// OVERLAY — the classic photo-editor combine, applied to LUMA only (so
// deck B keeps its own hue while borrowing deck A's contrast curve):
// per pixel, if A's luma is dark the result folds toward 2*a*b (a
// multiply), if light it folds toward 1-2*(1-a)*(1-b) (a screen) — one
// seamless S-curve between the two. DEPTH pushes A's luma contrast
// before the fold, SAT keeps or flattens B's own colour in the result,
// BAL shifts the fold's pivot off the classic 0.5 midpoint.
{
    name: "Overlay"
    engine: "transition"
    engage: "ramp"
    dials: [
        {name: "DEPTH", bind: "p0", default: 0.5},
        {name: "SAT", bind: "p1", default: 0.6},
        {name: "BAL", bind: "p2", default: 0.5}
    ]
    shader: draw.DrawVjFxDuo {
        overlay1: fn(a: float, b: float, piv: float) -> float {
            let lo = 2.0 * a * b
            let hi = 1.0 - 2.0 * (1.0 - a) * (1.0 - b)
            return mix(lo, hi, step(piv, a))
        }
        trans: fn(uv: vec2, t: float) -> vec4 {
            let a = self.deck_a(uv)
            let b = self.deck_b(uv)
            let w = vec3(0.2126, 0.7152, 0.0722)
            let luma_a = dot(a.xyz, w)
            let luma_b = dot(b.xyz, w)
            let depth = 0.2 + self.user.x * 1.8
            let la = clamp(0.5 + (luma_a - 0.5) * depth, 0.0, 1.0)
            let piv = 0.1 + self.user.z * 0.8
            let luma_out = self.overlay1(la, luma_b, piv)
            let ratio = luma_out / max(luma_b, 0.02)
            let flat = vec3(luma_out, luma_out, luma_out)
            let toned = flat.mix(b.xyz * ratio, self.user.y)
            let composite = a.xyz.mix(toned, clamp(t, 0.0, 1.0))
            let cx = clamp(composite.x, 0.0, 1.0)
            let cy = clamp(composite.y, 0.0, 1.0)
            let cz = clamp(composite.z, 0.0, 1.0)
            return vec4(cx, cy, cz, 1.0)
        }
    }
}