Vector Blur and alpha

I’m rendering a 3D object to be composited over video. Mblur looks very good, but I can’t stomach the rendering time. I setup a Vector Blur node but discovered a problem. When I render it with premul, the blur effect is only visible over the object itself, but not beyond its limits. In other words, the object’s surface looks smeared, but not past it’s edges. There is no blur trailing the object.

It looks like vector blur is not functioning in the alpha channel. Is this a limitation of the vector blur, or is there a way to make it work?

I can post an example if my description isn’t making sense.

I figured out a solution: add a separate Vector Blur node, run the alpha from the render layer into the image input, then the output goes to the alpha input of the composite node. Pretty simple.

Tahat’s the long way around because both nodes require sample calculations. Try this Add>Converter>Separate RGBA, then connect the alpha output socket of this node to the alpha input socket of a Set Alpha node and the image output socket of the Vector Blur node to the set alpha’s image input. conect the result to the composite node.

Complex node trees can quickly tax system memory limits and/or OS memory constraints and the above setup is much more efficient.