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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.

Video is an excellent vehicle for astronomy education and outreach. Usage patterns and user demographics are presented for short videos covering a variety of astronomy topics, delivered to public audiences from three websites, three YouTube channels, and three massive open online classes, or MOOCs. The data spans over a decade in some cases. The modality of the content ranges from short lecture presentations of sub-topics in astronomy to longer scripted pieces created by students to Q&A sessions held by the MOOC instructor with live audiences of 100-200. In the aggregate, the videos have attracted 1.2 million views, and those viewers have watched 77,300 hours of astronomy content. Most of the viewers are not based in the United States. Viewership rose dramatically at the outset of the COVID-19 pandemic and has not yet returned to pre-pandemic levels. The videos watched by lifelong learners taking a MOOC show a decline in usage as they progress through the online course. But on these YouTube channels, when viewers can choose among the topics, the most popular are cosmology and exoplanets. Suggestions are made for the effective ways to create and disseminate astronomy videos.

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