Add semi-transparent box around subtitle
Use

ffmpeg -i movie.mkv -strict -2 -vf "subtitles=subtitle.srt:force_style='FontName=ubuntu,fontsize=30,BorderStyle=3,OutlineColour=&amp;H80000000',scale=720:-1" -qscale:v 3  Movie.avi


See Style Fields and Data Types for tag info and color syntax.