将RGB转换为HSB (又名HSV)

Question

我正在尝试实现一个简单的RGB到HSB例程，我已经严格遵循了维基百科参考文章的指示。

我所写的代码是：

#include <iostream>
#include <cmath>

struct RGB { float red, green, blue; };
struct HSB { float hue, saturation, brightness; }; // aka HSV

// https://en.wikipedia.org/wiki/HSL_and_HSV#Converting_to_RGB
static void HSBToRGB(HSB const & hsb, RGB & rgb )
{
  /*
   * Given a color with hue H ∈ [0°, 360°), saturation SHSV ∈ [0, 1], and value
   * V ∈ [0, 1], we first find chroma:
   */
  const float H = hsb.hue;
  const float S_HSV = hsb.saturation;
  const float V = hsb.brightness;
  const float C = V * S_HSV;
  /*
  * Then we can find a point (R1, G1, B1) along the bottom three faces of the RGB
  * cube, with the same hue and chroma as our color (using the intermediate value X
  * for the second largest component of this color):
  */
  const float H_prime = H / 60.;
  const float X = C * ( 1 - std::abs( (int)H_prime % 2 - 1) );
  float R1, G1, B1;
  if( isnan( H ) )
    {
    R1 = G1 = B1 = 0;
    }
  else if( 0 <= H_prime && H_prime < 1 )
    {
    R1 = C; G1 = X; B1 = 0;
    }
  else if( 1 <= H_prime && H_prime < 2 )
    {
    R1 = X; G1 = C; B1 = 0;
    }
  else if( 2 <= H_prime && H_prime < 3 )
    {
    R1 = 0; G1 = C; B1 = X;
    }
  else if( 3 <= H_prime && H_prime < 4 )
    {
    R1 = 0; G1 = X; B1 = C;
    }
  else if( 4 <= H_prime && H_prime < 5 )
    {
    R1 = X; G1 = 0; B1 = C;
    }
  else if( 5 <= H_prime && H_prime < 6 )
    {
    R1 = C; G1 = 0; B1 = X;
    }

  /*
   * Finally, we can find R, G, and B by adding the same amount to each component,
   * to match value:
   */
  const float m = V - C;
  rgb.red   = R1 + m;
  rgb.green = G1 + m;
  rgb.blue  = B1 + m;
}

int main()
{
  HSB const hsb = { 251.1f, 0.887f, 0.918f };
  RGB rgb = { 0.255f , 0.104f , 0.918f };
  std::cout << "Reference: " << rgb.red << "," << rgb.green << "," << rgb.blue << std::endl;
  HSBToRGB(hsb, rgb);
  std::cout << "Computed:  " << rgb.red << "," << rgb.green << "," << rgb.blue << std::endl;

  return 0;
}

在linux /jessie amd64上编译和运行时，我得到的是：

$ ./ref
Reference: 0.255,0.104,0.918
Computed:  0.103734,0.103734,0.918

现在，如果您查找正确的引用值(同一条，表中的第11行)，它应该是相同的。我不明白的是，Hue的差值很大(0.255 != 0.103734)。

我一直在重复检查我的实现与维基百科文章描述的内容，并且找不到任何不同之处。

使用java作为参考，我可以检查RGB值是否正确，例如：

import java.awt.Color;

public class HSBToRGBExample {
  public static void main(String[] args) {
    float hue = 251.1f / 360;
    float saturation = 0.887f;
    float brightness = 0.918f;
    int rgb = Color.HSBtoRGB(hue, saturation, brightness);
    int red = (rgb >> 16) & 0xFF;
    int green = (rgb >> 8) & 0xFF;
    int blue = rgb & 0xFF;
    System.out.println((float)red / 255 + " " + (float)green / 255 + " " + (float)blue / 255);
  }
}

维基百科的文章中有错误吗？或者在我的实现中缺少什么？

Answer 1

您需要浮点模数，但您正在转换到一个int (可能是为了平息编译器关于%只适用于整数的抱怨)。

在C++中

std::abs(std::fmod(H_prime, 2) - 1);

(由于浮点不精确，您仍然会看到差异。)