[x3d-public] Calculation of normal from normalTexture.

[Michalis Kamburelis]

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> napisał(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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