import { transformedNormalView } from '../../accessors/NormalNode.js';
import { positionViewDirection } from '../../accessors/PositionNode.js';
import { sheen, sheenRoughness } from '../../core/PropertyNode.js';
import { tslFn, float } from '../../shadernode/ShaderNode.js';

// https://github.com/google/filament/blob/master/shaders/src/brdf.fs
const D_Charlie = ( roughness, dotNH ) => {

	const alpha = roughness.pow2();

	// Estevez and Kulla 2017, "Production Friendly Microfacet Sheen BRDF"
	const invAlpha = float( 1.0 ).div( alpha );
	const cos2h = dotNH.pow2();
	const sin2h = cos2h.oneMinus().max( 0.0078125 ); // 2^(-14/2), so sin2h^2 > 0 in fp16

	return float( 2.0 ).add( invAlpha ).mul( sin2h.pow( invAlpha.mul( 0.5 ) ) ).div( 2.0 * Math.PI );

};

// https://github.com/google/filament/blob/master/shaders/src/brdf.fs
const V_Neubelt = ( dotNV, dotNL ) => {

	// Neubelt and Pettineo 2013, "Crafting a Next-gen Material Pipeline for The Order: 1886"
	return float( 1.0 ).div( float( 4.0 ).mul( dotNL.add( dotNV ).sub( dotNL.mul( dotNV ) ) ) );

};

const BRDF_Sheen = tslFn( ( { lightDirection } ) => {

	const halfDir = lightDirection.add( positionViewDirection ).normalize();

	const dotNL = transformedNormalView.dot( lightDirection ).clamp();
	const dotNV = transformedNormalView.dot( positionViewDirection ).clamp();
	const dotNH = transformedNormalView.dot( halfDir ).clamp();

	const D = D_Charlie( sheenRoughness, dotNH );
	const V = V_Neubelt( dotNV, dotNL );

	return sheen.mul( D ).mul( V );

} );

export default BRDF_Sheen;