[x3d-public] visual shape differencing with x3d

Wed Jul 5 14:36:36 PDT 2023

Thanks for the nice glTF example.

It turns that the broken window example uses another technique to model
holes. It uses a single shape for the window and then uses an alpha texture
to define the hole:

https://github.com/KhronosGroup/glTF-Sample-Assets/blob/main/Models/GlassBrokenWindow/glTF/WindowGlass_ColorAlpha.png

Textures are probably a more standard way to have cutouts but are less
applicable to shape differencing or CSG. So the depthmask plus sort order
technique solves a different problem which glTF cannot directly address.

Andreas

>
> Date: Wed, 5 Jul 2023 08:33:41 -0700
> From: Leonard Daly <Leonard.Daly at realism.com>
> To: x3d-public at web3d.org
> Subject: Re: [x3d-public] visual shape differencing with x3d
> Message-ID: <2b27d08a-6db9-4f5b-27f6-5409fc631dc4 at realism.com>
> Content-Type: text/plain; charset="utf-8"; Format="flowed"
>
> Hi Andreas,
>
> The BrokenWindow glTF model
> (
> https://github.com/KhronosGroup/glTF-Sample-Assets/blob/main/Models/GlassBrokenWindow/README.md)
>
> shows the use of transparency (for the glass) and alpha masking (not
> blending) for the "holes" (actually breaks). You can see the model and
> work with various backgrounds at
>
> https://github.khronos.org/glTF-Sample-Viewer-Release/?model=https://raw.GithubUserContent.com/KhronosGroup/glTF-Sample-Assets/main/./Models/GlassBrokenWindow/glTF-Binary/GlassBrokenWindow.glb
>
> There is a discussion of various alpha modes at
>
> https://github.com/KhronosGroup/glTF-Sample-Assets/blob/main/Models/AlphaBlendModeTest/README.md
>
> glTF uses transparency for "see through" material. Alpha blending is use
> when a transition from one material to another is required. For example
> metallic coating on a surface that goes from fully opaque to fully
> transparent over a visible distance.
>
> Custom and embedded shaders were removed in glTF 2.0 for security and
> other reasons.
>
> Leonard
>
>
>
> On 7/3/2023 8:51 AM, Andreas Plesch wrote:
> > Thanks, Don, for the positive feedback.
> >
> > Let me just adjust expectations a bit. This is a useful technique but
> > only approximates actual CSG. The full hole shape is always rendered
> > which means it may obscure other shapes in addition to the intended
> > shape. This can be mitigated by making the hole shape only protrude a
> > little bit outside of the solid shape intended to become cut out.
> > Also, the way sorting works in x3dom the solid shape has to have a
> > nominal amount of transparency to ensure it is rendered after the
> > transparent hole shape, using the explicit sortkey.
> >
> > On the plus side, the technique allows for material inside the
> > cutouts, eg. tinted, shiny glass windows.
> >
> >
> https://andreasplesch.github.io/Library/Viewer/index.html?url=https://gist.githubusercontent.com/andreasplesch/415d151abee114858c79c20c8c4b49e0/raw/0e8e270901077a2165d9d99b139eab27c10b0584/BoxWithHoles183.x3d
> > is slightly updated with a longer cone.
> >
> > With regards to glTF, it looks like glTF v.1.0 had the option to
> > define a custom transparent rendering technique with depthmask=true.
> > But the now prevalent glTF v.2.0 does not seem to have explicit
> > depthmask control.
> >
> > Here is a related, complicated discussion which mentions cutouts:
> > https://github.com/KhronosGroup/glTF/issues/822
> >
> > It may be that glTF has an alphablend mode which enables cutouts,
> > instead of standard blending. This may then also imply enabled
> > z-buffer rendering. But the official glTF alpphablendmode test does
> > not test for cutouts:
> >
> https://github.com/KhronosGroup/glTF-Sample-Models/blob/master/2.0/AlphaBlendModeTest/README.md
> .
> > So I do not think currently glTF has that capability although it had
> > been discussed and there may be a glTF extension for that.
> >
> > Like depthmask sorting seems to be considered exclusively in the
> > domain of the viewer which implements glTF.
> >
> > All the best, Andreas
> >
> > On Sun, Jul 2, 2023 at 10:13?PM Brutzman, Donald (Don) (CIV)
> > <brutzman at nps.edu>  wrote:
