Monochrome Effect - Filter with transparent PNG

Dec 17, 2009 at 4:01 PM

I am using the Monochrome effect to strip the color from transparent PNGs. I notice that all the pure transparent pixels are rendered as black when I do this. I see a filter paramter, but not sure what value I should supply and if that is even intended to allow a particular value to avoid the effect being applied which could in theory maintain the alpha channel