Modify the drawtext filter in Ffmpeg

I am trying to embed various texts into video files, e.g. GPS coordinates for each second. As the text changes during the video, I can't specify it statically for the drawtext ffmpeg filter, and would like to implement the solution proposed here: [login to view URL] You should be already familiar with Ffmpeg, but I specify here the documentation about the drawtext filter: [login to view URL] With the modified filter, I'd like to use the "timecode" parameter to specify the file name which contains the text to be drawn (one line per second). The "textfile" parameter is a good indication of how files are read by this filter. The source code can be explored here: [login to view URL] Besides this "timecode" change, I'd also like to have a slightly larger box drawn below the text as the background. A 3-5 pixel margin should probably look OK. Please see the "box" and "boxcolor" parameters, and look for "draw_box" in the code.