Skip Connection

#import "@preview/cetz:0.5.3": canvas, draw
#import draw: content, hobby, line

#set page(width: auto, height: auto, margin: 8pt, fill: none)
#set text(size: 12pt)

#canvas({
  let node-sep = 2.5 // Horizontal separation between nodes
  let arrow-style = (mark: (end: "stealth", fill: black, scale: 0.5))
  // Layers get a colored box, activations a transparent one with a caption below.
  for (column, body, name, fill) in (
    (0, [layer 1], "l1", rgb("#ffd699")),
    (1, $a(arrow(x))$, "act1", none),
    (2, [layer 2], "l2", rgb("#7dc3c3")),
    (3.75, $a(arrow(x))$, "act2", none),
  ) {
    let padding = if fill == none { (0, 3pt) } else { (3pt, 6pt) }
    content(
      (column * node-sep, 0),
      body,
      fill: fill,
      name: name,
      frame: "rect",
      padding: padding,
      stroke: none,
    )
    if fill == none { content((rel: (0, -0.3), to: name + ".south"), "activation") }
  }
  content((3 * node-sep, 0), text(size: 1.8em)[$plus.o$], name: "add")
  content((rel: (0, -0.3), to: "add.south"), "add")

  for (start, end) in (
    ("l1", "act1"),
    ("act1", "l2"),
    ("l2", "add.west"),
    ("add.east", "act2"),
  ) { line(start, end, ..arrow-style) }

  line((rel: (-2, 0), to: "l1"), "l1", name: "input", ..arrow-style)
  content((rel: (0, -0.2), to: "input.10%"), $arrow(x)$)

  hobby(
    (rel: (-1.5, 0), to: "l1"),
    (rel: (0, 2.2), to: "act1"),
    "add.north",
    close: false,
    tension: 0.8,
    ..arrow-style,
    name: "skip",
  )
  content((rel: (0, 0.3), to: "skip.mid"), $arrow(x)$)
  content(
    (rel: (0, -0.3), to: "skip.mid"),
    align(center)[skip connection\ (identity)],
    anchor: "north",
  )

  // Residual branch F(x) spans both layers
  content(
    (rel: (0, -1.2), to: "act1"),
    math.underbrace(box(width: 17em), text(size: 1.4em)[$cal(F)(arrow(x))$]),
  )

  content(
    (rel: (0.8, 0.8), to: "add"),
    $cal(F)(arrow(x)) + arrow(x)$,
    name: "fx-label",
    frame: "rect",
    stroke: none,
    padding: 1pt,
  )
  line("fx-label.south", "add.north-east", stroke: .2pt)
})