Problem of AviDemux rendering subtitle with opaque box

[compusic]

Hi, I'm used to making subtitles with Aegisub, and then using AviDemux to render videos with those subtitles burned-in.

I have encountered a problem of subtitles with opaque box.

As you can see, the 1st image is what I see in Aegisub. Also when using a software player to playback video with this external subtitle, the result is the same, that is no problem .



Then when I load the subtitle into AviDemux, what I preview in AviDemux is the 2nd image. And the rendered video has the same problem.



How can it be like that? Any suggestions? BTW, I do need transparent box, and have tried all the combinations of different parameter settings. The problem persists.

Thanks for any help.