Of course this won't look right in all cases, but for backdrops it will often work if you set Ambient to 1 and a white colour, then plug the texture map into the ambient channel as well.

The ambient will override the shadows. BUT it will make the backdrop emit light if you use it with IDL. So in that particular case it's not a good idea.

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  Using Poser since 2002. Currently at Version 11.1 - Win 10.