@von Suppé: Some nice info about bdsup is available here:
http://blog.thescorpius.com/index.ph...btitle-format/
bdsup does not have frame number (it does have a "frame rate" but that is mostly not used), so it's time based like SRT.
Each image has a palette with 255 colors and each entry also has an "alpha" (lumianance) - much like an 8-bit png.
A bdsup entry can have multiple images (but mostly don't)