[Openal-devel] Doppler Velocity; revised

[Alexandre Mah]

Perhaps we could simply deprecate AL_DOPPLER_VELOCITY, 
AL_DOPPLER_FACTOR, alDopplerVelocity() and alDopplerFactor() and treat 
them as NOPs, since the current doppler is broken anyway?  That was one 
of Andrew's suggestions, I think.

I've come up with a suggestion I think is very nice.  (Note the default 
value for AL_VELOCITY_SCALE.)  I've put down my opinions and reasoning 
below.

Feel free to comment.

VS = AL_VELOCITY_SCALE (default value 0.0)
SD = AL_SOURCE_DOPPLER (default value 1.0; per source parameter)
vl = listener velocity (scalar value along source-to-listener vector)
vs = source velocity (scalar value along source-to-listener vector)
f = frequency of sample
f' = Doppler shifted frequency
f' = f * (1 - VS * SD * vl) / (1 - VS * SD * vs)

The reason I think this is nice is that you can turn Doppler shift on 
and off globally with AL_VELOCITY_SCALE, and you can turn Doppler shift 
on and off on a per source basis with AL_SOURCE_DOPPLER.

Now for my reasoning.

1.  Why use VELOCITY_SCALE rather than SPEED_OF_SOUND?
Using VELOCITY_SCALE gives you a way to globally turn off Doppler shift 
by setting VELOCITY_SCALE to 0 (and SPEED_OF_SOUND doesn't have this 
ability).

2.  Why use a default of 0 for VELOCITY_SCALE?
This is a much more sensible initial value than 1.0, since a default 
value of 0 allows OpenAL to optimise away Doppler calculations by 
default if the application isn't using Doppler effects.
If a default of 1.0 were used instead, OpenAL would carry out all 
Doppler calculations (uselessly) by default even if the application 
doesn't use any doppler effects and leaves all velocity vectors as 
(0,0,0).  So 1.0 would be a more computationally costly default than 0.
Also a default of 1.0 wouldn't be very useful since any application 
wanting to use Doppler shifting would probably have to set 
AL_VELOCITY_SCALE to a different value anyway.
Another advantage to having Doppler off by default is that any 
applications written for OpenAL 1.0 that used Doppler will still run 
well (just with Doppler effect disabled).

3.  Why the name "AL_VELOCITY_SCALE"?
The parameter simply scales all listener and source velocity vectors.

(Note: It's easy to see that AL_VELOCITY_SCALE does not scale distances 
at all since the default value is 0, and the OpenAL universe has not 
collapsed into a point. :)