[NTG-context] quick and shallow question about layers

Hi, this is pretty shallow, but maybe someone knows a way: for one of my presentation, every slide has a (different) picture as background. The title is typeset within a narrow colored frame which extends across the entire slide. Is there any way to make the background picture show through the letters of the title, i.e. make the letters transparent within the colored frame? I thought of metapost’s infill, but it appears that this only adds a background color, not real transparency. Hope the question is clear enough - just in case someone has a suggestion; my life doesn’t depend on it! Thanks and all best Thomas