VRM MToon Outline Thickness Adjustment

This skill covers how to adjust outline thickness for VRM models using MToon materials in @pixiv/three-vrm.

Common Symptoms

1. Outline too thick when model is scaled up or camera is close
2. Outline thickness inconsistent at different zoom levels
3. Need to adjust outline programmatically at runtime

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Key Concepts

Outline Width Modes

MToon materials have two main outline width modes:

Mode	Description	Behavior
'worldCoordinates'	Outline width is a physical world-space size	Outline appears thicker when model is scaled up or camera is closer
'screenCoordinates'	Outline width is relative to screen pixels	Outline stays consistent size regardless of zoom/scale

Critical Properties

material.outlineWidthMode    // 'none' | 'worldCoordinates' | 'screenCoordinates'
material.outlineWidthFactor  // Number - the actual width value
material.isMToonMaterial     // Boolean - true for MToon materials
material.needsUpdate         // Set to true after modifying properties

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Solution: Switch to Screen Coordinates

To make outlines stay consistent regardless of zoom/scale:

function adjustOutlineThickness(vrm, screenFactor = 0.005) {
    vrm.scene.traverse((object) => {
        if (object.isMesh || object.isSkinnedMesh) {
            const materials = Array.isArray(object.material) ? object.material : [object.material];
            
            materials.forEach(material => {
                if (!material || !material.isMToonMaterial) return;
                
                // Check if this material has outlines enabled
                const hasOutline = material.outlineWidthFactor > 0 && 
                                   material.outlineWidthMode !== 'none';
                
                if (hasOutline) {
                    // Switch to screen coordinates mode
                    material.outlineWidthMode = 'screenCoordinates';
                    material.outlineWidthFactor = screenFactor;
                    material.needsUpdate = true;
                }
            });
        }
    });
}

Factor Value Guidelines

For screenCoordinates mode:

Factor Value	Approximate Effect
0.002 - 0.003	Very thin outline (1 pixel)
0.005	Thin outline (1-2 pixels)
0.01	Medium outline (2-3 pixels)
0.02+	Thick outline

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Detection Pattern

To diagnose outline issues, log all materials:

function debugOutlineMaterials(vrm) {
    let count = 0;
    vrm.scene.traverse((object) => {
        if (!object.isMesh && !object.isSkinnedMesh) return;
        
        const materials = Array.isArray(object.material) ? object.material : [object.material];
        materials.forEach(material => {
            if (material?.isMToonMaterial || 'outlineWidthFactor' in material) {
                count++;
                console.log({
                    name: material.name || '未命名',
                    type: material.type,
                    isMToonMaterial: material.isMToonMaterial,
                    outlineWidthMode: material.outlineWidthMode,
                    outlineWidthFactor: material.outlineWidthFactor
                });
            }
        });
    });
    console.log(`Found ${count} MToon/Outline materials`);
}

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Important Notes

[!IMPORTANT]
Call timing matters! The function must be called AFTER currentModel is set. In vrm-core.js, the loadModel() function sets manager.currentModel near line 923. Any function that accesses currentModel must be called after this point.

[!NOTE]
Materials named "XXX (Outline)" are outline pass materials automatically created by three-vrm. They share properties with the main material but render the outline effect.

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API Reference (three-vrm MToon)

The enum values for outlineWidthMode:

// From three-vrm.module.min.js
{
    None: "none",
    WorldCoordinates: "worldCoordinates", 
    ScreenCoordinates: "screenCoordinates"
}

Property	Type	Description
outlineWidthMode	string	Width calculation mode
outlineWidthFactor	number	Width value (meaning depends on mode)
outlineColorFactor	Color	Outline color
outlineLightingMixFactor	number	How much lighting affects outline
outlineWidthMultiplyTexture	Texture	Texture to modulate outline width