You can also achieve the same result using paint tools.
1 - Eyedrop the background color.
2 - Switch to the Paintbrush and the "color erase" blending mode. Color erase is also a color-to-alpha transition.
3 - Start painting the background.
Note that with any color-to-alpha transition you may wind up with "residual" alpha values (very low transparency) in areas. If you need to clean these up you can do so with the eraser tool or with a Levels/Curves adjustment (shadow end specifically) on the alpha channel.
-- Stratadrake
strata_ranger hotmail com
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Numbers may not lie, but neither do they tell the whole truth.