Many of the stock agencies do not support embedding of a true alpha channel in their stock footage due to the fact that the codecs that allow for it would produce much larger file sizes. In order to work around this many stock video contributors include an "alpha mask" sequence (correctly called a luma channel) as part of their video clip that contains the clip's transparency. To see how to use this in your own projects have a look at this tutorial:

93films.blogspot.com/2012/04/how-to-use-alpha-mask-embedded-in-stock.html

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