Etcher

Etcher is the annotation layer for Fresco-based image viewers in Phoenix.

Users draw shapes (rectangle, circle, polygon, freehand) on top of any Fresco viewer; your LiveView receives geometry events; you decide what to persist. A bundled Ecto schema + migration generator covers the common case; consumers with richer needs implement a behaviour and plug in their own storage.

An etcher is the tool that incises marks into a surface — Etcher does the same digitally.

┌─────────────────────────────────────────────────────┐
│  <Fresco.viewer id="photo" src="/uploads/img.jpg"/> │
│   ┌──┐                                              │
│   │+ │  ← fresco's nav column                       │
│   │- │                                              │
│   │⟲ │                                              │
│   │⛶ │                                              │
│   │✎ │  ← added by <Etcher.layer />                 │
│   └──┘                                              │
│                                                     │
│         ┌───┐  ┌────────┐                           │
│         │   │  │        │   ← drawn annotations     │
│         │   │  │        │                           │
│         └───┘  └────────┘                           │
│                                                     │
│         [⌖] [▭] [○] [⬡] [〰] [×]   ← bottom toolbar  │
└─────────────────────────────────────────────────────┘

Installation

Add :fresco (the viewer) and :etcher to your mix.exs:

def deps do
  [
    {:fresco, "~> 0.2"},
    {:etcher, "~> 0.1"}
  ]
end

Wire the JS hooks in your assets/js/app.js:

import "../../deps/fresco/priv/static/fresco.js"
import "../../deps/etcher/priv/static/etcher.js"

let liveSocket = new LiveSocket("/live", Socket, {
  hooks: { ...window.FrescoHooks, ...window.EtcherHooks, ...colocatedHooks }
})

If you want the bundled etcher_annotations table, run:

mix etcher.gen.migration
mix ecto.migrate

And point Etcher at your Repo in config/config.exs:

config :etcher, repo: MyApp.Repo

(You can skip both steps if you're implementing custom storage — see below.)

Quick start

defmodule MyAppWeb.PhotoLive do
  use MyAppWeb, :live_view

  def render(assigns) do
    ~H"""
    <Fresco.viewer id="photo" src={~p"/uploads/photo.jpg"} class="w-full h-[80vh]" />

    <Etcher.layer
      fresco_id="photo"
      target_type="file"
      target_uuid={@file.uuid}
      initial_annotations={@annotations}
    />
    """
  end

  def handle_event("etcher:created", attrs, socket) do
    case Etcher.create_annotation(Map.put(attrs, "creator_uuid", socket.assigns.current_user.uuid)) do
      {:ok, annotation} ->
        # Reflect the persisted uuid back to the client so subsequent
        # updates/deletes can reference the saved row.
        {:noreply,
         push_event(socket, "etcher:annotation-saved", %{
           tmp_id: attrs["tmp_id"],
           uuid: annotation.uuid
         })}

      {:error, _changeset} ->
        {:noreply, put_flash(socket, :error, "Could not save annotation")}
    end
  end

  def handle_event("etcher:selected", %{"uuid" => uuid}, socket) do
    {:noreply, assign(socket, :selected_annotation_uuid, uuid)}
  end
end

Open the page, click the pencil in Fresco's nav column → the bottom toolbar appears with the four drawing tools. Pick rectangle, drag on the image, release — handle_event("etcher:created", …) fires with the geometry in image pixel coordinates.

The component

<Etcher.layer
  fresco_id="photo"
  target_type="file"
  target_uuid={@file.uuid}
  initial_annotations={@annotations}
  tools={[:rectangle, :circle, :polygon, :freehand]}
/>

Events

The component emits four LiveView events. The consumer's LiveView handles whichever ones it cares about.

def handle_event("etcher:created", attrs, socket), do: ...
def handle_event("etcher:updated", %{"uuid" => uuid, "geometry" => geom}, socket), do: ...
def handle_event("etcher:deleted", %{"uuid" => uuid}, socket), do: ...
def handle_event("etcher:selected", %{"uuid" => uuid}, socket), do: ...

The etcher:created payload includes:

%{
  "target_type" => "file",
  "target_uuid" => "...",
  "kind" => "rectangle" | "circle" | "polygon" | "freehand",
  "geometry" => %{ ... },              # shape-specific, image-pixel coords
  "tmp_id" => "tmp-abc123-..."          # client-side temp id
}

After persisting, push back the saved uuid so the client can adopt it:

push_event(socket, "etcher:annotation-saved", %{tmp_id: tmp_id, uuid: annotation.uuid})

Geometry shapes:

All coordinates are in image pixels — Fresco's pan/zoom rescales them automatically.

Custom storage

Etcher.Storage is a behaviour. The default implementation is fine for most consumers, but you can swap in your own — useful when annotations need to be linked to other tables (comments, notifications, audit trails) inside the same transaction.

defmodule MyApp.AnnotationStorage do
  @behaviour Etcher.Storage

  alias MyApp.Repo
  alias MyApp.{Annotation, Comment}

  def create(attrs) do
    Repo.transaction(fn ->
      {:ok, comment} = %Comment{}
                       |> Comment.changeset(%{kind: "annotation", author_uuid: attrs.creator_uuid})
                       |> Repo.insert()

      {:ok, annotation} = %Annotation{}
                          |> Annotation.changeset(Map.put(attrs, :comment_uuid, comment.uuid))
                          |> Repo.insert()

      annotation
    end)
  end

  def list_for(target_type, target_uuid), do: ...
  def update(uuid, attrs), do: ...
  def delete(uuid), do: ...
end

Then in your LiveView:

def handle_event("etcher:created", attrs, socket) do
  {:ok, annotation} = MyApp.AnnotationStorage.create(attrs)
  # ...
end

Etcher's component doesn't run any persistence itself — it fires events and trusts the consumer. The bundled Etcher.create_annotation/1 is just a shortcut for Etcher.Storage.Default.create/1.

