declare shader
    color "threshold_volume" (
	color  "color" default 1 1 1 1,
	vector "center" default 0 0 0,
	scalar "radius" default 1,
	scalar "density_threshold" default 0,
	scalar "unit_density" default 1,
	scalar "march_increment" default 0.1
    )
    version 1
    apply volume
end declare