I changed the macro to this and it seems to work OK:
#macro gamma_color_adjust(in_color)
#local out_gamma = 2.2;
#local in_color = in_color + <0,0,0,0,0>;
<
pow(in_color.red, out_gamma),
pow(in_color.green, out_gamma),
pow(in_color.blue, out_gamma),
in_color.filter,
in_color.transmit
>
#end
--
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