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fglFog(3G)		       OpenGL Reference			    fglFog(3G)

NAME
     fglFogf, fglFogi, fglFogfv, fglFogiv - specify fog parameters

FORTRAN SPECIFICATION
     SUBROUTINE fglFogf( INTEGER*4 pname,
			 REAL*4 param )
     SUBROUTINE fglFogi( INTEGER*4 pname,
			 INTEGER*4 param )

PARAMETERS
     pname   Specifies a single-valued fog parameter.  GL_FOG_MODE,
	     GL_FOG_DENSITY, GL_FOG_START, GL_FOG_END, and GL_FOG_INDEX are
	     accepted.

     param   Specifies the value that pname will be set to.

FORTRAN SPECIFICATION
     SUBROUTINE fglFogfv( INTEGER*4 pname,
			  CHARACTER*8 params )
     SUBROUTINE fglFogiv( INTEGER*4 pname,
			  CHARACTER*8 params )

PARAMETERS
     pname
	  Specifies a fog parameter.  GL_FOG_MODE, GL_FOG_DENSITY,
	  GL_FOG_START, GL_FOG_END, GL_FOG_INDEX, GL_FOG_COLOR, and
	  GL_FOG_OFFSET_VALUE_SGIX are accepted.

     params
	  Specifies the value or values to be assigned to pname.  GL_FOG_COLOR
	  requires an array of four values to specify the RGBA components of
	  the fog color.  GL_FOG_OFFSET_VALUE_SGIX requires an array of four
	  values to specify a reference point in eye-space coordinates, and a
	  Z offset in eye-space coordinates.  All other parameters accept an
	  array containing only a single value.

DESCRIPTION
     Fog is initially disabled.	 While enabled, fog affects rasterized
     geometry, bitmaps, and pixel blocks, but not buffer clear operations. To
     enable and disable fog, call fglEnable and fglDisable with argument
     GL_FOG.

     fglFog assigns the value or values in params to the fog parameter
     specified by pname.  The following values are accepted for pname:

     GL_FOG_MODE	 params is a single integer or floating-point value
			 that specifies the equation to be used to compute the
			 fog blend factor, f.  Four symbolic constants are
			 accepted:  GL_LINEAR, GL_EXP, GL_EXP2, and

									Page 1

fglFog(3G)		       OpenGL Reference			    fglFog(3G)

			 GL_FOG_FUNC_SGIS.  The equations corresponding to
			 these symbolic constants are defined below.  The
			 initial fog mode is GL_EXP.

     GL_FOG_DENSITY	 params is a single integer or floating-point value
			 that specifies density, the fog density used in both
			 exponential fog equations.  Only nonnegative
			 densities are accepted.  The initial fog density is
			 1.

     GL_FOG_START	 params is a single integer or floating-point value
			 that specifies start, the near distance used in the
			 linear fog equation.  The initial near distance is 0.

     GL_FOG_END		 params is a single integer or floating-point value
			 that specifies end, the far distance used in the
			 linear fog equation.  The initial far distance is 1.

     GL_FOG_INDEX	 params is a single integer or floating-point value
			 that specifies i , the fog color index.  The initial
					 f
			 fog index is 0.

     GL_FOG_COLOR	 params contains four integer or floating-point values
			 that specify C , the fog color.  Integer values are
				       f
			 mapped linearly such that the most positive
			 representable value maps to 1.0, and the most
			 negative representable value maps to -1.0.
			 Floating-point values are mapped directly.  After
			 conversion, all color components are clamped to the
			 range [0,1].  The initial fog color is (0, 0, 0, 0).

     GL_FOG_OFFSET_VALUE_SGIX
			 params contains four integer or floating-point
			 values.  The first three specify a reference point as
			 X, Y, and Z eye coordinates.  The fourth specifies a
			 Z offset in eye coordinates.  The offset is used to
			 modify z in the fog equations described below, so
			 that bright objects (such as emissive objects
			 representing light sources) will have a more
			 realistic appearance in heavily-fogged environments.
			 The significance of the reference point is discussed
			 below.	 Integer values are mapped linearly such that
			 the most positive representable value maps to 1.0,
			 and the most negative representable value maps to
			 -1.0.	Floating-point values are mapped directly.
			 The initial value is (0,0,0,0).

     Fog blends a fog color with each rasterized pixel fragment's
     posttexturing color using a blending factor f.  Factor f is computed in
     one of four ways, depending on the fog mode.  Let z be the distance in
     eye coordinates from the origin to the fragment being fogged.  If
     GL_FOG_OFFSET_SGIX is enabled, subtract the Z offset specified by

									Page 2

fglFog(3G)		       OpenGL Reference			    fglFog(3G)

     GL_FOG_OFFSET_VALUE_SGIX from z, and clamp the result so that it is
     greater than or equal to zero.

