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opacity masking... yes

good call Frances :)

no problem - we got there in the end ;)

yep feathering looks good.. although there is a pixel limit on how much you can apply

many meanings of the word contour - cartographic contour is another - guess there was confusion as you showed...

ok - but contours are steps - if you don't want steps, don't use contours, or if you need to increase the size of a non-uniform shape by means of contours then make one shape of it before you apply...

or:

or like this...

or maybe more like ... ?

you mean like the attached ?