Tool to add opaque box and change transparency

[kodec]

In short, is there a tool that has all the options to edit and save subtitles in the way MPC has the options to show them (pic below)? It would be ideal if MPC had the option to save the loaded subtitles with the created look, ha ha; they have it perfect. I'm searching for the tool that would allow me to change the transparency of the opaque box AND to retain white letters inside the box (I've found an old thread here that explained how to change the transparency inside the .ssa using the notepad but it also changes the actual colour of the letters inside the box and makes it darker).

[jagabo]

I've done it with SSA subs and some tricks in AviSynth:


[Attachment 51232 - Click to enlarge]

As normally rendered by TextSub() on the left, with additional semi-transparent box.

[sneaker]

In ASS (see e.g. Aegisub Styles Manager) the alpha levels of the 4 colors (Primary, Secondary, Outline(=box color), Shadow) are independent.