// SCREEN MIX — the soft additive (1-(1-a)(1-b)): lights combine without
// clipping to white the way the raw add does. The gentle MX50 overlay.
{
    name: "Screen 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"
    p0: 0.5  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 one = vec3(1.0, 1.0, 1.0)
            // GAIN pushes B hotter into the screen (1:1 at mid-knob);
            // CURVE bends the fader response; KEY gates by B's luma so
            // only its bright parts screen through when raised.
            let gain = 0.4 + self.user.x * 1.2
            let tt = pow(clamp(t, 0.0, 1.0), pow(4.0, self.user.y * 2.0 - 1.0))
            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 screened = one - (one - a.xyz) * (one - b.xyz * gain * tt * gate)
            return vec4(screened, 1.0)
        }
    }
}