How did superimposing effects against white card work?

[Oller Visuals]

Felt this was more of a topic for the Film forum rather than special effects.

Just watching documentary about Blade Runner and they did some effects (like props) against white card, and then superimposed it somehow. I don't get it... how does this actually work? How did they get rid of the white without keying?

 

[Liam Hall]

Not sure about Blade Runner, but on film, you'd use the white reversed to produce black and use that as a solid matte on an optical printer since black won't let light through.