Currently, as far as I know, the only way to display the current frame number for each frame is with the Info() filter. It would be really really nice if there was a FrameNumber() clip property, so that it could be added to a string being passed to Subtitle (for instance, if you're combining pieces of clips from multiple sources together using Trim, it would let you make a test to ensure that you did it correctly). This would be kind of different from the other clip properties, since it changes on a frame by frame basis; however I think it would be more than worth making a special case for.