// ADDITIVE MIX — the MX50's luminance add: deck B's light POURS INTO deck
// A instead of replacing it (white areas bloom, black adds nothing).
// GAIN pushes B hotter before the add. Ride the fader for how much light.
{
    name: "Additive Mix"
    engine: "transition"
    // Overlay semantics: ridden to the fader's end the key STAYS fully
    // applied (the composite is the destination) — never a snap to
    // plain B. Zero only at the A end.
    engage: "ramp"
    p1: 0.5
    dials: [
        {name: "GAIN", bind: "p0", default: 0.5},
        {name: "CURVE", bind: "p1", default: 0.5},
        {name: "KEY", bind: "p2", default: 0.0}
    ]
    shader: draw.DrawVjFxDuo {
        trans: fn(uv: vec2, t: float) -> vec4 {
            let a = self.deck_a(uv)
            let b = self.deck_b(uv)
            let gain = 0.5 + self.user.x * 1.5
            // CURVE bends the pour's fader response (1:1 at mid-knob).
            let tt = pow(clamp(t, 0.0, 1.0), pow(4.0, self.user.y * 2.0 - 1.0))
            // KEY gates the add by B's own luminance: 0 = plain add,
            // full = only B's bright parts pour.
            let luma = dot(b.xyz, vec3(0.2126, 0.7152, 0.0722))
            let gate = mix(1.0, smoothstep(0.25, 0.75, luma), self.user.z)
            let c = a.xyz + b.xyz * gain * tt * gate
            return vec4(min(c.x, 1.0), min(c.y, 1.0), min(c.z, 1.0), 1.0)
        }
    }
}