[PATCH v2] dsound: Correctly calculate angle between vectors with equal and opposite directions

[Andrew Eikum]

On Mon, Jul 23, 2018 at 10:51:49PM +0300, Ruslan Kabatsayev wrote:
> Just curious, which cases does this fix? The ones when fabs(cos)>1 due
> to rounding errors? Or is acos broken for arguments near but not
> exceeding ±1?
> 

Pretty sure it's rounding errors that result in fabs(cos)>1. That
causes acos to return NAN, and bad things happen to audio volumes. I'd
be surprised if acos is broken like the 2nd case you mention.

Andrew

> On 23/07/2018, Andrew Eikum <aeikum at codeweavers.com> wrote:
> > Signed-off-by: Andrew Eikum <aeikum at codeweavers.com>
> > ---
> >
> > v2: Remove old calculation, thanks Nikolay.
> >
> >  dlls/dsound/sound3d.c | 8 +++++++-
> >  1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/dlls/dsound/sound3d.c b/dlls/dsound/sound3d.c
> > index 03fd3d4678..6ca79c1bbc 100644
> > --- a/dlls/dsound/sound3d.c
> > +++ b/dlls/dsound/sound3d.c
> > @@ -111,7 +111,13 @@ static inline D3DVALUE AngleBetweenVectorsRad (const
> > D3DVECTOR *a, const D3DVECT
> >  		return 0;
> >
> >  	cos = product/(la*lb);
> > -	angle = acos(cos);
> > +	if(fabs(1 - cos) < 0.0001f){
> > +		angle = 0;
> > +	}else if(fabs(-1 - cos) < 0.0001f){
> > +		angle = M_PI;
> > +	}else{
> > +		angle = acos(cos);
> > +	}
> >  	TRACE("angle between (%f,%f,%f) and (%f,%f,%f) = %f radians (%f
> > degrees)\n",  a->x, a->y, a->z, b->x,
> >  	      b->y, b->z, angle, RadToDeg(angle));
> >  	return angle;	
> > --
> > 2.18.0
> >
> >
> >
> >