// Based on GPU Gems
// Optimised by Alan Zucconi
inline fixed3 bump3y (fixed3 x, fixed3 yoffset)
{
    float3 y = 1 - x * x;
    y = saturate(y-yoffset);
    return y;
}

fixed3 spectral_zucconi6 (float w)
{
    // w: [400, 700]
    // x: [0,   1]
    fixed x = saturate((w - 400.0)/ 300.0);
    const float3 c1 = float3(3.54585104, 2.93225262, 2.41593945);
    const float3 x1 = float3(0.69549072, 0.49228336, 0.27699880);
    const float3 y1 = float3(0.02312639, 0.15225084, 0.52607955);
    const float3 c2 = float3(3.90307140, 3.21182957, 3.96587128);
    const float3 x2 = float3(0.11748627, 0.86755042, 0.66077860);
    const float3 y2 = float3(0.84897130, 0.88445281, 0.73949448);
    return
        bump3y(c1 * (x - x1), y1) +
        bump3y(c2 * (x - x2), y2) ;
}

// --- Diffraction grating effect ---
    float3 L = gi.light.dir;
    float3 V = viewDir;
    float3 N = worldNormal;

    // Reminder:
    //     thetaL = angle from L to N
    //     thetaR = angle from reflected L inside material to N
    // From Snell's Law:
    //     N1 * sin(thetaL) = N2 * sin(thetaR)
    float cos_thetaL = dot(N, L);
    float thetaL = acos(cos_thetaL);

    float sin_thetaR = (_N1 / _N2) * sin(thetaL);
    float thetaR = asin(sin_thetaR);
		float u = _N2 * 2 * d * abs(cos(thetaR));//光程差

    fixed3 color = 0;
    for (int n = 1; n <= _Order; n++)
    {
        // Constructive interference
        float wavelength = u / n;
        color += spectral_zucconi6(wavelength);
    }
    color = saturate(color);

half NdotV =  dot(normal_dir, view_dir * _Tile);
// 蓝色
half3 Mask01 = saturate(acos(NdotV)*2/UNITY_PI * half3(0,0,1));
// 蓝色和白色叉乘得到绿色，这里是为了拿到绿色通道 --- 可以手动计算下看结果，有saturate所以-1会修正
// --- 绿色和白色叉乘得到红色
// --- 红色和白色叉乘得到蓝色
half3 CrossColor = cross (half3(0,0,1) , half3(1,1,1));
// 绿色和红色
half3 Mask02 = saturate (asin(NdotV)*2/UNITY_PI * CrossColor);
// 得到红，绿，蓝三色通道
half3 AddMask = Mask01 + Mask02;
// 转灰度
half Rgb2Gray = saturate(0.2989 * AddMask.r + 0.587 * AddMask.g + 0.114 * AddMask.b);

half4 col = tex2D(_BubbleTex, half2(NdotV,NdotL));
half4 basecolor = tex2D(_MainTex,i.uv);
outcolor = lerp(col ,basecolor ,Rgb2Gray );