I happened to me too and it's pretty frustrating. Some clips have just a bit of semi-transparent pixels at the edges. Cropping will not do. If you have After Effects you can load the clip in a comp, add a oval mask, keep it smaller than the size of the clip, about 8/10 of the size, feather the mask, probably around 20 pixels. Move the mouse outside the mask so that you can see the value of the pixels in the verify info panel. If the register at 0,0,0 then you are all set. Save the clip with a new name, load it in Carrara and it should work.

Paolo

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