EXT_mesh_primitive_edge_visibility's material and line string

[danielzhong]

Description

This is a follow up PR to #12859. Primarily implementing the missing functionality for the material and line string section of EXT_mesh_primitive_edge_visibility:

• Parse EXT_mesh_primitive_edge_visibility materials and lineStrings during glTF load, extracting baseColorFactor as the default edge color and honoring per-lineString overrides.

• Expand the edge visibility pipeline to merge the bitfield and line-string edge data, generate optional edge-color attributes, and pass them through to the edge-rendering shader.

• Update the edge fragment stage to use the override colors from material section

Material:

Defines an accessor to the glTF material that supplies the edge color. This field is optional, when it is omitted the edges fall back to the mesh’s per‑vertex colors.

LineStrings

Defines accessors containing one or more line strings. Each entry may reference its own material; if not specified, the material defined in the section above is used, or otherwise the mesh’s vertex colors. Edges listed in this section must not be duplicated in the visibility bitfield (Handling during encoding: https://github.com/iTwin/imodel-native-internal/pull/888).

For more details please check:
EXT_mesh_primitive_edge_visibility README: Edge Visibility

Issue number and link

Previous PR: #12859
backlog issues: https://github.com/iTwin/itwin-graphics-backlog/issues/742
EXT_mesh_primitive_edge_visibility README: Edge Visibility
Khronos Group PR: Edge Visibility

Testing plan

Example Test files:

Two triangles: both have red fill color — one with a green outline and the other with a blue outline.

example.zip

"materials": [
  {
    "pbrMetallicRoughness": {
      "metallicFactor": 0.0
    },
    "doubleSided": true
  },
  {
    "pbrMetallicRoughness": {
      "baseColorFactor": [0.0, 0.0, 1.0, 1.0],  // blue
      "metallicFactor": 0.0
    },
    "doubleSided": true
  },
  {
    "pbrMetallicRoughness": {
      "baseColorFactor": [0.0, 1.0, 0.0, 1.0],  // green
      "metallicFactor": 0.0
    },
    "doubleSided": true
  }
]

"EXT_mesh_primitive_edge_visibility": {
  "lineStrings": [
    { "indices": 5, "material": 1}, 
    { "indices": 6, "material": 2 }
  ]
}

2. Expecting: 1 blue cylinder (Both outline and fill color are the same)

cylinder.zip

"EXT_mesh_primitive_edge_visibility": {
  "visibility": 5,
  "silhouetteNormals": 6
}

Live DEMO

Author checklist

• I have submitted a Contributor License Agreement
• I have added my name to CONTRIBUTORS.md
• I have updated CHANGES.md with a short summary of my change
• I have added or updated unit tests to ensure consistent code coverage
• I have updated the inline documentation, and included code examples where relevant
• I have performed a self-review of my code
• Live DEMO
• Add example glb file to the repo

[github-actions]

Thank you for the pull request, @danielzhong!

✅ We can confirm we have a CLA on file for you.

[Copilot]

Pull Request Overview

This PR implements missing material and line string functionality for the EXT_mesh_primitive_edge_visibility glTF extension, extending edge rendering capabilities to support per-edge colors and explicit line string definitions.

• Parses materials and lineStrings sections from the extension, extracting edge colors from baseColorFactor and supporting per-lineString color overrides
• Updates edge visibility pipeline to merge bitfield and line-string edges, generate edge-color attributes, and apply material colors in the shader
• Simplifies edge fragment shader to use override colors and remove debug color coding

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
EdgeVisibilityStageFS.glsl	Simplified fragment shader to apply edge colors from attributes instead of debug color coding
ModelComponents.js	Added edgeVisibility property to Primitive for storing extension data
EdgeVisibilityPipelineStage.js	Extended pipeline to handle line strings, generate edge color attributes, and process vertex colors
GltfLoader.js	Added parsing of material colors and line strings from extension during glTF load

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.