WebGLPrograms: Allow flat shading when material.wireframe == undefined

[visansm]

PR #31242 introduced an issue where flat shading would not work for a MeshMatcapMaterial without explicitly setting material.wireframe = false, because by default material.wireframe is undefined for MeshMatcapMaterial . This PR makes it so flat shading can be enabled when wireframe is undefined.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	338.24 79.14	338.24 79.14	+0 B +2 B
WebGPU	584.78 161.56	584.78 161.56	+0 B +0 B
WebGPU Nodes	583.39 161.32	583.39 161.32	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	469.7 113.74	469.7 113.75	+0 B +2 B
WebGPU	654.13 177.06	654.13 177.06	+0 B +0 B
WebGPU Nodes	608.25 166.24	608.25 166.24	+0 B +0 B

[Mugen87]

@WestLangley Any reasons why you didn't add wireframe to MeshMatcapMaterial? It seems all Mesh* materials support it.

[WestLangley]

Actually, I am much more concerned that flat shading no longer works with MeshMatcapMaterial.

Git bisect indicates flat shading was broken on MeshMatcapMaterial by #31242.

//

Any reasons why you didn't add wireframe to MeshMatcapMaterial?

I think MeshMatcapMaterial should be used to render meshes -- not lines.

[Mugen87]

Materials like MeshNormalMaterial also have this property. The wireframe property has debug purposes so I would say it's more consistent to add it to MeshMatcapMaterial as well.

I think MeshMatcapMaterial should be used to render meshes -- not lines.

Given this argumentation, we would have to remove wireframe from all Mesh* materials. None of these are intended for points and lines.

[WestLangley]

I have never liked the use of "lit" materials on wireframes. I think it is better to render lines with unlit.

I would prefer to keep MeshMatcapMaterial as-is, rendering meshes, not lines.

I'd focus on fixing the broken flat-shading, instead.

EDIT: I decided to add support for wireframe. See #31917.

[Mugen87]

TBH, I think it's better to add wireframe to MeshMatcapMaterial for consistency reasons. This should automatically fix flat shading since the matcap material is now treated like all other mesh materials. Also the wireframe property is an important debugging feature that we definitely should keep.

@mrdoob What is your preference here?

[Mugen87]

Closing in favor of #31917.

[mrdoob]

TBH, I think it's better to add wireframe to MeshMatcapMaterial for consistency reasons. This should automatically fix flat shading since the matcap material is now treated like all other mesh materials. Also the wireframe property is an important debugging feature that we definitely should keep.

@mrdoob What is your preference here?

Adding wireframe to MeshMacapMaterial is the best solution for this yep!