VobSub files are subtitle files found on DVDs. They are actually a set of bitmap/raw-images. They are not text. This filter can overlay them into the video and make them a permanent part of it. The subtitle images will be drawn into the underlying images of the video. They cannot be turned on or off. If you want to have that sort of functionality, you should look into using what is called softsubs (continue reading for more information below).
VobSub subtitles can be extracted from DVD files using the Avidemux “Tools→VOB(nbsp)->(nbsp)VobSub” pulldown menu. The extraction will produce an *.idx and *.sub file.
If you want to use softsubs, after you have extracted, use the Avidemux “Tools→OCR (VobSub(nbsp)->(nbsp)srt)” pulldown menu. This will open a feature for using the automated tools to create a text-based subtitle file that can be used in softsubs. There is not yet a section in the documentation about adding soft subtitles in Avidemux.
Subtitles in general should be added last of all filters (thus at the bottom of the filter list). Select the *.idx file from the extracted VobSub files. You can select language you like. The default is the first language found in the idx file.
The Shrink factor can be set for making the text smaller. Set as you like and use the preview window to test.
The Shift is used to change the time that the text images will appear. It will shift the appearance forward in time according to the time entered in milliseconds (ms).