Rendering menu > Video Post > Video Post toolbar > Add Image Filter Event > Choose Pseudo Alpha Filter from the Filter Plug-In
list.
Rendering menu > Video Post > Video Post window > Select a Contrast Filter. > Video Post toolbar > Edit Current Event
The Edit Pseudo Alpha filter creates an alpha channel for the image based on the image's first pixel (the upper-left corner pixel). All pixels that have the same color as this pixel become transparent.
Because only one pixel color becomes clear, edges of the opaque areas are aliased. The main use for this filter is when you want to composite a bitmap whose format does not have an alpha channel.
There is also a layer event called the Pseudo Alpha Compositor.