k3d/share/shaders/k3d_rustymetal.sl:13: *   rustKa, rustKd, rustcolor - control the appearance of the rust.
k3d/share/shaders/k3d_rustymetal.sl:45:	    float rustKa = 1, rustKd = 1;
k3d/share/shaders/k3d_rustymetal.sl:86:      Crust = rustcolor * (rustKa*ambient() + rustKd*diffuse(Nf));
