Re: [Algorithms] Depth-Sorting Axis-Aligned Boxes

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> Since you are using flash you could
> use masking capabilities.

Great, this definitely looks like a good solution; I've never actually used mask layers in any form (in the tool or via code) so I completely failed to consider them. Thanks!!

> 1. roughly sort all boxes using their centroid

I'm unclear about what exactly the view direction *is* for this sort of perspective; it can't be axis-aligned along z since three faces of an axis-aligned cube are visible, but worldspace x and y are parallel to screenspace x and y, which implies an axis-aligned view along z..?!

thanks,
raigan

--- On Tue, 8/4/09, Mathi Nagarajan (2K Australia) <Mat...@2k...> wrote:

> From: Mathi Nagarajan (2K Australia) <Mat...@2k...>
> Subject: Re: [Algorithms] Depth-Sorting Axis-Aligned Boxes
> To: ra...@ha..., "Game Development Algorithms" <gda...@li...>
> Received: Tuesday, August 4, 2009, 12:19 AM
> Since you are using flash you could
> use masking capabilities.
> 
> So in the example picture you provided, you could:
> 
> 1. render the dark grey object first with a mask that
> excludes the medium grey object (cos it is considered
> in-front of the current object)
> 2. render the medium grey object next with a mask that
> excludes the light grey object (cos it is considered
> in-front of the current object)
> 3. render the light grey object next with a mask that
> excludes the dark grey object (cos it is considered in-front
> of the current object)
> 
> In this case, only step 3 needs to use a mask, cos steps 1
> and 2 have their "front" objects rendered after them in
> sequence.
> 
> So this shows a simple approach:
> 1. roughly sort all boxes using their centroid (this is
> actually an optimization to minimize masking, you could use
> something more sophisticated to optimize further)
> 2. loop over each object from back to front 
>     2a. collect objects that are considered
> in front of the current object that have already been
> rendered
>     2b. build a mask excluding the collected
> "in-front" objects
>     2c. render the current object using the
> mask
> 
> I'm not that fluent with flash masking, but I think it can
> do that...
> 
> I'm relying on the property that you can render 2 convex
> non-intersecting objects sorted correctly without splitting
> either.
> 
> -----Original Message-----
> From: metanet software [mailto:met...@ya...]
> 
> Sent: Tuesday, August 04, 2009 12:41 PM
> To: Game Development Algorithms
> Subject: Re: [Algorithms] Depth-Sorting Axis-Aligned Boxes
> 
> > The neat thing is that you only need to use the
> spatial data structure 
> > to "discover" the sort order, since certain classes of
> geometry sets 
> > (AABB's in particular) can be rendered correctly
> without splits.
> 
> Is this right? I *definitely* might be overthinking, but it
> seems like you could easily construct the classic "three
> mutually overlapping/overlapped sticks" out of AABBs.. which
> would imply that you _do_ need to split them in some cases.
> I'll attach an example of what I mean.
> 
> 
> > Also, if you always keep track of the "last sort", you
> can get the 
> > next sort order very quickly since it will usually
> only diverge a 
> > little bit.
> 
> Yes, I'm hoping that I can somehow combine/piggyback the
> depth-sorting with the incremental sweep-and-prune for
> collision.
> 
> thanks,
> raigan
> 
> 
> 
> --- On Mon, 8/3/09, John Ketchpaw <jke...@pa...>
> wrote:
> 
> > From: John Ketchpaw <jke...@pa...>
> > Subject: Re: [Algorithms] Depth-Sorting Axis-Aligned
> Boxes
> > To: ra...@ha...,
> "Game Development Algorithms" 
> > <gda...@li...>
> > Received: Monday, August 3, 2009, 7:22 PM You may be
> over thinking it.
> > 
> > Non overlapping convex polyhedra are usually correctly
> sorted using 
> > their centroids, assuming they are of similar
> volume.  Along the way, 
> > if you generate a axis aligned bounding box in camera
> space for each 
> > one, you can detect the groups of objects that violate
> the simple sort 
> > + use a more expensive algorithm like a kd tree
> without feeling dirty 
> > since the sets will be small.
> > 
> > The neat thing is that you only need to use the
> spatial data structure 
> > to "discover" the sort order, since certain classes of
> geometry sets 
> > (AABB's in particular) can be rendered correctly
> without splits.
> > 
> > Also, if you always keep track of the "last sort", you
> can get the 
> > next sort order very quickly since it will usually