     The equation for GL_LINEAR fog is

				      ___________
				  f =
				      end - start

     The equation for GL_EXP fog is

					       .
				     -(density	 z)
				f = e

     The equation for GL_EXP2 fog is

					      .	  2
				    -(density	z)
			       f = e

     The blending factor for GL_FOG_FUNC_SGIS fog is determined by
     interpolating a set of application-defined control points.
     fglFogFuncSGIS is used to specify the control points.  Each control point
     consists of a pair of floating-point numbers.  The first number of the
     pair specifies a value of z, and the second number of the pair specifies
     a value of f.  The GL fits a curve through all the control points.	 This
     curve may be piecewise linear or it may be smoothed, but it will pass
     through the control points exactly (limited only by the resolution of the
     implementation).  If a given z is less than or equal to the z of the
     first control point, then the resulting value of f is the f of the first
     control point.  If z is greater than or equal to the z of the last
     control point, then the resulting value of f is the f of the last control
     point.  Otherwise, f is determined by finding the point on the curve that
     corresponds to the given z.

     Regardless of the fog mode, f is clamped to the range [0, 1] after it is
     computed.	Then, if the GL is in RGBA color mode, the fragment's red,
     green, and blue colors, represented by C , are replaced by
					     r

				  '
				C   = fC  + (1-f)C
				 r	r	  f

     Fog does not affect a fragment's alpha component.

     In color index mode, the fragment's color index i	is replaced by
						      r

				  '
				i   = i	 + (1-f)i
				 r     r	 f

NOTES
     In perspective projections, a Z offset in eye coordinates will map to
     different Z offsets in window coordinates at various points in the range
     of Z.  Machines that evaluate the fog blending factor by using a window Z
     coordinate to index a table must therefore compute the window-coordinate

									Page 3

fglFog(3G)		       OpenGL Reference			    fglFog(3G)

     Z offset relative to some reference point.	 This is the purpose of the
     reference point specified by the first three values in params when pname
     is GL_FOG_OFFSET_VALUE_SGIX.  For best fog accuracy in perspective
     projections, this reference point must be close to the objects being
     fogged.

     In parallel projections the window-coordinate Z offset does not vary with
     position in the Z range, so the reference point is ignored.

     GL_FOG_FUNC_SGIS is part of the SGIS_fog_func extension, not part of the
     core GL command set.  If GL_SGIS_fog_func is included in the string
     returned by fglGetString when called with argument GL_EXTENSIONS,
     extension SGIS_fog_func is supported by the connection.  See fglIntro for
     more information about using extensions.

     GL_FOG_OFFSET_VALUE_SGIX is part of the SGIX_fog_offset extension, and
     similar considerations apply.

ERRORS
     GL_INVALID_ENUM is generated if pname is not an accepted value, or if
     pname is GL_FOG_MODE and params is not an accepted value.

     GL_INVALID_VALUE is generated if pname is GL_FOG_DENSITY, and params is
     negative.

     GL_INVALID_OPERATION is generated if fglFog is executed between the
     execution of fglBegin and the corresponding execution of fglEnd.

ASSOCIATED GETS
     fglIsEnabled with argument GL_FOG
     fglIsEnabled with argument GL_FOG_OFFSET_SGIX
     fglGet with argument GL_FOG_COLOR
     fglGet with argument GL_FOG_FUNC_SGIS
     fglGet with argument GL_MAX_FOG_FUNC_POINTS_SGIS
     fglGet with argument GL_FOG_INDEX
     fglGet with argument GL_FOG_DENSITY
     fglGet with argument GL_FOG_START
     fglGet with argument GL_FOG_END
     fglGet with argument GL_FOG_MODE
     fglGet with argument GL_FOG_OFFSET_VALUE_SGIX

MACHINE DEPENDENCIES
     Fog mode GL_FOG_FUNC_SGIS is supported only on InfiniteReality systems.

     The fog offset capability is supported only on InfiniteReality systems.

     On InfiniteReality systems, fog cannot be used with color-index visuals
     that are 2 or 4 bits deep.	 Accordingly, these visuals are marked as
     non-conforming.

									Page 4

fglFog(3G)		       OpenGL Reference			    fglFog(3G)

SEE ALSO
     fglEnable, fglFogFuncSGIS

									Page 5

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