[Openal] Filter

[Andy Patrick]

Hello all,
 
We're looking at playing around with the per-voice filter in the Effects
Extension, in preparation for occlusion etc. We'd also like to change
the filter over distance using a method that offers more control than
the in-built distance filter in OpenAL, which I seem to remember reading
about but can't now find a reference to!
 
I understand that the low pass filter cut-off (-3dB point) is fixed at
4KHz, and the slope is fixed at -6dB/octave. What I don't understand is
how the available parameters - AL_LOWPASS_GAIN and AL_LOWPASS_GAINHF -
relate to that. I assumed that GAIN would modify the gain to the left of
the cut-off point, and GAINHF would modify the gain to the right of the
cut-off point. However, my attempts at varying GAINHF from 0 to 1 by
distance doesn't seem to change the sound very much, and based on this
assumption setting both GAIN and GAINHF to 0 would silence the sound,
which it doesn't.
 
Could someone please explain what the intent of these settings is, and
briefly suggest how I might be able to implement the effects I'm after?
I'm using OpenAL 1.1 on the Windows platform, primarily.
 
Many thanks
 
Andy P
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