k3d/share/shaders/k3d_velvet.sl:27: *			"backscatter" parameter to control it; added
k3d/share/shaders/k3d_velvet.sl:40:	    float backscatter = 0.1,
k3d/share/shaders/k3d_velvet.sl:61:    shiny += pow ( cosine, 1.0/roughness ) * backscatter
