blend.h

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#ifndef COLOR_UTIL_BLEND_H
#define COLOR_UTIL_BLEND_H

#include <color_util/types.h>
#include <algorithm>

namespace color_util
{
template <typename rgba>
inline rgba rgbaBlend(const rgba& color_a, const rgba& color_b, double ratio)
{
  ratio = std::min(ratio, 1.0);
  ratio = std::max(ratio, 0.0);

  double i_ratio = 1.0 - ratio;

  rgba color;
  color.r = color_a.r * i_ratio + color_b.r * ratio;
  color.g = color_a.g * i_ratio + color_b.g * ratio;
  color.b = color_a.b * i_ratio + color_b.b * ratio;
  color.a = color_a.a * i_ratio + color_b.a * ratio;
  return color;
}

template <typename hsva>
inline hsva hueBlend(const hsva& color_a, const hsva& color_b, double ratio)
{
  ratio = std::min(ratio, 1.0);
  ratio = std::max(ratio, 0.0);

  double i_ratio = 1.0 - ratio;

  hsva color;
  color.h = color_a.h * i_ratio + color_b.h * ratio;
  color.s = color_a.s * i_ratio + color_b.s * ratio;
  color.v = color_a.v * i_ratio + color_b.v * ratio;
  color.a = color_a.a * i_ratio + color_b.a * ratio;
  return color;
}

inline color_util::ColorHSVA hueBlendPlus(const color_util::ColorHSVA& color_a,
                                          const color_util::ColorHSVA& color_b,
                                          double ratio)
{
  ratio = std::min(ratio, 1.0);
  ratio = std::max(ratio, 0.0);

  double i_ratio = 1.0 - ratio;

  // Direct interpolation for saturation/value/alpha
  color_util::ColorHSVA color;
  color.s = color_a.s * i_ratio + color_b.s * ratio;
  color.v = color_a.v * i_ratio + color_b.v * ratio;
  color.a = color_a.a * i_ratio + color_b.a * ratio;

  // Hue interpolation
  double start_h, end_h, diff;
  if (color_a.h > color_b.h)
  {
    start_h = color_b.h;
    end_h = color_a.h;
    ratio = i_ratio;
  }
  else
  {
    start_h = color_a.h;
    end_h = color_b.h;
  }
  diff = end_h - start_h;

  // If the hue difference is greater than 0.5, interpolate the other way around
  if (diff > 0.5)  // 180deg
  {
    start_h = start_h + 1;  // 360deg
    double ipart;
    color.h = modf(start_h + ratio * (end_h - start_h), &ipart);  // 360deg
  }
  else
  {
    color.h = start_h + ratio * diff;
  }
  return color;
}


}  // namespace color_util

#endif  // COLOR_UTIL_BLEND_H