declare shader 
    color "ground_fog" (
	color   "fog_color"          default 1 1 1,
	color   "fog_density"        default 1 1 1,
	scalar  "fade_start"         default 0,
	scalar  "fade_end"           default 1,
	scalar  "unit_density"       default 1,
	scalar  "march_increment"    default 0.1,
    )
    version 1
    apply volume
end declare