> only diverge a 
> > little bit.
> > 
> > -john
> > 
> > -----Original Message-----
> > From: metanet software [mailto:met...@ya...]
> > Sent: Monday, August 03, 2009 1:32 PM
> > To: Game Development Algorithms
> > Subject: Re: [Algorithms] Depth-Sorting Axis-Aligned
> Boxes
> > 
> > 
> > > > The picture you included in a subsequent
> post HAD
> > the
> > > > boxes overlapping.
> > >
> > > Sorry, I meant that in 3D the boxes wouldn't be
> interpenetrating, 
> > > they would be separated or resting
> > on each
> > > other; when projected onto the viewing plane
> they
> > will
> > > almost definitely overlap -- hence the need to
> sort
> > them
> > > somehow.
> > 
> > I just realized you must have been referring to how
> the blue box was 
> > within the extents of the red shape; in this case the
> red shape would 
> > be modeled as three boxes (top and sides are all
> separate AABBs of 
> > minimum thickness).
> > 
> > 
> > 
> > 
> > 
> > --- On Mon, 8/3/09, metanet software <met...@ya...>
> > wrote:
> > 
> > > From: metanet software <met...@ya...>
> > > Subject: Re: [Algorithms] Depth-Sorting
> Axis-Aligned
> > Boxes
> > > To: "Game Development Algorithms"
> > <gda...@li...>
> > > Received: Monday, August 3, 2009, 3:15 PM
> > >
> > > > The picture you included in a subsequent
> post HAD
> > the
> > > > boxes overlapping.
> > >
> > > Sorry, I meant that in 3D the boxes wouldn't be
> > > interpenetrating, they would be separated or
> resting
> > on each
> > > other; when projected onto the viewing plane
> they
> > will
> > > almost definitely overlap -- hence the need to
> sort
> > them
> > > somehow.
> > >
> > >
> > > > Building a BSP tree makes sense for static
> > geometry
> > > but
> > > > perhaps
> > > > less for dynamic geometry (though you could
> > merge
> > > dynamic
> > > > objects
> > > > into a prebuilt static tree).
> > >
> > > I figured that a combination of using
> sweep-and-prune
> > for
> > > broadphase collision detection, and the fact that
> all
> > of the
> > > polygons are parallel to world planes (xy,xz, or
> yz),
> > > building a BSP would be simplified: we'd already
> have
> > all
> > > the faces sorted in order along each axis, as
> well as
> > > knowing which pairs overlapped along which axes.
> > >
> > > Possibly "kd-tree" is a better description, since
> all
> > the
> > > planes would be axis-aligned.
> > >
> > > So far my fall-back plan is to use a head-on
> > perspective as
> > > in SNES Zelda; this avoids the cyclical
> > > three-mutually-overlapping-faces problem. Sadly
> it
> > doesn't
> > > really look or work as well when you're trying
> to
> > depict
> > > depth :(
> > >
> > > thanks again,
> > > raigan
> > >
> > >
> > >
> > >
> > >
> > > --- On Mon, 8/3/09, chr...@pl...
> > > <chr...@pl...>
> > > wrote:
> > >
> > > > From: chr...@pl...
> > > <chr...@pl...>
> > > > Subject: Re: [Algorithms] Depth-Sorting
> > Axis-Aligned
> > > Boxes
> > > > To: ra...@ha...,
> > > "Game Development Algorithms" <gda...@li...>
> > > > Received: Monday, August 3, 2009, 2:14 PM
> > > >
> > > > Raigan wrote:
> > > > > Sadly I don't have a z-buffer, so I
> need to
> > > determine
> > > > a way to sort
> > > > > the boxes back-to-front. The boxes will
> all
> > be
> > > > non-overlapping,
> > > > > although they may be touching, and
> will
> > move
> > > smoothly
> > > > (i.e not by
> > > > > grid-sized steps as is common in some
> > games).
> > > >
> > > > The picture you included in a subsequent
> post HAD
> > the
> > > > boxes
> > > > overlapping.
> > > >
> > > >
> > > > > Is there a simple solution to this
> problem?
> > I.e
> > > a
> > > > formula which
> > > > > considers the extents and/or a
> reference
> > point on
> > > each
> > > > box to
> > > > > produce a back-to-front order? Or am I
> stuck
> > with
> > > the
> > > > general
> > > > > solution: build a BSP tree?
> > > >
> > > > Building a BSP tree makes sense for static
> > geometry
> > > but
> > > > perhaps
> > > > less for dynamic geometry (though you could
> > merge
> > > dynamic
> > > > objects
> > > > into a prebuilt static tree).
> > > >
> > > > There's also Newell-Newell-Sancha:
> > > >
> > > > http://en.wikipedia.org/wiki/Newell%27s_algorithm
> > > >
> > > > which, from the limited description you
> gave,
> > sounds
> > > like
> > > > a
> > > > better option for you.
> > > >
> > > >
> > > > Christer Ericson, Director of Tools and
> > Technology
> > > > Sony Computer Entertainment, Santa Monica
> > > >
> > > >
> > > >
> > >
> >
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