6, p. 12) Second paragraph changed to:

        "Each vertex is specified with two, three, or four coordinates.
        In addition, a current normal, current texture coordinates,
        current color, and current fog coordinate may be used in
        processing each vertex."

  - 2.6.3, p. 19) First paragraph changed to

        "The only GL commands that are allowed within any Begin/End
        pairs are the commands for specifying vertex coordinates, vertex
        colors, normal coordinates, texture coordinates, and fog
        coordinates (Vertex, Color, Index, Normal, TexCoord,
        FogCoord)..."

  - (2.7, p. 20) Insert the following paragraph following the third
        paragraph describing current normals:

        "   The current fog coodinate is set using
                void FogCoord[fd]EXT(T coord)
                void FogCoord[fd]vEXT(T coord)."

    The last paragraph is changed to read:

        "The state required to support vertex specification consists of
        four floating-point numbers to store the current texture
        coordinates s, t, r, and q, one floating-point value to store
        the current fog coordinate, four floating-point values to store
        the current RGBA color, and one floating-point value to store
        the current color index. There is no notion of a current vertex,
        so no state is devoted to vertex coordinates. The initial values
        of s, t, and r of the current texture coordinates are zero; the
        initial value of q is one. The initial fog coordinate is zero.
        The initial current normal has coordinates (0,0,1). The initial
        RGBA color is (R,G,B,A) = (1,1,1,1). The initial color index is
        1."

  - (2.8, p. 21) Added fog coordinate command for vertex arrays:

    Change first paragraph to read:

        "The vertex specification commands described in section 2.7
        accept data in almost any format, but their use requires many
        command executions to specify even simple geometry. Vertex data
        may also be placed into arrays that are stored in the client's
        address space. Blocks of data in these arrays may then be used
        to specify multiple geometric primitives through the execution
        of a single GL command. The client may specify up to seven
        arrays: one each to store edge flags, texture coordinates, fog
        coordinates, colors, color indices, normals, and vertices. The
        commands"

    Add to functions listed following first paragraph:

        void FogCoordPointerEXT(enum type, sizei stride, void *pointer)

    Add to table 2.4 (p. 22):

        Command                     Sizes   Types
        -------                     -----   -----
        FogCoordPointerEXT          1       float,double

    Starting with the second paragraph on p. 23, change to add
    FOG_COORDINATE_ARRAY_EXT:

        "An individual array is enabled or disabled by calling one of

            void EnableClientState(enum array)
            void DisableClientState(enum array)

        with array set to EDGE_FLAG_ARRAY, TEXTURE_COORD_ARRAY,
        FOG_COORDINATE_ARRAY_EXT, COLOR_ARRAY, INDEX_ARRAY,
        NORMAL_ARRAY, or VERTEX_ARRAY, for the edge flag, texture
        coordinate, fog coordinate, color, color index, normal, or
        vertex array, respectively.

        The ith element of every enabled array is transferred to the GL
        by calling

            void ArrayElement(int i)

        For each enabled array, it is as though the corresponding
        command from section 2.7 or section 2.6.2 were called with a
        pointer to element i. For the vertex array, the corresponding
        command is Vertex<size><type>v, where <size> is one of [2,3,4],
        and <type> is one of [s,i,f,d], corresponding to array types
        short, int, float, and double respectively. The corresponding
        commands for the edge flag, texture coordinate, fog coordinate,
        color, color, color index, and normal arrays are EdgeFlagv,
        TexCoord<size><type>v, FogCoord<type>v, Color<size><type>v,
        Index<type>v, and Normal<type>v, respectively..."

    Change pseudocode on p. 27 to disable fog coordinate array for
    canned interleaved array formats. After the lines

            DisableClientState(EDGE_FLAG_ARRAY);
            DisableClientState(INDEX_ARRAY);

        insert the line

            DisableClientState(FOG_COORDINATE_ARRAY_EXT);

    Substitute "seven" for every occurence of "six" in the final
    paragraph on p. 27.

  - (2.12, p. 41) Add fog coordinate to the current rasterpos state.

    Change the first sentence of the first paragraph to read

        "The state required for the current raster position consists of
        three window coordinates x_w, y_w, and z_w, a clip coordinate
        w_c value, an eye coordinate distance, a fog coordinate, a valid
        bit, and associated data consisting of a color and texture
        coordinates."

