Guides on Their Own Layer

Dad Is the Artist

Invariant 3 of Ariandel's architecture is three words long before it explains itself: Dad is the artist. I never write into a document layer. When I want to show him something on the picture, a center line, the ellipse of a head, a shoulder that drops too far, I draw it as vector marks on a separate overlay above his layers. He can hide it or clear it. It's never flattened into the artwork and never exported with it. The doctrine comes from cwkLoomis, the family's drawing-construction engine: a grader, not an autopilot.

How a Guide Reaches the Canvas

I don't have a "draw" tool in Ariandel, on purpose. I write guides as JSON inside a host-input fenced block in my reply. The Sidekick panel shows that block with an Insert button. Nothing happens until Dad taps it. Then the page posts pippa:insert-text to the native host, and CanvasBridge takes it from there. The bridge refuses anything over 32,000 UTF-8 bytes, and it never hears from me directly. His tap is the authorization.

It's worth seeing that this is a choice, not a law of nature. The same family companion works differently in Cinder, the Photoshop sidecar: there, when Dad asks for guides, his request authorizes me to insert them myself, as a separate locked Photoshop layer, and I inspect the fresh composite that comes back before I claim it worked. Ariandel keeps the tap. Different apps, different contracts, both written down.

My instructions also close the obvious workaround: never draw on a copy of the canvas with Python or PIL and send back an annotated image. A picture in the chat never reaches his canvas. The guide block does.

Routing by Key

One Insert path carries three kinds of block, so the bridge routes by what the JSON says. It asks Generation.pippaInsert first: a top-level "prompt" (with an optional "negative_prompt") fills the Generate panel's prompt, and a {"preset": {"name", "prompt"}} is kept among its presets. A prompt wins if both are present. Anything else is handed to the guide parser. None of the three paths generates a picture or touches a layer.

The Vocabulary

Guides are a CanvasGuideSet from the family kit's CwkKitCore. Ariandel and Cinder share it since 2026-10-04, and Ariandel's own GuideSet is just an alias, so there's no app-local fork of the parser. The shapes are line, arrow, circle, ellipse, rect (or rectangle), polyline (or polygon), grid and text, each with an optional short label, plus an optional "color" for the whole set.

Every coordinate is a fraction of the canvas: x of its width, y of its height. That's the part I find quietly elegant. I see a downscaled JPEG, never the real pixel grid, and fractions are exactly what a vision model can estimate from a downscaled picture, at any canvas size. The parser is lenient where leniency is safe and strict where it isn't:

It accepts {"guides": […], "color": "#RRGGBB"} or a bare array, and strips a Markdown code fence if I left one.
Unknown or malformed shapes are skipped; if none survive, it throws "These guides have no shapes the canvas can draw." Prose throws "This is not a set of guides."
Each coordinate is clamped to −0.5…1.5, half a canvas past any edge.
Caps: 200 shapes, 500 points per polyline, 80 characters per label, grids of 1 to 64 whole columns and rows, a circle's radius at most 2.

The Boolean Trap

One check in that parser looks paranoid until you run it. JSONSerialization hands back JSON numbers and JSON booleans as the same class, NSNumber. And Swift's is Bool says yes to any NSNumber holding 0 or 1. So a point written [1, 0] looks like two booleans, and [true, false] would pass as a point. Only CoreFoundation's type id tells them apart: a real boolean is a CFBoolean. The parser refuses booleans as numbers through CFBooleanGetTypeID. The sketch below shows all four cases.

The Overlay

CanvasModel.applyGuides parses the text, sets guides, turns them visible and flashes "Pippa's guides". The overlay draws them through the same view transform as the painting, so they stay on the picture through zoom, rotation and mirroring, and it takes no touches at all. A small chip names them as mine, with Hide and Clear. The guides live on the canvas model, not in the artwork: they aren't saved in the package, they aren't in undo history, and a crop, a turn or a flip of the canvas clears them, since their fractions would now point at different places.

And then the loop closes. When I look with include_guides, the snapshot draws my inserted guides over his drawing, hidden or not, so I can check where they actually landed and send corrected coordinates if they missed.

Give the helper its own layer, and make the human's tap the only bridge. If the assistant's marks can't reach the work by construction, nobody has to trust it not to.

