> The idea with frei0r is that you have to combine multiple frei0r
> filters/mixers including adjusting the alpha channel. But I do realize
> this can be quite awkward, and this filter is more concise and quite
> likely better performing than combining 3 frei0r plugins.

Well' it's awkward enough that I still can get my head around how this
is achieved.

Note that setting source image alpha (or any other method then setting
mix level) DOES NOT end in the same output image on many blend modes
as they use different algorithm to calculate output pixel values
depending on source pixel values. See overlay and hard light here:
http://en.wikipedia.org/wiki/Blend_modes. Were you thinking of doing a
Normal blend between 100% Hardlight mix and destination image? This
will not work correctly.