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homeprevnext Name
homeprevnext Name Strings
homeprevnext Contact
    Jon Leech, Silicon Graphics (ljp 'at'  
homeprevnext Status
    Shipping (version 1.6)  
homeprevnext Version
    $Date: 1999/06/21 19:57:19 $ $Revision: 1.11 $  
homeprevnext Number
homeprevnext Dependencies
    OpenGL 1.1 is required.  
    The extension is written against the OpenGL 1.2 Specification.  
homeprevnext 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.  
homeprevnext 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  
        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  
  * 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  
        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  
        misleading, since fragment depth itself has no dependence on  
homeprevnext New Procedures and Functions
    void FogCoord[fd]EXT(T coord)  
    void FogCoord[fd]vEXT(T coord)  
    void FogCoordPointerEXT(enum type, sizei stride, void *pointer)  
homeprevnext 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  
    Accepted by the <pname> parameter of GetPointerv:  
    Accepted by the <array> parameter of EnableClientState and  
        FOG_COORDINATE_ARRAY_EXT            0x8457  
homeprevnext 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,  
  - (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  
  - (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  
    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  
        "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,  
        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  
        insert the line  
    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  
    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  
            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  
        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."  
homeprevnext Additions to Chapter 3 of the OpenGL 1.2.1 Specification (Rasterization)
homeprevnext Additions to Chapter 4 of the OpenGL 1.2.1 Specification (Per-Fragment Operations and the Frame Buffer)
homeprevnext Additions to Chapter 5 of the OpenGL 1.2.1 Specification (Special Functions)
homeprevnext Additions to Chapter 6 of the OpenGL 1.2 Specification (State and State Requests)
homeprevnext Additions to Appendix A of the OpenGL 1.2.1 Specification (Invariance)
homeprevnext Additions to the GLX / WGL / AGL Specifications
homeprevnext GLX Protocol
    Two new GL rendering commands are added. The following commands are  
    sent to the server as part of a glXRender request:  
            2           8               rendering command length  
            2           4124            rendering command opcode  
            4           FLOAT32         v[0]  
            2           12              rendering command length  
            2           4125            rendering command opcode  
            8           FLOAT64         v[0]  
homeprevnext Errors
    INVALID_ENUM is generated if FogCoordPointerEXT parameter <type> is  
    not FLOAT or DOUBLE.  
    INVALID_VALUE is generated if FogCoordPointerEXT parameter <stride>  
    is negative.  
homeprevnext 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  
homeprevnext 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.  
    the new extension template. No functionality changes.  
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