// Movement breaks the picture into horizontal bands that jump sideways and flash
// with shifted colours. Still frames pass through unchanged.
// order 70
// slider band 2.0 200.0 48.0
// slider intensity 0.0 2.0 1.0
// slider sensitivity 1.0 60.0 24.0
// slider shift 0.0 0.3 0.08
// slider split 0.0 0.06 0.012
// slider flash 0.0 1.0 0.7
// slider rate 1.0 30.0 12.0

// A repeatable pseudo random number in 0..1 from a 2D seed.
static float hash21(float2 p) {
    p = fract(p * float2(123.34, 456.21));
    p += dot(p, p + 45.32);
    return fract(p.x * p.y);
}

// How much the picture moved near this spot, in 0..1.
static float motionAt(float2 uv, Inputs in) {
    float2 flow = in.flow.sample(in.smp, uv).xy;
    float3 now = in.camera.sample(in.smp, uv).rgb;
    float3 before = in.last.sample(in.smp, uv).rgb;
    float moved = length(now - before);
    return clamp(length(flow) * in.p.sensitivity + moved * in.p.sensitivity * 0.05, 0.0, 1.0);
}

float4 effect(float2 uv, Inputs in) {
    float bandPixels = max(in.p.band, 1.0);
    float bandHeight = bandPixels / in.resolution.y;
    float band = floor(uv.y / bandHeight);
    float centre = (band + 0.5) * bandHeight;

    // The whole band reacts to the motion measured at its middle.
    float strength = clamp(motionAt(float2(uv.x, centre), in) * in.p.intensity, 0.0, 1.0);

    // Each band gets fresh random values a few times per second.
    float tick = floor(in.time * in.p.rate);
    float r1 = hash21(float2(band, tick));
    float r2 = hash21(float2(band + 71.3, tick));
    float r3 = hash21(float2(band + 13.7, tick + 5.0));

    float slide = (r1 * 2.0 - 1.0) * in.p.shift * strength;
    float split = (r2 * 2.0 - 1.0) * in.p.split * strength;
    float2 base = float2(uv.x + slide, uv.y);

    float3 col = in.image.sample(in.smp, base).rgb;
    col.r = in.image.sample(in.smp, float2(base.x + split, base.y)).r;
    col.b = in.image.sample(in.smp, float2(base.x - split, base.y)).b;

    // Flash with a shifted copy of the channels.
    float3 other = mix(col.gbr, col.brg, step(0.5, r3));
    col = mix(col, other, in.p.flash * strength);
    col *= 1.0 + (r1 - 0.5) * in.p.flash * strength;

    return float4(col, 1.0);
}