ASS subtitles can have a semi-transparent background for the text.

With aegisub

The easiest way to do this is with aegisub.

1. Open your subtitles file with aegisub.
2. Click Subtitle → Styles manager.
3. Under Current Script choose Default, then press the Edit button.
4. Experiment with the Outline and Shadow values. Check Opaque box.
5. Under Colors click the color under Outline or Shadows. A window will appear. Adjust the value of the Alpha box to change transparency.
6. Save the subtitles as an .ass file.

Now you can use the AAS file to make hardsubs or softsubs with ffmpeg.

Without aegisub

If you want hardsubs you can use the subtitles filter to add the transparent background with the force_style option.

ffmpeg -i input -filter_complex "subtitles=subs.ass:force_style='OutlineColour=&H80000000,BorderStyle=3,Outline=1,Shadow=0,MarginV=20'" output

• This will work with any text based subtitles supported by FFmpeg because the filter will automatically convert them to ASS.

• See SubStation Alpha (ASS) style fields for formatting options.

Issue with multiple lines

If your subtitles contains multiple lines, due to auto-wrapping of long lines or an intentional line break, the backgrounds will overlap and potentially look ugly as shown below:

You can avoid this by:

1. Changing the Outline and Shadow sizes to 0.
2. The alpha settings of the shadow will control the transparency of the background box. Click on the shadow color to adjust the Alpha of the shadow color to your desired transparency level.

3. Edit the ASS file in a text editor. In the Style line change the value corresponding with BorderStyle to 4. This will fill the bounding box background of each subtitle event. Example Style line:

   Style: Default,Arial,20,&H00FFFFFF,&H000000FF,&H80000000,&H80000000,-1,0,0,0,100,100,0,0,4,0,0,2,10,10,10,1
   

Example:

Note that BorderStyle=4 is a non-standard value, so it may not work properly in all players.

_{Thanks to sup and wm4 for the BorderStyle suggestion.}

Using drawbox

The drawbox filter can be used to create a background box. This may be useful if you want the box to span the width.

ffmpeg -i input -filter_complex "drawbox=w=iw:h=24:y=ih-28:t=max:color=black@0.4,subtitles=subs.ass" output

Downside is that you have to account for line breaks or word wrapping for long subtitles. Simply making the box taller to compensate will suffice, but will look ugly because the subtitles baseline remains static: the single line subtitles will have more padding on the top than the bottom.