Support SF2.04 absolute value modulators

[derselbst]

Related discussion

--

Is your feature request related to a problem?

Fluidsynth doesn't support absolute value modulator transforms. In fact, it actively disables them during loading:

fluidsynth/src/sfloader/fluid_defsfont.c

Lines 1651 to 1658 in ce97c6d

	/* *** Transform *** */
	/* SF2.01 only uses the 'linear' transform (0).
	* Deactivate the modulator by setting the amount to 0 in any other case.
	*/
	if(mod_src->trans != 0)
	{
	mod_dest->amount = 0;
	}

Describe the solution you'd like

SF2.04 8.3:

The output value of the multiplier is to be the absolute value of the input value, as
defined by the relationship:

Describe alternatives you've considered

--

Additional context

--

Please upvote this issue if you're lacking this functionality.

[spessasus]

Hi,

I'm sorry if I misunderstood something, but reading the SF spec isn't this just Math.abs of the value?

The output value of the multiplier is to be the absolute value of the input value, as
defined by the relationship:
output = square root ((input value)^2)

Square root of squared value is just abs of that value.

So making:

/* it's as simple as that: */
return (fluid_real_t) mod->amount * v1 * v2;

this:

if(mod-> trans == 2) return fabs((fluid_real_t) mod->amount * v1 * v2);
return (fluid_real_t) mod->amount * v1 * v2;

would implement it? Or am I reading the SFspec incorrectly?

[derselbst]

Square root of squared value is just abs of that value.

Yes, that's also my understanding. I'm just wondering what would be the purpose / use-case behind this.

[derselbst]

Implementing this would require to introduce a new field in the fluid_mod struct, which should also be modifiable via the API. Such an API change to the modulator API should be considered together with feature #1355.

[spessasus]

Here's a test soundfont:
absolute value test.zip

NOTE: Rename .zip to .sf2, zip is so github will accept this

[spessasus]

It works exactly like the I described above (tested on polyphone and spessasynth)