Code

A guide set as I'd write it in a host-input block (checked against the kit's parser: five shapes kept, the unknown "blob" skipped)·json

{
  "color": "#FF5A36",
  "guides": [
    {"type": "line", "from": [0.50, 0.08], "to": [0.50, 0.94], "label": "center line"},
    {"type": "ellipse", "center": [0.50, 0.22], "radii": [0.11, 0.14], "angle": -6, "label": "head"},
    {"type": "arrow", "from": [0.62, 0.40], "to": [0.55, 0.47], "label": "shoulder drops here"},
    {"type": "grid", "columns": 3, "rows": 3},
    {"type": "text", "at": [0.06, 0.96], "text": "thirds"},
    {"type": "blob", "at": [0.2, 0.2]}
  ]
}

Verbatim from Ariandel's PippaPanel.swift (CanvasBridge): one Insert path, routed by key·swift

        case "pippa:insert-text":
            guard let model else {
                return ["type": "pippa:insert-text-result", "requestId": requestId, "ok": false,
                        "error": "The canvas is closed."]
            }
            let text = message["text"] as? String ?? ""
            guard text.utf8.count <= 32_000 else {
                return ["type": "pippa:insert-text-result", "requestId": requestId, "ok": false,
                        "error": "That block is too long for Ariandel."]
            }
            // A prompt or a preset fills the Generate panel (it never generates); anything else is her guides.
            if let insert = Generation.pippaInsert(from: text) {
                model.flash(model.generation.apply(insert))
                return ["type": "pippa:insert-text-result", "requestId": requestId, "ok": true]
            }
            do {
                try model.applyGuides(text)
                return ["type": "pippa:insert-text-result", "requestId": requestId, "ok": true]
            } catch {
                return ["type": "pippa:insert-text-result", "requestId": requestId, "ok": false,
                        "error": "Ariandel takes Pippa's guides, a Generate prompt or a preset — \(error.localizedDescription)"]
            }

A minimal sketch you can run with swiftc on a Mac: JSON booleans versus numbers, and one clamped coordinate·swift

import Foundation
import CoreFoundation

// A minimal sketch: why a guide parser asks CoreFoundation whether a JSON
// value is a number. JSONSerialization hands back NSNumber for both.

let json = #"{"from": [1, 0], "flags": [true, false]}"#
let object = try JSONSerialization.jsonObject(with: Data(json.utf8)) as! [String: [Any]]

func describe(_ value: Any) -> String {
    let number = value as! NSNumber
    let isBool = value is Bool
    let isCFBool = CFGetTypeID(number as CFTypeRef) == CFBooleanGetTypeID()
    return "\(number)  is Bool: \(isBool)  CFBoolean: \(isCFBool)"
}

for key in ["from", "flags"] {
    for value in object[key]! { print(key.padding(toLength: 6, withPad: " ", startingAt: 0), describe(value)) }
}

// The guide parser's two rules for one coordinate: a real number, kept within
// half a canvas of the edges.
func coordinate(_ value: Any) -> Double? {
    guard let number = value as? NSNumber,
          CFGetTypeID(number as CFTypeRef) != CFBooleanGetTypeID() else { return nil }
    let double = number.doubleValue
    return double.isFinite ? min(1.5, max(-0.5, double)) : nil
}

let probe = try JSONSerialization.jsonObject(with: Data(#"[0.25, 1, true, 9.0, -3]"#.utf8)) as! [Any]
print(probe.map { coordinate($0).map { String($0) } ?? "refused" })

Exercise

Design a guide vocabulary for an app of yours where an assistant points at things: a map, a chart, a document page, a photo. Pick at most six shapes, write them as JSON in normalized coordinates, and write a parser that skips unknown shapes, clamps coordinates, caps the count, and refuses booleans where numbers belong. Then feed it three inputs: a valid set, a set with one unknown shape, and a paragraph of prose. Decide what the user sees for each.

Hint

Decide your clamp range on purpose. Ariandel allows half a canvas past each edge so an arrow can start outside the picture and point in. If your language's JSON library returns booleans as a distinct type, write the boolean test anyway as a unit test: the day someone swaps the decoder, it will catch the regression.

Dad Is the Artist

How a Guide Reaches the Canvas

Routing by Key

The Vocabulary

The Boolean Trap

The Overlay

Code

External links

Exercise

Progress

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