// Glowing outlines with a slowly cycling colour.
// order 20
// slider detail 1.0 12.0 5.0
// slider glow 0.0 6.0 2.5
// slider source 0.0 1.0 0.35
// slider hue 0.0 1.0 0.55
// slider cycle 0.0 1.0 0.3
// hue 0..1 -> a fully saturated colour (0 red, 0.08 orange, 0.33 green, 0.66 blue)
static float3 hueColor(float h) {
    return clamp(abs(fract(h + float3(0.0, 2.0 / 3.0, 1.0 / 3.0)) * 6.0 - 3.0) - 1.0, 0.0, 1.0);
}

static float luma(Inputs in, float2 uv) {
    return dot(in.image.sample(in.smp, uv).rgb, float3(0.299, 0.587, 0.114));
}

// how strongly brightness changes around this point
static float edge(Inputs in, float2 uv) {
    float2 t = 1.5 / in.resolution;
    float dx = luma(in, uv + float2(t.x, 0.0)) - luma(in, uv - float2(t.x, 0.0));
    float dy = luma(in, uv + float2(0.0, t.y)) - luma(in, uv - float2(0.0, t.y));
    return length(float2(dx, dy));
}

float4 effect(float2 uv, Inputs in) {
    float sharp = smoothstep(0.1, 0.5, edge(in, uv) * in.p.detail);
    float halo = 0.0;  // edges found in a ring around this pixel = a cheap glow
    for (int i = 0; i < 12; i++) {
        float a = 6.2832 * float(i) / 12.0;
        float2 dir = float2(cos(a), sin(a)) / in.resolution;
        halo += edge(in, uv + dir * 4.0) + edge(in, uv + dir * 9.0);
    }
    halo = halo / 24.0 * in.p.detail * in.p.glow;

    float3 tint = hueColor(in.p.hue + in.time * in.p.cycle * 0.2);
    float3 base = in.image.sample(in.smp, uv).rgb * in.p.source;
    return float4(base + tint * (sharp + halo), 1.0);
}