target map comes from a depth map of a previous stage.
It is called "dual" because it only uses 2 accumalative passes (2 stages @ 1 pass).

store a gradient or gamma multiplier in the alpha channel;
this effects all pixels in the view rather than the rgb values that are only multiplied by the lit pixels.

For aliasing, expect a version 2.0 soon with a linear search solution for both the loss of z-buffer precision and limitation of pixel space.