To separate my posers from the background I do this: I render the characters with any background color. I remove the lights and save as a different file. Then I render against a white background. I open combustion and bring both clips in. I apply a "set matte" effect to the character layer and use the black-white clip as matte source. I set to "inverse luminance" and voila :)

nemirc
Renderosity Magazine Staff Writer
https://renderositymagazine.com/users/nemirc
https://about.me/aris3d/