three.js/src/renderers/shaders/ShaderChunk/morphnormal_vertex.glsl.js

export default /* glsl */`
#ifdef USE_MORPHNORMALS

	// morphTargetBaseInfluence is set based on BufferGeometry.morphTargetsRelative value:
	// When morphTargetsRelative is false, this is set to 1 - sum(influences); this results in normal = sum((target - base) * influence)
	// When morphTargetsRelative is true, this is set to 1; as a result, all morph targets are simply added to the base after weighting
	objectNormal *= morphTargetBaseInfluence;

	for ( int i = 0; i < MORPHTARGETS_COUNT; i ++ ) {

		if ( morphTargetInfluences[ i ] != 0.0 ) objectNormal += getMorph( gl_VertexID, i, 1 ).xyz * morphTargetInfluences[ i ];

	}

#endif
`;