Option to interpolate volume between two sample control points

[MightyHelper]

Describe the new feature:
The name says it all:
Add a button that will do a linear interpolation of the volume between the volumes of 2 time frames.

This would make some volume transitions more natural.
Proposal designs of the feature:
Add a new checkbox in the time frame options.

[smoogipoo]

What is a "time frame"? Do you mean global game volume or in-game (control point) volume?

[MightyHelper]

I meant the timing section time frames, where you can specify volume for hit objects, kiai, samples, etc.
What would be the right terminology?

[smoogipoo]

The correct terminology is sample control points. I've adjusted the PR title.