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I am new to blender and since i am unaware of the terminology i dont know what to search for
I have a 'quick smoke' icosphere as a rendered mpeg file.
I have another video in which a man shows his hands as if he is holding the smoke ball.
But in Video Sequence Editor when i add both the clips and add the transform effect to the smoke clip with 'alpha over' and scale it,I get a black box (background) around the smoke.
I have tried to make it transparent as given here
Can Blender render pngs with the background transparent?
How should i display only the smoke without background?
Note:I have also tried changing the opacity of both the transform and smoke strips to no avail

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    $\begingroup$ As far as I know Mpeg format does not support alpha channel information. Try rendering the smoke as a png sequence. $\endgroup$ – cegaton Sep 7 '14 at 16:24
  • $\begingroup$ @cegaton thank you for the quick answer.Does AVI support alpha channel transformation?Because i tried saving it as an avi raw file ,this also doesnt show the smoke properly $\endgroup$ – Raghav Subramanian Sep 7 '14 at 16:36
  • $\begingroup$ @cegaton thanks a lot for helping a newbie :) I'd be glad to accept your answer if you post it,since it could help newbies like me $\endgroup$ – Raghav Subramanian Sep 7 '14 at 18:13
  • $\begingroup$ Note that it's a bad idea to render directly to a video anyway, as you cannot pause/stop the render without corrupting the video and losing all the progress you made. $\endgroup$ – gandalf3♦ Sep 7 '14 at 19:27
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Most video formats do not support alpha channels,at least not the ones built into blender. (there is one option if you are using OSX)

You have two options:

1.- Export your scene as an image sequence with in RBGA (in a format like PNG, Tiff, TGA, Jpeg2000 or OpenEXR) and then import it into the Video sequence editor to combine your shots

2.- Do the compositing directly in blender either by importing the smoke scene as a strip into the video editor or using the compositor located in the Node Editor

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