TSL: Fix bitangent* if used material.flatShading

[sunag]

Related issue: #30828 (comment)

Description

Fix bitangent* if used material.flatShading.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	336.36 78.34	336.36 78.34	+0 B +0 B
WebGPU	533.07 148.15	533.25 148.18	+175 B +28 B
WebGPU Nodes	532.54 148.05	532.71 148.08	+175 B +29 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	465.37 112.21	465.37 112.21	+0 B +0 B
WebGPU	604.68 164.12	604.83 164.15	+149 B +30 B
WebGPU Nodes	559.67 153.56	559.82 153.58	+149 B +26 B

[Copilot]

Pull Request Overview

This PR fixes issues with bitangent computations when materials use flat shading. It updates various accessors to conditionally assign varyings based on the material’s flatShading flag.

• Update TextureNode to pass the builder parameter to getUV
• Update Normal to conditionally assign a varying for normalWorld based on flatShading
• Update Bitangent accessor implementation to correctly handle flatShading and adjust type annotations

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/nodes/accessors/TextureNode.js	Passes builder to getUV to ensure the correct context is available
src/nodes/accessors/Normal.js	Refactors normalWorld creation to use conditional varying assignment based on material.flatShading
src/nodes/accessors/Bitangent.js	Wraps getBitangent in Fn, adds a varyingName argument, and standardizes bitangent types

Comments suppressed due to low confidence (3)

src/nodes/accessors/TextureNode.js:331

• Confirm that the updated call to getUV with the additional 'builder' parameter matches the expected API and is consistent with all call sites.

uvNode = builder.context.getUV( this, builder );

src/nodes/accessors/Normal.js:78

• Ensure that omitting the varying assignment for 'normal' when flat shading is enabled is the intended behavior and aligns with the shading strategy.

if ( builder.material.flatShading !== true ) {

src/nodes/accessors/Bitangent.js:21

• Consider using the computed 'bitangent' (from crossNormalTangent.mul(tangentGeometry.w).xyz) instead of 'crossNormalTangent' when assigning the varying to ensure the correct value is propagated.

bitangent = varying( crossNormalTangent, varyingName );

[WestLangley]

If the front side is extruded by a normal map, then the back side should be indented. With this PR, both sides are extruded when tangents are present and flat shading is true.

You can fix that in a separate PR if you wish.

[WestLangley]

@sunag Are you planning on fixing the problem in this PR, or in another one?

[sunag]

If the front side is extruded by a normal map, then the back side should be indented.

I agree, but apparently I'm getting these results here.

gltf.scene.children[ 0 ].geometry.computeTangents();
gltf.scene.children[ 0 ].material.side = THREE.DoubleSide;
gltf.scene.children[ 0 ].material.flatShading = true;

Front side

Back side

[sunag]

In fact something is wrong, it seems to be related to the camera angle, I will have to check the code.

[sunag]

@WestLangley I will make this fix in another PR due to the amount of change.