gistfile1.cs

// Copyright 2013 Matvei Stefarov <me@matvei.org>

using System;
using JetBrains.Annotations;
using RgbColor = System.Drawing.Color;

namespace fCraft.Drawing {
    /// <summary> Represents a palette of Minecraft blocks,
    /// that allows matching RGB colors to their closest block equivalents. </summary>
    public class BlockPalette {
        // XN/YN/ZN are illuminant D65 tristimulus values
        const double XN = 95.047,
                     YN = 100.000,
                     ZN = 108.883;

        // these constant are used in CIEXYZ -> CIELAB conversion
        const double LinearThreshold = (6/29d)*(6/29d)*(6/29d),
                     LinearMultiplier = (1/3d)*(29/6d)*(29/6d),
                     LinearConstant = (4/29d);

        // CIE76 formula for Delta-E, over CIELAB color space
        static double ColorDifference( LabColor color1, LabColor color2 ) {
            return
                // NOTE: multiplying delta-L by 1.2 is optional and not part of vanilla deltaE formula.
                // It is done here subjectively to emphasize luminance difference over hue/saturation difference.
                Math.Sqrt( (color2.L - color1.L)*(color2.L - color1.L)*1.2 +
                           (color2.a - color1.a)*(color2.a - color1.a) +
                           (color2.b - color1.b)*(color2.b - color1.b) );
        }


        // Conversion from RGB to CIELAB, using illuminant D65.
        static LabColor RgbToLab( RgbColor color ) {
            // RGB are assumed to be in [0...255] range
            double R = color.R/255d;
            double G = color.G/255d;
            double B = color.B/255d;

            // CIEXYZ coordinates are normalized to [0...1]
            double x = 0.4124564*R + 0.3575761*G + 0.1804375*B;
            double y = 0.2126729*R + 0.7151522*G + 0.0721750*B;
            double z = 0.0193339*R + 0.1191920*G + 0.9503041*B;

            double xRatio = x/XN;
            double yRatio = y/YN;
            double zRatio = z/ZN;

            LabColor result = new LabColor {
                // L is normalized to [0...100]
                L = 116*XyzToLab( yRatio ) - 16,
                a = 500*(XyzToLab( xRatio ) - XyzToLab( yRatio )),
                b = 200*(XyzToLab( yRatio ) - XyzToLab( zRatio ))
            };
            return result;
        }


        static double XyzToLab( double ratio ) {
            if( ratio > LinearThreshold ) {
                return Math.Pow( ratio, 1/3d );
            } else {
                return LinearMultiplier*ratio + LinearConstant;
            }
        }
        

        protected struct LabColor {
            public double L, a, b;
        }
    }
}