virtualx-engine/thirdparty/oidn/core/transfer_function.h

// ======================================================================== //
// Copyright 2009-2019 Intel Corporation                                    //
//                                                                          //
// Licensed under the Apache License, Version 2.0 (the "License");          //
// you may not use this file except in compliance with the License.         //
// You may obtain a copy of the License at                                  //
//                                                                          //
//     http://www.apache.org/licenses/LICENSE-2.0                           //
//                                                                          //
// Unless required by applicable law or agreed to in writing, software      //
// distributed under the License is distributed on an "AS IS" BASIS,        //
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. //
// See the License for the specific language governing permissions and      //
// limitations under the License.                                           //
// ======================================================================== //

#pragma once

#include "image.h"
#include "node.h"

namespace oidn {

  __forceinline float luminance(float r, float g, float b)
  {
    return 0.212671f * r + 0.715160f * g + 0.072169f * b;
  }

  // Color transfer function base class
  class TransferFunction
  {
  public:
    virtual ~TransferFunction() = default;

    virtual float forward(float y) const = 0;
    virtual float inverse(float x) const = 0;
  };

  // HDR transfer function base class
  class HDRTransferFunction : public TransferFunction
  {
  protected:
    static constexpr float yMax = 65504.f;

    float exposure;
    float rcpExposure;

  public:
    HDRTransferFunction(float exposure = 1.f)
    {
      setExposure(exposure);
    }

    void setExposure(float exposure)
    {
      this->exposure = exposure;
      this->rcpExposure = (exposure != 0.f) ? (1.f / exposure) : 0.f;
    }
  };

  // Linear transfer function (LDR)
  class LinearTransferFunction : public TransferFunction
  {
  public:
    __forceinline float forward(float y) const override
    {
      return min(y, 1.f);
    }

    __forceinline float inverse(float x) const override
    {
      return min(x, 1.f);
    }
  };

  // 2.2 gamma transfer function (LDR)
  class GammaTransferFunction : public TransferFunction
  {
  public:
    __forceinline float forward(float y) const override
    {
      return min(pow(y, 1.f/2.2f), 1.f);
    }

    __forceinline float inverse(float x) const override
    {
      return min(pow(x, 2.2f), 1.f);
    }
  };

  // Logarithmic transfer function (HDR)
  // Compresses [0..65504] to [0..1]
  class LogTransferFunction : public HDRTransferFunction
  {
  private:
    static const float xScale;

  public:
    LogTransferFunction(float exposure = 1.f)
      : HDRTransferFunction(exposure)
    {
    }

    __forceinline float forward(float y) const override
    {
      return log(y * exposure + 1.f) * xScale;
    }

    __forceinline float inverse(float x) const override
    {
      return (exp(x * (1.f/xScale)) - 1.f) * rcpExposure;
    }
  };

  // PQX transfer function (HDR)
  // Compresses [0..65504] to [0..1]
  class PQXTransferFunction : public HDRTransferFunction
  {
  private:
    static constexpr float m1 = 2610.f / 4096.f / 4.f;
    static constexpr float m2 = 2523.f / 4096.f * 128.f;
    static constexpr float c1 = 3424.f / 4096.f;
    static constexpr float c2 = 2413.f / 4096.f * 32.f;
    static constexpr float c3 = 2392.f / 4096.f * 32.f;
    static constexpr float  a = 3711.f / 4096.f / 8.f;

    static constexpr float yScale = 100.f / 10000.f;
    static const float     xScale;

  public:
    PQXTransferFunction(float exposure = 1.f)
      : HDRTransferFunction(exposure)
    {
    }

    __forceinline float forward(float y) const override
    {
      return pqxForward(y * exposure * yScale) * xScale;
    }

    __forceinline float inverse(float x) const override
    {
      return pqxInverse(x * (1.f/xScale)) * (1.f/yScale) * rcpExposure;
    }

  private:
    static __forceinline float pqForward(float y)
    {
      const float yp = pow(y, m1);
      return pow((c1 + c2 * yp) * rcp(1.f + c3 * yp), m2);
    }

    static __forceinline float pqxForward(float y)
    {
      if (y <= 1.f)
        return pqForward(y);
      else
        return a * log(y) + 1.f;
    }

    static __forceinline float pqInverse(float x)
    {
      const float xp = pow(x, 1.f/m2);
      return pow(max((xp - c1) * rcp(c2 - c3 * xp), 0.f), 1.f/m1);
    }

    static __forceinline float pqxInverse(float x)
    {
      if (x <= 1.f)
        return pqInverse(x);
      else
        return exp((x - 1.f) * (1.f/a));
    }
  };

  // Autoexposure node
  class AutoexposureNode : public Node
  {
  private:
    Image color;
    std::shared_ptr<HDRTransferFunction> transferFunc;

  public:
    AutoexposureNode(const Image& color,
                     const std::shared_ptr<HDRTransferFunction>& transferFunc)
      : color(color),
        transferFunc(transferFunc)
    {}

    void execute(stream& sm) override
    {
      const float exposure = autoexposure(color);
      //printf("exposure = %f\n", exposure);
      transferFunc->setExposure(exposure);
    }

  private:
    static float autoexposure(const Image& color);
  };

} // namespace oidn