fogvertex(3G)							 fogvertex(3G)

NAME    [Toc]    [Back]

     fogvertex - specify atmospheric fogging effects

C SPECIFICATION    [Toc]    [Back]

     void fogvertex(long mode, float params[])

PARAMETERS    [Toc]    [Back]

     mode     expects one of eight valid symbolic constants:

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

	      FG_VTX_EXP2:  FG_PIX_EXP2:  interpret params as a	specification
	      for fog density and color.  These	modes are suited to simulate
	      distant haze.

	      FG_VTX_LIN:  FG_PIX_LIN:	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 depthcue:

	      FG_ON:  enable the previously defined fog	calculation.

	      FG_OFF:  disable fog calculations. (default)

     params   Expects an array of floats containing value settings.  For
	      FG_VTX_EXP, FG_PIX_EXP, FG_VTX_EXP2, and FG_PIX_EXP2 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 FG_VTX_LIN and
	      FG_PIX_LIN 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 FG_ON, or FG_OFF.

DESCRIPTION    [Toc]    [Back]

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

     Calculation of the	blend factor for:

     FG_VTX_EXP:

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




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fogvertex(3G)							 fogvertex(3G)



     FG_PIX_EXP:

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


     FG_VTX_EXP2:

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


     FG_PIX_EXP2:

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


     FG_VTX_LIN:

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


     FG_PIX_LIN:

	  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.





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fogvertex(3G)							 fogvertex(3G)



     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    [Toc]    [Back]

     gRGBcolor,	mmode

NOTES    [Toc]    [Back]

     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, FG_PIX_EXP, FG_PIX_EXP2,
     and FG_PIX_LIN, while the VGXT and	SkyWriter support all fog options.
     Use getgdesc(GD_FOGVERTEX)	to determine whether fog support is available.

     Fog only works for	mmode(MVIEWING).

     For FG_PIX_EXP, FG_PIX_EXP2, and FG_PIX_LIN, 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
     lsetdepth range.

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


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