node_modules/three/src/renderers/shaders/ShaderChunk/metalnessmap_fragment.glsl.js

export default /* glsl */`
float metalnessFactor = metalness;

#ifdef USE_METALNESSMAP

	vec4 texelMetalness = texture2D( metalnessMap, vUv );

	// reads channel B, compatible with a combined OcclusionRoughnessMetallic (RGB) texture
	metalnessFactor *= texelMetalness.b;

#endif
`;