PBR extension: anisotropy

[alemuntoni]

TODO:

• fix mipmap creation for normal texture images (assigned to @malomo)
  use floats instead of uint8 per channel, trough bimg functions
  • fixed in commit 6c2f4ce
• check commit 6c2f4ce

[alemuntoni]

test: ANISOTROPY_ROTATION_TEST in 950-pbr

[Copilot]

Pull request overview

Adds initial support for the glTF KHR_materials_anisotropy extension across the asset pipeline (glTF import → material/uniform plumbing → PBR shader evaluation) and provides new sample assets to validate anisotropy strength/rotation/texture behavior.

Changes:

• Implement anisotropic GGX specular + reflection “bent normal” path in the PBR shader, and plumb anisotropy strength/rotation + anisotropy texture through uniforms.
• Extend material/texture typing to include an anisotropy texture and serialize/deserialize anisotropy parameters.
• Update texture-stage/sampler layout and add new glTF examples/assets for anisotropy testing.

Reviewed changes

Copilot reviewed 12 out of 23 changed files in this pull request and generated 10 comments.

Show a summary per file

File	Description
vclib/render/shaders/vclib/bgfx/pbr_common.sh	Adds anisotropic GGX functions and threads anisotropy parameters into pbrColorLights; also changes default light settings.
vclib/render/shaders/vclib/bgfx/drawable/uniforms/material_uniforms.sh	Adds u_anisotropyPack uniform macros for strength/rotation.
vclib/render/shaders/vclib/bgfx/drawable/uniforms/drawable_mesh_texture_uniforms.sh	Adds anisotropy texture sampling helpers and shifts BRDF LUT to stage position 6.
vclib/render/shaders/vclib/bgfx/drawable/drawable_mesh/surface/fs_surface_uber_pbr.sc	Computes anisotropy tangent frame + reflection bending; passes anisotropy data into pbrColorLights.
vclib/render/include/vclib/bgfx/drawable/uniforms/material_uniforms.h	Adds/binds the anisotropy uniform pack on the C++ side.
vclib/render/include/vclib/bgfx/drawable/uniforms/drawable_mesh_uniforms.h	Adds ANISOTROPY texture type to the packed-stage enum.
vclib/render/include/vclib/bgfx/drawable/drawable_environment.h	Changes BRDF LUT sampler uniform from s_tex5 → s_tex6.
vclib/core/include/vclib/space/core/material.h	Adds anisotropy parameters and anisotropy texture type + serialization support.
vclib/core/include/vclib/io/mesh/gltf/detail/load_mesh.h	Loads KHR_materials_anisotropy extension values/texture into Material.
examples/render/950-pbr/main.cpp	Adds anisotropy-related sample selections and prints anisotropy material info.
assets/example_meshes/gltf/CarbonFibre/CarbonFibre_occlusion.png	New sample texture (CarbonFibre).
assets/example_meshes/gltf/CarbonFibre/CarbonFibre_anisotropy.png	New sample anisotropy texture (CarbonFibre).
assets/example_meshes/gltf/CarbonFibre/CarbonFibre.gltf	New CarbonFibre glTF sample using KHR_materials_anisotropy.
assets/example_meshes/gltf/AnisotropyRotationTest/Heights_1d_Normals_v2.png	New anisotropy rotation test texture.
assets/example_meshes/gltf/AnisotropyRotationTest/GridWithMarkers.png	New anisotropy rotation test texture.
assets/example_meshes/gltf/AnisotropyRotationTest/AnisotropyRotationTest.gltf	New anisotropy rotation test scene using KHR_materials_anisotropy.
assets/example_meshes/gltf/AnisotropyRotationTest/AnisoRotation30_Linear.png	New anisotropy rotation test texture.
assets/example_meshes/gltf/AnisotropyRotationTest/AnisoRotation10_Linear.png	New anisotropy rotation test texture.
assets/example_meshes/gltf/AnisotropyRotationTest/AnisoDonutLabels.png	New anisotropy rotation test texture.

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[alemuntoni]

Fixed mipmap downsampling for normal maps (commit 6c2f4ce):

Normal Map Downsampling Fixes

• Custom Downsampling Logic: Extracted the normal map downsampling routine into a dedicated lambda (downsampleNormalMap2x2) outside of setTexture to improve readability and isolation of logic.
• Correct Vector Math: Instead of relying on standard image downsampling (which shortens normal vectors by linear interpolation), the new logic fetches the 4 adjacent pixels, remaps their RGB channels from [0, 255] to [-1.0, 1.0], averages them, and then explicitly normalizes the resulting vector. This ensures normal maps maintain unit length across all mip levels, preventing shading artifacts in PBR anisotropy.
• Alpha Channel Preservation: The custom lambda correctly applies standard bilinear interpolation to the alpha channel. This fixes a major issue where bimg's normal map downsample function would discard or corrupt the alpha channel entirely.
• Integration: Updated the setTexture lambda signature to accept an isNormalMap boolean flag. At invocation, it checks if textureType == Material::TextureType::NORMAL to seamlessly route the data to either the custom normal map downsampler or the standard bimg::imageRgba8Downsample2x2 for color/data textures.