[x3d-public] HAnim Displacer examples

[Andreas Plesch]

Looking through the glTF spec. github issues is also insightful. For
example, I learned that it is not possible to share a skeleton with
multiple skinned meshes. Also the weights are expected to sum to 1 for each
vertex. Overall the glTF skinning seems less capable, and there is some
amount of confusion.

Andreas

On Tue, Jul 17, 2018, 8:09 PM John Carlson <yottzumm at gmail.com> wrote:

> Wow, I wish I could read specifications better!
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> Thanks for an email introduction!
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> John
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> Sent from Mail <https://go.microsoft.com/fwlink/?LinkId=550986> for
> Windows 10
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> *From: *Andreas Plesch <andreasplesch at gmail.com>
> *Sent: *Tuesday, July 17, 2018 8:01 PM
> *To: *John Carlson <yottzumm at gmail.com>
> *Cc: *X3D Graphics public mailing list <x3d-public at web3d.org>
> *Subject: *Re: [x3d-public] HAnim Displacer examples
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> I found this tutorial the best introduction into how skinning is
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> performed with glTF:
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> https://github.com/KhronosGroup/glTF-Tutorials/blob/master/gltfTutorial/gltfTutorial_019_SimpleSkin.md
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> The spec. itself is short on detail:
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> https://github.com/KhronosGroup/glTF/blob/master/specification/2.0/README.md#skins
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> The principles at work in both HAnim and glTF are very similar:
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> joints, weights, transforming from and to joint space, taking into
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> account the root trafo.
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> There are some important differences. Apparently, the weights in glTF
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> are direct factors for the joint matrices, and not applied to the
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> transformed vertex positions like in HAnim. That is how the tutorial
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> explains it although let's check Three.js implementation to be sure.
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> The inverse bind matrix needs to be precomputed and provided (although
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> I think it could be derived from the joint local coordinate system).
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> The mapping of vertices to be deformed to weighted joints is
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> different. In glTF each vertex of a skinned mesh is mapped, perhaps
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> most with 0 weights. In HAnim the joint defines the vertices it
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> influences.
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> I think the shader idea in glTF can be perhaps applied to HAnim: Per
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> vertex two additional vec4 attributes which are four indices into a
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> joint matrix array and four weights (as floats).
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> -Andreas
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> On Tue, Jul 17, 2018 at 4:52 PM John Carlson <yottzumm at gmail.com> wrote:
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> > GPU assisted HAnim would be best coordinated with glTF skinned
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> > deformation support in x3dom. This may require first thinking about
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> > how both compare in some detail.
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> > -Andreas
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> > Do we have some good glTF examples?
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> > Thanks,
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> > John
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> --
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> Andreas Plesch
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> Waltham, MA 02453
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