// Copyright 2009 Intel Corporation
// SPDX-License-Identifier: Apache-2.0

#pragma once

#include "MicrofacetDistribution.ih"

OSPRAY_BEGIN_ISPC_NAMESPACE

// Beckmann distribution
struct BeckmannDistribution
{
  float alpha;
};

inline BeckmannDistribution make_BeckmannDistribution(float alpha)
{
  BeckmannDistribution m;
  m.alpha = alpha;
  return m;
}

inline vec3f sample(
    const BeckmannDistribution &self, float &pdf, const vec2f &s)
{
  float alpha2 = sqr(self.alpha);
  float tanTheta2 = -alpha2 * log(1.f - s.x);
  float cosTheta = rsqrt(1.f + tanTheta2);
  float sinTheta = cos2sin(cosTheta);

  float phi = 2.f * (float)pi * s.y;

  float cosTheta3 = sqr(cosTheta) * cosTheta;
  pdf = (1.f - s.x) * rcp((float)pi * alpha2 * cosTheta3);

  float sinPhi, cosPhi;
  sincos(phi, &sinPhi, &cosPhi);

  float x = cosPhi * sinTheta;
  float y = sinPhi * sinTheta;
  float z = cosTheta;
  return make_vec3f(x, y, z);
}

OSPRAY_END_ISPC_NAMESPACE
