Client-side backprojection of presentation slides into educational video

Abstract

A significant part of many videos of lectures is presentation slides that occupy much of the field of view. Further, for a student studying the lecture, having the slides sharply displayed is especially important, compared with the speaker, background, and audience. However, even if the original capture supports it, the bandwidth required for real time viewing is substantive, especially in the context of mobile devices. Here we propose reconstructing the video on the client side by backprojecting high resolution slide images into the video stream with the slide area blacked out. The high resolution slide deck can be sent once, and inserted into the video on the client side based on the transformation (a homography) computed in advance. We further introduce the idea that needed homography transformations can be approximated using affine transformations, which allows it to be done using built-in capabilities of HTML 5. We find that it is possible to significantly reduce bandwidth by compressing the modified video, while improving the slide area quality, but leaving the non-slide area roughly the same.