FBX incorrect diffuse color

[LeXXik]

While looking at #1113, I noticed an FBX import pipeline generates a material with incorrect diffuse color. No vertex colors used here. Diffuse color should be white, or whatever the default material color is.

The GLB generates material with diffuse color:

The FBX generates material with:

Same model, just different format exports.

Project: https://playcanvas.com/project/1436770/overview/blank-project

[willeastcott]

The Issue: Color Space Conversion

This is a linear vs sRGB issue.

The Math

• CCCCCC = 204/255 ≈ 0.8 per channel
• E6E6E6 = 230/255 ≈ 0.902 per channel

If you convert 0.8 linear → sRGB: 0.8^(1/2.2) ≈ 0.902 → E6

What's Happening

Blender works internally in linear color space. The default Principled BSDF base color is 0.8 linear RGB.

GLTF/GLB Import (correct):

std::array<float, 3> linearTosRGB(const std::array<float, 3> &value)
{
    return std::array<float, 3>{
        powf(value[0], 1.0f / 2.2f),
        powf(value[1], 1.0f / 2.2f),
        powf(value[2], 1.0f / 2.2f)
    };
}

And it's applied to the diffuse color:

                // base color factor
                auto &bcf = pbr.baseColorFactor;
                if (bcf.size() == 4)
                {
                    result->diffuse = linearTosRGB({ bcf[0], bcf[1], bcf[2] });
                    result->opacity = bcf[3];

GLTF spec requires colors in linear space, so the importer correctly converts 0.8 linear → 0.902 sRGB (E6E6E6).

FBX Import (no conversion):

        lDouble3 = ((FbxSurfacePhong *)lMaterial)->Diffuse;
        lDouble = ((FbxSurfacePhong *)lMaterial)->DiffuseFactor;
        double blah = lDouble.Get();
        diffuse[0] = static_cast<float>(lDouble3.Get()[0] * lDouble.Get());
        diffuse[1] = static_cast<float>(lDouble3.Get()[1] * lDouble.Get());
        diffuse[2] = static_cast<float>(lDouble3.Get()[2] * lDouble.Get());

The FBX colors are used directly without any gamma conversion. Blender exports 0.8 (the raw linear value) to FBX, and it stays as 0.8 → CCCCCC.

Calling @mvaligursky and @slimbuck to see whether the right approach would be to update our fbxscenereader.cpp file in FACT to convert linear colors to sRGB.

[willeastcott]

I've submitted a PR to our back-end to fix this. Review, merge and deploy might take a little time over the Christmas break. I'm guessing first week of January.