Re: [ggt-devel] Sphere emptiness

[Paul M. <pm...@sk...>]

On 10/24/2012 10:46 AM, Sebastian Messerschmidt wrote:
> Hello Kevin,
>> What does it mean to have an empty sphere? Or aabox for that matter. Is
>> it same as sphere volume == 0. Or is it a flag to signify something
>> else (I.e. No objects contained in).
>>
>> if vol == 0 then why not set the size to 0... If it is so you can
>> remember previous size, why distinguish 0 volume, why not and an N
>> volume switch... Why a switch at all...
>>
>> Excuse me for not knowing/remembering the purpose of mEmpty...
>
> Imagine you have BoundingVolume (Doesn't matter if it is a box or
> sphere) that is default initialized.
> Now if you use this object to extend an existing volume it will expand
> it in a wrong way.
> Since an empty sphere (i.e. radius == 0.0) still has an position it will
> expand the volume to be extended incorrectly.
> (For example you have sphere1 with pos(10,0,0) and r = 1 and an empty
> sphere (pos(0,0,0), radius=0). The extendVolume will produce a sphere of
> pos(5.5,0,0) radius = 6, which is wrong)
> The main purposes of the empty flag is to ignore those primitives and to
> check if it was initialized.

Good summary. Also, an mEmpty member variable will differentiate between a 
sphere that contains nothing (uninitialized), and a sphere that contains exactly 
one point. Radius == 0 alone doesn't provide this functionality.

As I mentioned previously, I could use a negative radius to indicate an empty 
sphere. But, we could also have AABox use mMax < mMin to indicate an empty box, 
and we don't -- instead we have AABox::mEmpty, and code to respect the setting 
of that variable in Containment.h. I think, for consistency, Sphere should be 
the same way.

I'll proceed with the code change, if no one objects.
    -Paul

>
> Your approach testing the volume might work in this case, but it is
> surely not a general solution.
> So I vote for Paul's approach.
>
> cheers
> Sebastian
>
> P.S. Hi Paul, nice to see you outside the OSG mailing list ;-)
>>
>> Thanks,
>>
>> Sent from my Phone
>> From: Paul Martz
>> Sent: 10/23/2012 5:15 PM
>> To: ggt...@li...
>> Subject: [ggt-devel] Sphere emptiness
>> Hi all -- My project uses both gmtl::Sphere and gmtl::AABox as
>> bounding volumes.
>> But I've noticed that Sphere lacks the 'bool mEmpty' member variable that is
>> present in AABox. Is there a reason for this? If not, I'd like to add it.
>>
>> I would also modify Containment.h's extendVolume(Sphere&,...) to respect that
>> variable (as is already done for extendVolume(AABox&,...) ). Finally, I'd also
>> add new extendVolume() functions to extend Sphere around an AABox and
>> vice versa.
>>
>> Just wanted to see if there was a logical reason for the current
>> design before I
>> embarked on changing the code. I know that I can emulate the desired behavior
>> with a negative radius, but it seems to me it would be desirable to have this
>> functionality in gmtl rather than only in my project.
>>
>> Thanks,
>>       -Paul
>>
>>
>>
>>
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