I think, ultimately, the way to make it work short of redesigning the entire compositing model, would be to have a "transition" effect plugin.
Thanks for all your answers. Nice to know, that this was not a beginners fault. BTW, I've come up with a workaround that produces acceptable results for dissolve transitions (but not for wipe-style transitions): I add keyframes to the video effect, such that the effect 'dissolves' to neutral (or to its target value on the second clip) during the duration of the dissolve. This works even well for histograms, given that the number of control points does not change from keyframe to keyframe. Jochen
