// PUSH — deck B shoves deck A off the screen, both actually MOVING
// (the difference from a wipe: the pictures slide, the edge does not
// just reveal). FLIP pushes from the left instead.
{
    name: "Push"
    engine: "transition"
    dials: [
        {name: "SOFT", bind: "p0", default: 0.0},
        {name: "FLIP", bind: "p1", default: 0.0},
        {name: "DIP", bind: "p2", default: 0.0}
    ]
    shader: draw.DrawVjFxDuo {
        trans: fn(uv: vec2, t: float) -> vec4 {
            let flip = step(0.5, self.user.y)
            let x = mix(uv.x, 1.0 - uv.x, flip)
            let u = x + t
            // SOFT feathers the seam between the sliding decks
            // (0 = the stock hard cut).
            let soft = 0.0005 + self.user.x * 0.2
            let m = smoothstep(1.0 - soft, 1.0 + soft, u)
            let ax = mix(clamp(u, 0.0, 1.0), 1.0 - clamp(u, 0.0, 1.0), flip)
            let bx = mix(clamp(u - 1.0, 0.0, 1.0), 1.0 - clamp(u - 1.0, 0.0, 1.0), flip)
            let ca = self.deck_a(vec2(ax, uv.y))
            let cb = self.deck_b(vec2(bx, uv.y))
            let c = ca.mix(cb, m)
            // DIP: duck through black mid-slide (0 = off).
            let dim = 1.0 - 4.0 * clamp(t, 0.0, 1.0) * (1.0 - clamp(t, 0.0, 1.0)) * self.user.z * 0.9
            return vec4(c.xyz * dim, 1.0)
        }
    }
}