there is a really simple way to get rid of the line.
the reason for the line is that inferno/flame/flint assumes that
the images needs to be multiplied by its alpha in order to be
composited on to the background. the problem is that a 3d
render (rendered on black) is to be regarded as multiplied by
its alpha and you get the black line because it gets multiplied
twice.
to get around this:
divide the front by its alpha and you get the information (most
of it anyway depending on the bit depth you are working in) back.
the edges look wierd but in action the line is gone and you can
cc your front as much as you like.
two problems though, one is that you need (incredible as it might
seem) a spark to do this, like math ops from sapphire. the other
is that if you use blur or antialias (softness) in action the relationship
alpha/matte is no longer 100% valid and you might get a slight
distorted edge (still better than before).
hope you find this useful...
t~