float4 src = texture0.sample(texture0Smplr, In.v_texcoord);
    float3 yuv = rgb2yuv(src.rgb);    
    src.rgb = mix(src.rgb, yuv.x, -uniform.amount);

static float3 rgb2yuv(float3 rgb) {
    float3x3 kRGBtoYUVMatrix = float3x3(
        0.29900,    0.58700,   0.11400,
       -0.14713,   -0.28886,   0.43600,
        0.61500,   -0.51499,  -0.10001
    );
    return transpose(kRGBtoYUVMatrix) * rgb;
}

static float3 yuv2rgb(float3 yuv) {
    float3x3 kYUVtoRGBMatrix = float3x3(
        1.00000,    0.00000,    1.13983,
        1.00000,   -0.39465,   -0.58060,
        1.00000,    2.03211,    0.00000
    );
    return transpose(kYUVtoRGBMatrix) * yuv;
}

res = src - t * (lightness - src)

t = -1: res = src + lightness - src = lightness
t = 0:  res = src
t = 1:  res = src + (src - lightness)

    float4 src = texture0.sample(texture0Smplr, In.v_texcoord);
    float3 yuv = rgb2yuv(src.rgb);
    yuv.yz *= 1 + uniform.amount;
    src.rgb = yuv2rgb(yuv);

float area(float center, float width, float x) {
    return smoothstep(center - width, center, x) - smoothstep(center, center + width, x);
}

float3 gradient(float t) {
    return float3(
        area(0.25, 0.5, t),
        area(0.5, 0.5, t),
        area(0.75, 0.5, t)
    );
}

fragment
float4 fs_main(
    FS_INPUT In [[stage_in]],
    constant FS_UNIFORM& uniform [[buffer(16)]])
{
    return float4(gradient(In.v_texcoord.x), 1);
}

float3 gradient4(float3 r, float3 g, float3 b, float x) {
    return select(select(mix(r, g, x * 3), 
                      mix(g, b, x * 3 - 1.0), 
                      x > 1.0 / 3.0),
                  mix(b, r, x * 3 - 2.0),
                  x > 2.0 / 3.0);
}