Customizing the tooltip

Hovering or clicking an annotation pops up a small tooltip with a trash button (for persisted shapes) and three content slots: header, footer, and body. The defaults read a few generic metadata keys and degrade to just the shape kind if those are absent, but a consumer can replace any slot with its own rendering by setting window.Etcher.tooltipSlots:

window.Etcher.tooltipSlots = {
  header: (shape) => Etcher.escapeHtml(shape.metadata.author || shape.kind),
  footer: (shape) => shape.metadata.last_edited || null,
  body:   (shape) => `<p>${Etcher.escapeHtml(shape.metadata.note || "")}</p>`
};

• Slots are functions (shape) => string | null.
• Returning null or undefined falls back to Etcher's default for that slot. An empty return for body / footer omits the row entirely.
• The whole shape object is passed ({uuid, kind, geometry, style, metadata, …}) so consumers can build whatever HTML their data supports.
• Etcher controls the wrapper, positioning, hover bridge, click-to-pin, and the trash button — slots only own content. This keeps delete + pin behavior consistent across consumers.
• window.Etcher.escapeHtml(value) is exposed as a stable escape helper.

Default slot keys

If you don't register custom slots but want a meaningful tooltip, populate these on each annotation's metadata (server-side, in initial_annotations):

Styling primitives

Etcher's stylesheet ships a handful of opt-in classes consumers can use inside their slot HTML for a layout consistent with the default look:

• .etcher-tooltip-body — flex row, thumbnail on the left, text column on the right (gap: 8px, max-width: 260px)
• .etcher-tooltip-thumb — 40×40 rounded box for an <img> or icon span
• .etcher-tooltip-thumb-icon — modifier that centers an SVG icon inside the thumb box (paperclip-style fallback)
• .etcher-tooltip-text — flex column container for the right-hand text
• .etcher-tooltip-quote — italic, two-line clamp for a quoted text preview

These are entirely optional. A slot that just returns <p>plain text</p> lays out fine without any of them.

Lifecycle events

Slot APIs cover content. For interaction wiring the existing LiveView events still fire:

• etcher:selected {uuid} on click (also pins the tooltip)
• etcher:deleted {uuid} when the user hits the trash button

etcher:tooltip-show / -hide / -pin events would be a natural follow-up if a consumer needs them; not in v0.1.

Hooks reference

All extension points beyond the LiveView events listed above. None are required — Etcher works with zero configuration.

window.Etcher.colorSwatches — palette override

Replace the bundled pastel rainbow + monochrome bookends with your own swatches:

window.Etcher.colorSwatches = [
  { key: "brand",   color: "#ff6f00", title: "Brand orange" },
  { key: "muted",   color: "#9ca3af", title: "Muted gray" },
  { key: "ink",     color: "#0f172a", title: "Ink" }
];

Falls back to the default palette if unset or not an array.

window.Etcher.defaultColor — initial active color

Override which swatch starts pre-selected when annotation mode opens:

window.Etcher.defaultColor = "#ff6f00";

Falls back to the "blue" swatch in the active palette (back-compat) or the first swatch.

window.Etcher.layerFor(frescoId) — programmatic control

Returns the layer's control surface, or null if no layer is mounted for that fresco id. Lets you drive Etcher from outside (URL handlers, keyboard shortcuts, command palettes):

const layer = window.Etcher.layerFor("photo");
if (layer) {
  layer.setMode(true);           // enter annotation mode (toolbar opens)
  layer.exitDrawing();           // back to cursor (annotation mode stays on)
  layer.selectShape("uuid-…");   // pin the tooltip for that shape
  const shapes = layer.getShapes();
  // → [{ uuid, kind, geometry, style, metadata }, ...]
}

Lifecycle DOM events

Etcher dispatches bubbling CustomEvents on the layer's host element so consumer JS can react without reaching into the hook. Listen on the host or any ancestor:

document.addEventListener("etcher:tooltip-show", (e) => {
  console.log("Tooltip showing for", e.detail.uuid, "at", e.detail.anchor);
});

Server → client LiveView events

In addition to the create / update / delete / selected client→server events documented above, the server can push state into a running viewer via Phoenix.LiveView.push_event/3:

window.Etcher.escapeHtml(value) — escape helper

Stable helper exposed for use inside consumer slot functions. HTML-escapes &, <, >, ", '.

How it fits with Fresco

Etcher uses Fresco 0.2's handle.appendNavButton/3 extension point to add the pencil button — no other extension surface required. Drawing input is delivered as plain pointerdown / pointermove / pointerup events on an SVG overlay anchored to Fresco's image coordinate space, so shapes stay locked to image pixels through pan and zoom.

Out of scope (for now)

• Editing existing shapes after commit (drag handles, vertex move). v0.1 is draw-and-commit; to change a shape, delete and redraw.
• Touch + pinch gesture coexistence with Fresco's pan/zoom — annotation mode currently disables Fresco's drag-to-pan; refinement comes later.
• Custom tools beyond the four built-ins. The geometry kind is a string, so adding a new kind is straightforward; the toolbar wiring isn't pluggable yet.
• Annotation export / import in W3C Web Annotation Data Model JSON-LD.

License

MIT. See LICENSE.