3DModel: Models with multiple vertex color attributes render inconsistently

[ToadLobe]

Checks

• I have read and understand the bug report guidelines
• I checked for existing similar reports to avoid submitting a duplicate
• I verified the issue occurs in the latest beta release to avoid submitting an issue that has already been resolved

Problem description

A glTF mesh that includes more than one vertex color attribute (e.g. COLOR_0 and COLOR_1) renders correctly in Construct 3's editor window, as well as every other gLTF viewer we've tested (Babylon.js, Three.js, Blender, macOS preview). Construct's editor behaviour is expected and consistent with the gLTF spec.

However, at runtime and in exports, these meshes behave strangely. Sometimes they appear completely white, at other times their texture color is distorted, tinted or blown out.

Upload a project

Repro.c3p.zip

Steps to reproduce

1. Open the project, observe appearance in editor
2. Preview the project and observe appearance

Observed result

Expected result

Affected platforms

macOS

Affected browsers

Chrome

Last unaffected release

No response

First affected release

No response

Additional remarks

No response

Platform information

Product: Construct 3 r487 (stable)
Browser: Chrome 148.0.7778.179
Browser engine: Chromium
Context: browser
Operating system: macOS 26.5.0
Device type: desktop
Device pixel ratio: 2
Logical CPU cores: 10
Approx. device memory: 16 GB
User agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/148.0.0.0 Safari/537.36
Language setting: en-US
Audio sample rate: 48000 Hz
Audio output channels: 2
Audio output interpretation: speakers
Renderer: WebGPU
Compatibility mode: no
Supports GPU profiling: yes
Major performance caveat: no
Maximum texture size: 16384
Adapter details: apple/metal-3
Adapter features: bgra8unorm-storage, clip-distances, core-features-and-limits, depth-clip-control, depth32float-stencil8, dual-source-blending, float32-blendable, float32-filterable, indirect-first-instance, primitive-index, rg11b10ufloat-renderable, shader-f16, subgroups, texture-component-swizzle, texture-compression-astc, texture-compression-astc-sliced-3d, texture-compression-bc, texture-compression-bc-sliced-3d, texture-compression-etc2, texture-formats-tier1, texture-formats-tier2, timestamp-query

[DiegoScirra]

Can you share the model with multiple color attributes?

[ToadLobe]

@DiegoScirra Sample.glb.zip

[DiegoScirra]

This will be fixed in the next beta.

The same issue that happens on preview also happens when the project is saved, this means that any previously saved projects will still be showing the problem because it has to do with data that is being lost during the save process and it's not possible to guess it during load.

Asides from that, the expected behavior is for COLOR_0 to be the only supported color, other color attributes will be ignored.