subtitling and flat video workflow

For subtitling and displaying a flat video on the planetarium dome,

• Aegisub for subtitling
  
  
• Avidemux for resizing - for a quick and dirty solution, adding 360 pixels of black padding above a 720p video (and suitable padding on left and right) gives an acceptable video.
  
  
• Better aesthetics can be done with adding a background video with an alpha mask (created in Gimp) and overlaid with Avisynth's Overlay and Mask. Or Layer, which I used, similar to the example in the documentation, where our video was the bg, and the "background video" was on top.
  

  bg  = AviSource("the-flat-video.avi").ConvertToRGB32      
  mk  = ImageSource("mask.jpg").ConvertToRGB32       
  top = AviSource("video-to-fill-rest-of-space.avi").ConvertToRGB32.Mask(mk) 
  Layer(bg, top)                                   

• For wider dome coverage - like 180 degrees horizontal coverage for a 1080p flat video - we can use OCVWarp after overlaying the video on a 4k background. Or use OCVvid2fulldome if final output is a square-frame fulldome file.