fogver(3G)

NAME
     fogver - specify atmospheric fogging effects

FORTRAN SPECIFICATION
     subroutine fogver(mode, params)
     integer*4 mode
     real params()

PARAMETERS
     mode     expects one of eight valid symbolic constants:

              FGVE:  FGPE:  interpret params as a specification for fog
              density and color.  These modes are suited to simulate heavy
              fog.

              FGVE2:  FGPE2:  interpret params as a specification for fog
              density and color.  These modes are suited to simulate distant
              haze.

              FGVL:  FGPL:  interpret params as a specification for eye space
              distance to start of fog, eye space distance at which fully
              fogged, and fog color.  These modes can be used as a more
              versatile alternative to depthc:

              FGON:  enable the previously defined fog calculation.

              FGOFF:  disable fog calculations. (default)

     params   Expects an array of floats containing value settings.  For FGVE,
              FGPE, FGVE2, and FGPE2 four floats are expected.  They are
              density, red, green, and blue.  density specifies the
              (thickness) of the fog (or haze).  A value of 0.0 results in no
              fog.  Increasing positive values result in fog of increasing
              density.  Values are normalized such that a density of 1.0
              results in the fog becoming completely opaque at a distance of
              1.0 in eye-coordinates.  For FGVL and FGPL five floats are
              expected.  They are eye space distance to start of fog, eye
              space distance at which fog is completely opaque, red, green,
              and blue.  The colors are specified in the range 0.0 through
              1.0.  The specified contents of params are insignificant when
              mode is

DESCRIPTION
     The effects of fog on shading are simulated by blending computed object
     colors into the specified fog color. shadem is ignored during fogging.

     Calculation of the blend factor for:

     FGVE:

          fog = e ** (5.5*density*Zeye)
          fog is computed at each vertex of the primitive


     FGPE:

          fog = e ** (5.5*density*Zeye)
          fog is computed at each pixel of the primitive


     FGVE2:

          fog = e ** (-5.5*(density*Zeye) ** 2)
          fog is computed at each vertex of the primitive


     FGPE2:

          fog = e ** (-5.5*(density*Zeye) ** 2)
          fog is computed at each pixel of the primitive


     FGVL:

          fog = (end_fog + Zeye)/(end_fog - start_fog)
          fog is computed at each vertex of the primitive


     FGPL:

          fog = (end_fog + Zeye)/(end_fog - start_fog)
          fog is computed at each pixel of the primitive


     Where:

     Zeye        is the Z coordinate in eye space (always negative).

     density     is the fog density.

     fog         is the computed fog blending factor, ranging from 0 to 1.

     start_fog   is the eye-space distance at which fog effect begins.

     end_fog     is the eye-space distance at which fog is opaque.

     The pixel color/fog color blend is done with the following equation:

          C = Cp*fog + Cf*(1.0-fog)


     Where:
     fog   is the computed fog blending factor, ranging from 0 to 1.

     C     is the resulting color component (red, green, or blue).

     Cp    is the incoming pixel color, already either Gouraud or flat shaded,
           and textured.

     Cf    is the fog color component as specified by the fog definition

     Eye-coordinates exist between ModelView transformation and Projection
     transformation (see mmode).  This space is right-handed, so visible
     vertices always have negative Z coordinates.  Thus the fog equation
     always raises e to a negative power.

     In all cases (including ortho) the viewer is considered to be at location
     0,0,0, looking down the negative z axis.

SEE ALSO
     gRGBco, mmode

NOTES
     IRIS-4D G, GT, GTX, and the Personal Iris models do not support any fog
     options. IRIS Entry, Indy, XL, XS, XS24, XZ, Elan, Extreme, and VGX
     systems do not support the per pixel fog modes, FGPE, FGPE2, and FGPL,
     while the VGXT and SkyWriter support all fog options.  Use getgde to
     determine whether fog support is available.

     Fog only works for mmode(MVIEWING).

     For FGPE, FGPE2, and FGPL, to maximize the accuracy of the fog, minimize
     the ratio of the distance to the far clipping plane over the distance to
     the near clipping plane.  In addition, maximize the lsetde range.

     The results of fog calculations are defined only while in RGB mode.