1. A method for combining multiple video streams, comprising: extracting a sequence of media frames 224 - 1 / 224 - 2 from presenter 222 - 1 video and from shared digital rich media 222 - 2 video 340 ; analyzing the content 226 on the media frames 224 - 1 / 224 - 2 to determine a set of space and time varying alpha values 228 based on the content analysis 226 / 342 ; and performing a compositing operation 230 to produce combined video frames 232 based on the alpha values 228 / 344 .