[sdiy] Fast envelope generation

[Neil Johnson]

Tom,

> You can read the curve as a interpolation index x, and then do the following linear interp calc:
>
> y = ((1-x) * start) + (x * end)
>
> This gives you the curve between the two points 'start' and 'end' and saves the scaling, for the cost of two multiplies. Since you can read any lookup table, you can insert any curve between the two points for no more work.

As always the devil is in the details....

Neil
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