Main.java

package be.twofold.test;

import java.util.function.*;

public class Main {
    public static void main(String[] args) {
        int diff61 = testFullRange((a, b) -> compare6(a, b, 1));
        int diff63 = testFullRange((a, b) -> compare6(a, b, 3));

        int diff41 = testFullRange((a, b) -> compare4(a, b, 1));
        int diff42 = testFullRange((a, b) -> compare4(a, b, 2));

        System.out.println("6:1: " + diff61);
        System.out.println("6:3: " + diff63);
        System.out.println("4:1: " + diff41);
        System.out.println("4:2: " + diff42);
    }

    private static int testFullRange(BiPredicate<Integer, Integer> compare) {
        int differenceCount = 0;
        for (int a = 0; a < 256; a++) {
            for (int b = 0; b < 256; b++) {
                differenceCount += compare.test(a, b) ? 1 : 0;
            }
        }
        return differenceCount;
    }

    private static boolean compare6(int a, int b, int offset) {
        float fa = fromUNorm8(a);
        float fb = fromUNorm8(b);

        int agt2 = (6 * a + 1 * b + offset) / 7;
        int agt3 = (5 * a + 2 * b + offset) / 7;
        int agt4 = (4 * a + 3 * b + offset) / 7;
        int agt5 = (3 * a + 4 * b + offset) / 7;
        int agt6 = (2 * a + 5 * b + offset) / 7;
        int agt7 = (1 * a + 6 * b + offset) / 7;

        int egt2 = toUNorm8(lerp(fa, fb, 1f / 7f));
        int egt3 = toUNorm8(lerp(fa, fb, 2f / 7f));
        int egt4 = toUNorm8(lerp(fa, fb, 3f / 7f));
        int egt5 = toUNorm8(lerp(fa, fb, 4f / 7f));
        int egt6 = toUNorm8(lerp(fa, fb, 5f / 7f));
        int egt7 = toUNorm8(lerp(fa, fb, 6f / 7f));

        boolean difference = false;
        difference |= agt2 != egt2;
        difference |= agt3 != egt3;
        difference |= agt4 != egt4;
        difference |= agt5 != egt5;
        difference |= agt6 != egt6;
        difference |= agt7 != egt7;
        return difference;
    }

    private static boolean compare4(int a, int b, int offset) {
        float fa = fromUNorm8(a);
        float fb = fromUNorm8(b);

        int agt2 = (4 * a + 1 * b + offset) / 5;
        int agt3 = (3 * a + 2 * b + offset) / 5;
        int agt4 = (2 * a + 3 * b + offset) / 5;
        int agt5 = (1 * a + 4 * b + offset) / 5;

        int egt2 = toUNorm8(lerp(fa, fb, 1f / 5f));
        int egt3 = toUNorm8(lerp(fa, fb, 2f / 5f));
        int egt4 = toUNorm8(lerp(fa, fb, 3f / 5f));
        int egt5 = toUNorm8(lerp(fa, fb, 4f / 5f));

        boolean difference = false;
        difference |= agt2 != egt2;
        difference |= agt3 != egt3;
        difference |= agt4 != egt4;
        difference |= agt5 != egt5;
        return difference;
    }

    private static float clamp(float value, float min, float max) {
        return Math.min(max, Math.max(value, min));
    }

    private static float lerp(float a, float b, float t) {
        return a + (b - a) * t;
    }

    private static float fromUNorm8(int value) {
        return value / 255f;
    }

    private static int toUNorm8(float value) {
        return (int) (clamp(value, 0f, 1f) * 255f + .5f);
    }
}