    Change the last paragraph to read

        "The current raster position requires six single-precision
        floating-point values for its x_w, y_w, and z_w window
        coordinates, its w_c clip coordinate, its eye coordinate
        distance, and its fog coordinate, a single valid bit, a color
        (RGBA color and color index), and texture coordinates for
        associated data. In the initial state, the coordinates and
        texture coordinates are both (0,0,0,1), the fog coordinate is 0,
        the eye coordinate distance is 0, the valid bit is set, the
        associated RGBA color is (1,1,1,1), and the associated color
        index color is 1. In RGBA mode, the associated color index
        always has its initial value; in color index mode, the RGBA
        color always maintains its initial value."

  - (3.10, p. 139) Change the second and third paragraphs to read

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

            f = exp(-d*c)       (3.24)
            f = exp(-(d*c)^2)   (3.25)
            f = (e-c)/(e-s)     (3.26)

        If the fog source (as defined below) is FRAGMENT_DEPTH_EXT, then
        c is the eye-coordinate distance from the eye, (0 0 0 1) in eye
        coordinates, to the fragment center. If the fog source is
        FOG_COORDINATE_EXT, then c is the interpolated value of the fog
        coordinate for this fragment. The equation and the fog source,
        along with either d or e and s, is specified with

            void Fog{if}(enum pname, T param);
            void Fog{if}v(enum pname, T params);

        If <pname> is FOG_MODE, then <param> must be, or <param> must
        point to an integer that is one of the symbolic constants EXP,
        EXP2, or LINEAR, in which case equation 3.24, 3.25, or 3.26,,
        respectively, is selected for the fog calculation (if, when 3.26
        is selected, e = s, results are undefined). If <pname> is
        FOG_COORDINATE_SOURCE_EXT, then <param> is or <params> points to
        an integer that is one of the symbolic constants
        FRAGMENT_DEPTH_EXT or FOG_COORDINATE_EXT. 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 d is
        specified less than zero, the error INVALID_VALUE results."

  - (3.10, p. 140) Change the last paragraph preceding section 3.11
    to read

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

Additions to Chapter 3 of the OpenGL 1.2.1 Specification (Rasterization)

    None

Additions to Chapter 4 of the OpenGL 1.2.1 Specification (Per-Fragment
Operations and the Frame Buffer)

    None

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

    None

Additions to Chapter 6 of the OpenGL 1.2 Specification (State and State
Requests)

    None

Additions to Appendix A of the OpenGL 1.2.1 Specification (Invariance)

    None

Additions to the GLX / WGL / AGL Specifications

    None

GLX Protocol

    Two new GL rendering commands are added. The following commands are
    sent to the server as part of a glXRender request:

        FogCoordfvEXT
            2           8               rendering command length
            2           4124            rendering command opcode
            4           FLOAT32         v[0]

        FogCoorddvEXT
            2           12              rendering command length
            2           4125            rendering command opcode
            8           FLOAT64         v[0]

Errors

    INVALID_ENUM is generated if FogCoordPointerEXT parameter <type> is
    not FLOAT or DOUBLE.

    INVALID_VALUE is generated if FogCoordPointerEXT parameter <stride>
    is negative.

New State

(table 6.5, p. 195)
    Get Value                   Type    Get Command     Initial Value   Description     Sec Attribute
    ---------                   ----    -----------     -------------   -----------     --- ---------
    CURRENT_FOG_COORDINATE_EXT  R       GetIntegerv,    0               Current         2.7 current
                                        GetFloatv                       fog coordinate

(table 6.6, p. 197)
    Get Value                           Type    Get Command     Initial Value   Description                     Sec Attribute
    ---------                           ----    -----------     -------------   -----------                     --- ---------
    FOG_COORDINATE_ARRAY_EXT            B       IsEnabled       False           Fog coord array enable          2.8 vertex-array
    FOG_COORDINATE_ARRAY_TYPE_EXT       Z8      GetIntegerv     FLOAT           Type of fog coordinate          2.8 vertex-array
    FOG_COORDINATE_ARRAY_STRIDE_EXT     Z+      GetIntegerv     0               Stride between fog coords       2.8 vertex-array
    FOG_COORDINATE_ARRAY_POINTER_EXT    Y       GetPointerv     0               Pointer to the fog coord array  2.8 vertex-array


(table 6.8, p. 198)
    Get Value                   Type    Get Command     Initial Value       Description     Sec     Attribute
    ---------                   ----    -----------     -------------       -----------     ---     ---------
    FOG_COORDINATE_SOURCE_EXT   Z2      GetIntegerv,    FRAGMENT_DEPTH_EXT  Source of fog   3.10    fog
                                        GetFloatv                           coordinate for
                                                                            fog calculation

Revision History

  * Revision 1.6 - Functionality complete

  * Revision 1.7-1.9 - Fix typos and add fields to bring up to date with
    the new extension template. No functionality changes.