Custom Log Transfer Function

custom_log.cpp

// https://colab.research.google.com/drive/1NwjaD0NzNMzQeNQqZECj33PdcYGkeBM4#scrollTo=d61Knj0O9Lwf
namespace CustomLog
{
static struct Params
{
  Params(float w0, float w1, float o0, float p0, float p1)
      : w0(w0), w1(w1), o0(o0), p0(p0), p1(p1)
  {
    computeConstants();
  }

  void computeConstants()
  {
    s0 = (w1 - p1) / glm::log((w0 - o0) / (p0 - o0));
    k = s0 / (p0 - o0);
    o1 = w1 - s0 * glm::log(w0 - o0);
    y0 = p1 - k * p0;
  }

  float w0, w1, o0, p0, p1;
  float s0, k, o1, y0;
} params_ = {200.000000f, 1.000000f, -0.0066f, 0.022051f, 0.08f};

inline glm::vec3 fromLinear(const glm::vec3& x)
{
  static const auto map = [](float x) {
    if (x < params_.p0)
      return params_.k * x + params_.y0;
    return params_.s0 * glm::log(x - params_.o0) + params_.o1;
  };

  return glm::vec3(map(x.x), map(x.y), map(x.z));
}

inline glm::vec3 toLinear(const glm::vec3& x)
{
  static const auto map = [](float x) {
    if (x < params_.p1)
      return (x - params_.y0) / params_.k;
    return glm::exp((x - params_.o1) / params_.s0) + params_.o0;
  };

  return glm::vec3(map(x.x), map(x.y), map(x.z));
}

void computeNonNegativeFit()
{
  params_.p0 = params_.p1 / params_.k;
  params_.computeConstants();
}
} // namespace CustomLog