Testing on 4:3 PAL DV.
One clip on video track 1. Added video effect "Speed", set it to 50%.
Second clip on video track 2, same procedure.
3s overleap, added a crossfade transition between the two clips.
-> Kdenlive 0.5 cannot give clean preview (when it comes to the transition, you merely see a flicker; afterwards, it's o.k.).
Using a Core 2 Duo notebook (2x 1.66GHz), 1024MB (2x 512MB), NVidia GeForce Go 7400 with "latest" NVidia binary driver, Gutsy (7.10).
What can I do?
