Is it possible to get Terrain voxels while using FillBall/Block/Cylinder/Wedge, without actually calling it?

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Hello. I’m want make Terrain Editor plugin with more options. One of options that I want make is Partially Shape Filling:

  1. What it looks actually:
    image
  2. Brush which will be visible:
    image
  3. How Terrain will be created:
    image

I know, many will people will suggest to firstly create sphere, and then evaporate part that I don’t need. But here’s 1 problem:
image
If I’ll try make it, then some part of terrain will be erased.

So, my question is this: is it possible to get voxels, which will be affected, without actual filling?