Soft Mask with Type 1 (Function-based) Shading

i want use custom function soft mask - type 1 (function-based) shading

 

but function, not generate alpha used smask, questions are

1. is there way make type 1 (function-based) shading return alpha?
2. is there way transfer r, g or b layer alfa?
3. if not, sould use soft mask luminosity?
4. any examples somewhere of using functions soft masks (with alpha or luminosity types)?

 

  thank you!

you should able fine using standard alpha-based soft mask.   use devicegray-based shading , should fine.

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