setupFog( builder, outputNode ) {

		const fogNode = builder.fogNode;

		if ( fogNode ) {

			output.assign( outputNode );

			outputNode = vec4( fogNode );

		}

		return outputNode;

	}
Setup the fog.

@param {NodeBuilder} builder - The current node builder.
@param {Node<vec4>} outputNode - The existing output node.
@return {Node<vec4>} The output node.