Fix vec3 variant of gltf_image to include scaling factor

[kwokcb]

Update

• Fixes glTF Vector3 image definition variant does not include a scale factor #3054

• Fix the vec3 version of gltf_image to have image scaling factor as with other variants.

• This allows for looksup like ORM (occlusion, roughness, metallic) to fully support the roughness and metallic scaling factors as defined by the glTF specification.

Change

• Add in factor interface input to gltf_image (vec3 variant) of glTF library
• Update existing BoomBox material to have this factor specified. ORM set to 1,1,1 so no difference for rendering.

Results

This is a sample glTF converted to MaterialX. The ORM texture is scaled in roughness and metallic channels.

glTF

• metallicFactor = 0.2, and roughnessFactor = 0.8 are the scaling factors to apply on specific channels on the ORM image outputs.

{
  "asset": {
    "generator": "pygltflib@v1.16.5",
    "version": "2.0"
  },
  "images": [
    {
      "uri": "resources/Images/brick_base_gray.jpg",
      "name": "gltf_color"
    },
    {
      "uri": "resources/Images/brick_roughness.jpg",
      "name": "gltf_metallic_roughness"
    }
  ],
  "materials": [
    {
      "pbrMetallicRoughness": {
        "baseColorFactor": [
          0.7714629769325256,
          0.0016784999752417207,
          0.011678500100970268,
          0.5
        ],
        "metallicFactor": 0.2,
        "roughnessFactor": 0.8,
        "baseColorTexture": {
          "index": 0,
          "texCoord": 0
        },
        "metallicRoughnessTexture": {
          "index": 1,
          "texCoord": 0
        }
      },
      "emissiveFactor": [
        0,
        0,
        0
      ],
      "alphaMode": "OPAQUE",
      "doubleSided": false,
      "name": "gltf_pbr_surfaceshader"
    }
  ],
  "samplers": [
    {
      "magFilter": 9729,
      "minFilter": 9986,
      "wrapS": 10497,
      "wrapT": 10497
    }
  ],
  "textures": [
    {
      "sampler": 0,
      "source": 0,
      "name": "gltf_color"
    },
    {
      "sampler": 0,
      "source": 1,
      "name": "gltf_metallic_roughness"
    }
  ]
}

MaterialX Equivalent

The factor input on the ORM image (image_orm) match to the glTF factors as a vector of
[ occluson factor = 1, roughness factor, metalic factor ]. The colorscaling factor on
image_base_color is already supported.

<?xml version="1.0"?>
<materialx version="1.39" doc="" >
  <gltf_pbr name="gltf_pbr_surfaceshader" type="surfaceshader" nodedef="ND_gltf_pbr_surfaceshader">
    <input name="base_color" type="color3" nodename="image_base_color" output="outcolor" />
    <input name="alpha" type="float" value="0.5" />
    <input name="metallic" type="float" nodename="extract_metallic" />
    <input name="roughness" type="float" nodename="extract_roughness" />
    <input name="occlusion" type="float" nodename="extract_occlusion" />
  </gltf_pbr>
  <surfacematerial name="MAT_gltf_pbr_surfaceshader" type="material">
    <input name="surfaceshader" type="surfaceshader" nodename="gltf_pbr_surfaceshader" />
  </surfacematerial>
  <gltf_colorimage name="image_base_color" type="multioutput">
    <input name="file" type="filename" value="resources/Images/brick_base_gray.jpg" colorspace="srgb_texture" />
    <output name="outcolor" type="color3" />
    <output name="outa" type="float" />
    <input name="color" type="color4" value="0.771463, 0.0016785, 0.0116785, 0.5" />
  </gltf_colorimage>
  <gltf_image name="image_orm" type="vector3">
    <input name="file" type="filename" value="resources/Images/brick_roughness.jpg" />
    <input name="filtertype" type="string" value="linear" />
    <input name="uaddressmode" type="string" value="periodic" />
    <input name="vaddressmode" type="string" value="periodic" />
    <input name="factor" type="vector3" value="1.0, 0.8, 0.2" /> <!-- new factor -->
  </gltf_image>
  <extract name="extract_occlusion" type="float">
    <input name="in" type="vector3" nodename="image_orm" />
    <input name="index" type="integer" value="0" />
  </extract>
  <extract name="extract_roughness" type="float">
    <input name="in" type="vector3" nodename="image_orm" />
    <input name="index" type="integer" value="1" />
  </extract>
  <extract name="extract_metallic" type="float">
    <input name="in" type="vector3" nodename="image_orm" />
    <input name="index" type="integer" value="2" />
  </extract>
</materialx>

MaterialX Graph Editor

The resulting graph looks like this in the graph Editor

Color + ORM factors	Roughness Modified	Metallic Modified

[fallenmi]

The vector3 gltf_image path now matches the existing color3, color4, and float variants: factor is declared on the NodeDef, its neutral default preserves existing documents, and the implementation graph applies it before returning the sampled value. This enables the intended per-channel ORM scaling without changing the default result.

I verified exact head a4de710993f61d0087c70f197b21367ae8dce07c against exact base 39e4c9e04580bb690235667f6b28686e60344773 and current GitHub merge 3f2db9abc2cc2d8cc23c442741a6bc58dde616a1 using the public MaterialX 1.39.5 APIs. A factor-bearing vector3 document is rejected at base with the expected NodeDef interface error; at head and merge it validates and generates factor * image_out in GLSL, MSL, OSL, and MDL. The combined library, boombox example, and neutral-factor compatibility control also validate. All 35 check runs and EasyCLA are green.

Disclosure: I used Codex to inspect the change and run exact-base, exact-head, and current-merge validation and shader-generation oracles. I verified the results and live pull-request state before preparing this review.