[x3d-public] Calculation of normal from normalTexture.

[Andreas Plesch]

It makes sense to stay compatible with glTF but what is the reason to scale
x and y instead of just only scaling z ?

normal_tangent *= vec3(1.0, 1.0, castle_normalScale);

It would have the same effect (just reciprocal) and is conceptually cleaner.

Thanks, Andreas



>
> In short, the spec equation is correct. Let me explain why and what is
> the purpose of it:
>
> ( BTW, this is consistent with glTF material.normalTextureInfo.scale,
>
> https://www.khronos.org/registry/glTF/specs/2.0/glTF-2.0.html#_material_normaltextureinfo_scale
> ).
>
> 1. We deliberately multiply only XY (and not Z) components.
> Intuitively, "normalScale" allows the normalmap effect to be:
>
> - more emphasized, when normalScale > 1 (larger normalScale => larger
> XY compared to Z in tangent space => the effect of the normal map is
> more visible, as the surface looks more "rough")
>
> - or deemphasized, , when normalScale < 1 (smaller normalScale => the
> normals resemble more (0,0,1) in tangent space => normalmap effect is
> less visible)
>
> Multiplying all 3 vector components by normalScale wouldn't have any
> effect, indeed. As you say, the whole vector is normalized anyway.
>
> Note that this means that, regardless of the normalScale value, the
> lighting equations *always* work with normalized vectors. Using the
> "normalScale" doesn't make them non-normalized. This is deliberate,
> lighting equations in general are prepared assuming that normals
> (after all processing) are normalized.
>
> 2. Here's CGE/view3dscene implementation of it:
>
>
> https://github.com/castle-engine/castle-engine/blob/master/src/scene/glsl/source/bump_mapping.fs#L28
>
> It's just
>
> """
>  vec3 normal_tangent = texture2D(castle_normal_map,
>     castle_TexCoord<NormalMapTextureCoordinatesId>.st).xyz * 2.0 -
> vec3(1.0);
>
>   normal_tangent *= vec3(castle_normalScale, castle_normalScale, 1.0);
> """
>
> We normalize later.
>
> 3. I created test model for it:
>
>
> https://github.com/michaliskambi/x3d-tests/blob/master/pbr/enhanced_phong_material/bump_mapping_normalscale.x3dv
>
> with correct output on
>
>
> https://github.com/michaliskambi/x3d-tests/blob/master/pbr/enhanced_phong_material/bump_mapping_normalscale.png
>
> 4. I did just confirm it quickly looking at glTF sample viewer
> implementation:
>
>
> https://github.com/KhronosGroup/glTF-Sample-Viewer/blob/master/source/Renderer/shaders/material_info.glsl#L141
>
> They do this (comments from me):
>
> """
> #ifdef HAS_NORMAL_MAP
>
> info.ntex = texture(u_NormalSampler, UV).rgb * 2.0 - vec3(1.0);  //
> get normal vector from texture
>
> info.ntex *= vec3(u_NormalScale, u_NormalScale, 1.0); // multiply XY
> by normalScale
>
> info.ntex = normalize(info.ntex); // normalize
>
> info.n = normalize(mat3(t, b, ng) * info.ntex); // convert from
> tangent space to proper space (didn't check now whether this is to
> object or eye space)
> ...
> """
>
> Regards,
> Michalis
>
>
> sob., 2 kwi 2022 o 10:06 Holger Seelig <holger.seelig at yahoo.de>
> napisa?(a):
> >
> > Looking at X3DOneSidedMaterialNode
> https://www.web3d.org/specifications/X3Dv4Draft/ISO-IEC19775-1v4-CD1/Part01/components/shape.html#X3DOneSidedMaterialNode
> I can find there the calculation of the normal as following:
> >
> > normal.xyz = normalize((textureSample(normalTexture).rgb * vec3(2,2,2)
> - vec3(1,1,1)) * vec3(normalScale, normalScale, 1))
> >
> > Think the handling of normalScale is wrong, the vector must include 3
> times normalScale, not z equal 1, like so:
> >
> > vec3(normalScale, normalScale, normalScale)
> >
> > The second issue I notices is that the normalScale is applied before
> normalization, but that means that it has not effect. Normalization means
> multiplying the vector with a factor, so that the length of the vector is
> one (1), this makes the multiplication with the normalScale factor undone.
> >
> > Best regards,
> > Holger.
> >
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