[x3d-public] x3d-public Digest, Vol 128, Issue 30

[Michalis Kamburelis]

(Adding x3d-public to Cc.)

Indeed, when animating topology (like animating "coord" using
CoordinateInterpolator) one has to also animate other vectors in local
space -- like normals (in "normal" field) or (if we add it) vectors in
the "tangent" field. In this sense, indeed the burden is just shifted.

Still, it seems easier to animate a set of vectors expressed as
MFVec3f, than to animate a set of vectors expressed in a texture.
Outside of X3D, others animate vectors too -- Unity AddBlendShapeFrame
( https://docs.unity3d.com/ScriptReference/Mesh.AddBlendShapeFrame.html
) takes as parameters vertices, normals and tangents for new "blend
shape".

I can't comment about the use-case of "shading a point set" -- I mean,
my knowledge is more about "what is common for normal maps on
polygons". Although glTF treats the normal maps consistently on
polygons, points and lines -- they are always in tangent space,
according to glTF spec. Unity also only allows normal maps in tangent
space, as far as I know.

Regards,
Michalis

czw., 5 gru 2019 o 04:48 Andreas Plesch <andreasplesch at gmail.com> napisał(a):
>
> I had intended to share this also on x3d-public. Please feel free to do so.
>
> As a tangent map would be necessary then for points, would the tangents then be in local space ?
>
> And if this is the case, would an animation then require animation of the tangents map ?
>
> So, in that sense, the burden would be just shifted, would it not?
>
> On another note, in my mind the purpose of using as normal map for a dense point cloud would not be providing a sense of detailed morphology as it is for a continuous surface. It would be simply a compact way of delivering normal data to each point, for shading purposes.
>
> Best, Andreas
>
>
>
> ---on the phone---
>
> On Wed, Dec 4, 2019, 5:07 PM Michalis Kamburelis <michalis.kambi at gmail.com> wrote:
>>
>> Normals in normalmaps are usually expressed in tangent space, because
>> this makes them useful even after mesh deformation (using
>> CoordinateInterpolator or H-Anim skinning). I think that this argument
>> would apply also to PointSet (as far as I understand, the rationale
>> for "lit" PointSet is that it approximates a surface, e.g. when
>> scanned by 3D scanner; so we may want to animate it, just like we
>> animate IndexedFaceSet).
>>
>> Requiring normalmaps in other space (like local space) in X3D is
>> possible but indeed would be somewhat "exotic".
>>
>> - It would be surprising to asset authors, I think, since so many
>> other software (that creates or renders normalmaps) defaults to
>> "normalmaps in tangent space".
>>
>> - It would also be incompatible with glTF, that says normalmaps are in
>> tangent space. So we could not use normalmaps from glTF.
>>
>> - As we lose the advantages I mentioned at the beginning: animating
>> mesh (by CoordinateInterpolator etc.) would break normalmaps.
>>
>> Regards,
>> Michalis
>>
>>
>> śr., 4 gru 2019 o 15:28 Andreas Plesch <andreasplesch at gmail.com> napisał(a):
>> >
>> > Hi Michalis,
>> >
>> > thanks for the thoughtful explanations. Agreed on both points. I had
>> > forgotten about tangent space.
>> >
>> > Explicit normals in a Normals node are provided in local space, not
>> > tangent space. Could we require that normals in normal maps for points
>> > also need to provided in local space, to avoid the requirement for
>> > tangents ? This may be too exotic ? I could imagine that point clouds
>> > from lidar and image reconstruction actually would come with normals
>> > in world space, rather than tangent space but that would be something
>> > to learn more about.
>> >
>> > Best, -Andreas
>> >
>> > On Wed, Dec 4, 2019 at 6:09 AM Michalis Kamburelis
>> > <michalis.kambi at gmail.com> wrote:
>> > >
>> > > sob., 30 lis 2019 o 06:00 Andreas Plesch <andreasplesch at gmail.com> napisał(a):
>> > > > Yes, agreed. uv mapped from the axis aligned bounding box would be a good default. u from X and v from Y.
>> > >
>> > > To be more precise, I proposed to use the same scheme as
>> > > IndexedFaceSet (
>> > > https://www.web3d.org/documents/specifications/19775-1/V3.3/Part01/components/geometry3D.html#IndexedFaceSet
>> > > ). It means:
>> > >
>> > > - U along the longest dimension
>> > > - V along the next dimension
>> > >
>> > > I agree that  "U from X, V from Y" would be simpler (maybe also more
>> > > useful)... but IndexedFaceSet already defines it a bit differently.
>> > > And being consistent with IndexedFaceSet seems proper here.
>> > >
>> > > >
>> > > > Are tangents really necessary for Phong shading ? see
>> > > > https://en.m.wikipedia.org/wiki/Blinn%E2%80%93Phong_reflection_model
>> > >
>> > > Tangents are not really related to Phong shading. Tangents are
>> > > necessary to interpret the information from the normal map. This, in
>> > > turn, is necessary to actually use normal maps at all.
>> > >
>> > > Explanation: the vectors in normal map are defined in "tangent space".
>> > > In this space,
>> > >
>> > > - The +Z vector is pointing outward from the polygon. (To be more
>> > > precise, +Z is pointing in the direction indicated by per-vertex or
>> > > per-face normal vectors.)
>> > >
>> > > - The +X vector should point in the direction where U texture
>> > > coordinate grows (and be fixed to be orthogonal to +Z). This is called
>> > > "tangent vector".
>> > >
>> > > - Similarly, +Y vector should point in the direction where V texture
>> > > coordinate grows (and again be fixed to be orthogonal to  +Z). This is
>> > > called "bitangent vector".
>> > >
>> > > Browser can calculate tangent and bitangent if your mesh is composed
>> > > from polygons (using MikkTSpace algorithm , or CGE
>> > > TAbstractBumpMappingGenerator.CalculateTangentVectors ). But in case
>> > > of point set, you don't know which points should  be considered
>> > > "adjacent", so you cannot calculate it.
>> > >
>> > > And without knowing where the "tangent vector" points, we don't know
>> > > how to interpret a vector in normalmap like (1, 0, 0).
>> > >
>> > > Regards,
>> > > Michalis
>> >
>> >
>> >
>> > --
>> > Andreas Plesch
>> > Waltham, MA 02453