> >> Andreas, thanks for sharing on x3d public mailing list.  Agreed that
> this is an extremely powerful rendering technique - high "wow factor" for
> sure.
> >>
> >>
> >>
> >> Perhaps of note is that the X3D working group spent a few years
> pursuing Constructive Solid Geometry (CSG) with little to show for it,
> since such approaches can be computationally intensive and not well suited
> to real-time rendering on diverse devices.
> >>
> >>
> >>
> >> Wikipedia: Constructive solid geometry (CSG)
> >> https://en.wikipedia.org/wiki/Constructive_solid_geometry
> >>
> >>
> >>
> >> Avoiding the need for any geometry modification whatsoever when
> creating visual "holes" - totally sidestepping CSG modeling - seems quite
> novel.
> >>
> >>
> >>
> >> Wondering if this differencing (see-through) technique is possible
> today using glTF 2.0 models?  If a supporting X3D4 player might load it as
> glTF, then that might be a fast path to usage.
> >>
> >>
> >>
> >> Looking ahead, when ready, this seems like the nodes you are using in
> X3DOM (or some version thereof) might be strong candidates for inclusion in
> a future X3D 4.1 specification.
> >>
> >>
> >>
> >> Meanwhile (as you also recently discussed) we might experiment with
> extensions to X3D 4.0, perhaps through metadata for forwards/backwards
> compatibility and validation.  (Wish list: the ImageAtlas node never made
> the cut for X3D 4.0 and seems like a candidate for broader implementation
> as well.)  The X in X3D is Extensibility...
> >>
> >>
> >> Congratulations on what seems to be a major achievement!  8)
> >>
> >>
> >>
> >> all the best, Don
> >>
> >> --
> >>
> >> Don Brutzman  Naval Postgraduate School, Code USW/Brbrutzman at nps.edu
> >>
> >> Watkins 270,  MOVES Institute, Monterey CA 93943-5000 USA
> +1.831.656.2149
> >>
> >> X3D graphics, virtual worlds, navy roboticshttps://
> faculty.nps.edu/brutzman
> >>
> >>
> >>
> >> -----Original Message-----
> >> From: x3d-public<x3d-public-bounces at web3d.org>  On Behalf Of Andreas
> Plesch
> >> Sent: Sunday, July 2, 2023 9:20 AM
> >> To: X3D Graphics public mailing list<x3d-public at web3d.org>
> >> Subject: [x3d-public] visual shape differencing with x3d
> >>
> >>
> >>
> >> Let me share that we found that it is possible to simulate differencing
> of shapes in x3dom to a useful degree. This allows for transparent cutouts
> or holes:
> >>
> >>
> >>
> >>
> https://andreasplesch.github.io/Library/Viewer/index.html?url=https://gist.githubusercontent.com/andreasplesch/415d151abee114858c79c20c8c4b49e0/raw/4c70463321df267b28298462af76b869adbfdfdb/BoxWithHoles183.x3d
> >>
> >>
> >>
> >>
> >>
> >> The idea is to draw first transparent shapes which define the holes,
> including to the depth buffer. Then standard drawing of the solid shape
> will be hidden where the holes 'obscure' it (although the holes are
> actually transparent).
> >>
> >>
> >>
> >> To do that requires two changes to default rendering of transparent
> shapes. For best rendering normally transparent shapes are rendered last
> and do not write to the depth buffer. Here they are drawn first and do
> write to the depth buffer.
> >>
> >>
> >>
> >> In x3dom this is implemented by allowing explicit definition of the
> sort (rendering) order of shapes (with a sortKey field), and by allowing
> direct access to the gl depthmask function with a Depthmode node.
> >>
> >>
> >>
> >> An easy way to generate cutouts or holes which is otherwise only
> possible with external CSG tools is a powerful use case for these
> lower-level x3d extensions. Perhaps other browsers have similar features or
> are inspired to consider those.
> >>
> >>
> >>
> >> Andreas
> >>
> >>
> >>
> >> --
> >>
> >> Andreas Plesch
> >>
> >> Waltham, MA 02453
> >>
> >>
> >>
> >> _______________________________________________
> >>
> >> x3d-public mailing list
> >>
> >> x3d-public at web3d.org
> >>
> >> http://web3d.org/mailman/listinfo/x3d-public_web3d.org
> >
> >
> --
> *Leonard Daly*
> 3D Systems Engineering
> President, Daly Realism - /Creating the Future/
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