HslColor.cs

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using System;
using System.Drawing;

namespace ReversiForm
{
    public class HslColor
    {
        private float _h;
        public float H
        {
            get { return this._h; }
        }

        private float _s;
        public float S
        {
            get { return this._s; }
        }

        private float _l;
        public float L
        {
            get { return this._l; }
        }

        public HslColor(float hue, float saturation, float lightness)
        {
            if (hue < 0f || 360f <= hue)
            {
                throw new ArgumentException("hueは0以上360未満の値です。", "hue");
            }
            if (saturation < 0f || 1f < saturation)
            {
                throw new ArgumentException("saturationは0以上1以下の値です。", "saturation");
            }
            if (lightness < 0f || 1f < lightness)
            {
                throw new ArgumentException("lightnessは0以上1以下の値です。", "lightness");
            }

            this._h = hue;
            this._s = saturation;
            this._l = lightness;
        }

        public static HslColor FromRgb(Color rgb)
        {
            float r = (float)rgb.R / 255f;
            float g = (float)rgb.G / 255f;
            float b = (float)rgb.B / 255f;

            float max = Math.Max(r, Math.Max(g, b));
            float min = Math.Min(r, Math.Min(g, b));

            float lightness = (max + min) / 2f;

            float hue, saturation;
            if (max == min)
            {
                //undefined
                hue = 0f;
                saturation = 0f;
            }
            else
            {
                float c = max - min;

                if (max == r)
                {
                    hue = (g - b) / c;
                }
                else if (max == g)
                {
                    hue = (b - r) / c + 2f;
                }
                else
                {
                    hue = (r - g) / c + 4f;
                }
                    hue *= 60f;
                if (hue < 0f)
                {
                    hue += 360f;
                }

                //saturation = c / (1f - Math.Abs(2f * lightness - 1f));
                if (lightness < 0.5f)
                {
                    saturation = c / (max + min);
                }
                else
                {
                    saturation = c / (2f - max - min);
                }
            }

            return new HslColor(hue, saturation, lightness);
        }

        public static Color ToRgb(HslColor hsl)
        {
            float s = hsl.S;
            float l = hsl.L;

            float r1, g1, b1;
            if (s == 0)
            {
                r1 = l;
                g1 = l;
                b1 = l;
            }
            else
            {
                float h = hsl.H / 60f;
                int i = (int)Math.Floor(h);
                float f = h - i;
                //float c = (1f - Math.Abs(2f * l - 1f)) * s;
                float c;
                if (l < 0.5f)
                {
                    c = 2f * s * l;
                }
                else
                {
                    c = 2f * s * (1f - l);
                }
                float m = l - c / 2f;
                float p = c + m;
                //float x = c * (1f - Math.Abs(h % 2f - 1f));
                float q; // q = x + m
                if (i % 2 == 0)
                {
                    q = l + c * (f - 0.5f);
                }
                else
                {
                    q = l - c * (f - 0.5f);
                }

                switch (i)
                {
                    case 0:
                        r1 = p;
                        g1 = q;
                        b1 = m;
                        break;
                    case 1:
                        r1 = q;
                        g1 = p;
                        b1 = m;
                        break;
                    case 2:
                        r1 = m;
                        g1 = p;
                        b1 = q;
                        break;
                    case 3:
                        r1 = m;
                        g1 = q;
                        b1 = p;
                        break;
                    case 4:
                        r1 = q;
                        g1 = m;
                        b1 = p;
                        break;
                    case 5:
                        r1 = p;
                        g1 = m;
                        b1 = q;
                        break;
                    default:
                        throw new ArgumentException("色相の値が不正です。", "hsl");
                }
            }

            return Color.FromArgb(
            (int)Math.Round(r1 * 255f),
            (int)Math.Round(g1 * 255f),
            (int)Math.Round(b1 * 255f));
        }
    }
}