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

[Ruslan Kabatsayev]

Hi,

On 24/07/2018, Andrew Eikum <aeikum at codeweavers.com> wrote:
> 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.

Then why not check for the condition explicitly instead of introducing
an arbitrary "tolerance" constant and working in symmetric
neighborhoods of ±1? I.e. I suggest using `cos>1` condition in the
first 'if' and `cos<-1` in the second one. IMHO it'd be easier to
understand for a casual reader. (This will change behavior in the
cases of |cos|≫1, but we aren't trying to guard against them, are we?)

Regards,
Ruslan

>
> 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
>> >
>> >
>> >
>> >
>