[PD] tanh~ (core dumped)

[Claude Heiland-Allen]

On 15/08/17 23:46, Matt Davey wrote to <pd-list at lists.iem.at>:
> I've always used:
>
> tanh(x) ~= x*(27+x*x) / (27+9*x*x)
>
> works well and is pretty CPU cheap

Nice.

That seems accurate to about 5 or 6 bits in my tests (over the range -4
to 4). A slightly more accurate (about 7 or 8 bits) variant is:

    x * (27.2837670294674 + 1.22070209109864 * x * x)
      / (27.2837670294674 + 9.86205301460732 * x * x)

A higher degree version is accurate to about 16 or 17 bits:

    x * (44220389.1067293 + 4672204.74255456 * x * x
                          + 34604.8550273853 * x * x * x * x)
      / (44220389.1067293 + 19409074.7654446 * x * x
                          + 612890.122925575 * x * x * x * x)

(Though I realize now, later, that there is some redundancy and they
could be normalized to have a leading "1 +" instead of these large
values...)

I found these using gnuplot's fit functionality:

    F(x) = x * (a + b * x * x) / (a + c * x * x)
    fit F(x) 'tanh.dat' via a,b,c

and similarly for the higher degree version.  'tanh.dat' contains 1024
equally spaced samples of x,tanh(x) in [-4,4], calculated in double
precision with a small C program, similar to the input generation in the
help patches.

I evaluated the accuracy visually in gnuplot:

    bits(x,y) = log(abs(y - tanh(x))+2**(-53)) / log(2.0)
    plot [-4:4] bits(x,F(x))

I don't know if single-precision rounding will affect the results much.
 I put the full precision coefficients into the patches using a text
editor, so  double precision Pd can benefit just in case.

It's probably also best to use [clip~ -4 4] before these functions, to
avoid possible Inf or NaN explosions, or maybe [clip~ -1 1] afterwards
would  be enough to be safe?

I haven't benchmarked yet.

Consider the patches placed into the Public Domain.  Message-rate
versions are left as an exercise.


Claude
-- 
https://mathr.co.uk
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