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

[Andrew Eikum]

On Tue, Jul 24, 2018 at 04:51:21PM +0300, Ruslan Kabatsayev wrote:
> 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?)
> 

Yeah, that makes sense, thanks for the suggestion. I'll re-send.

Andrew