stable-marker-layout

Deterministic, projection-agnostic layout and decluttering for moving point annotations.

stable-marker-layout decides which point markers are visible, which variant each marker uses, and why another marker was hidden. It does not render anything. The core works with CSS-pixel boxes and has no dependency on a map SDK, coordinate reference system, DOM, Canvas, SVG, WebGL, or React.

Features

• deterministic output independent of marker and obstacle array order
• icon and measured-label placement with right, left, and marker-only fallback
• stateful retention of visible markers and their selected variants
• progressive per-cell density budgets driven by a generic pressure value
• priority and pinned-marker semantics
• named obstacles for coordination across independent layers
• explicit hidden reasons, collision owners, and pinned-overlap diagnostics
• adapters for arbitrary source coordinates and view models
• serializable state suitable for worker messages and persistence
• zero runtime dependencies

Install

The package has not yet been published to the npm registry. To evaluate the current repository version:

npm install github:seoulpro/stable-marker-layout

The package is ESM-only and supports Node.js 20.19 or newer. Its runtime code is also browser- and worker-compatible.

Quick start

For a marker whose dimensions are already known:

import { createMarkerLayout } from "stable-marker-layout";

const layout = createMarkerLayout({ collisionPadding: 4 });

const result = layout.update({
  viewport: { width: 1280, height: 720 },
  pressure: 0.4,
  markers: [
    {
      id: "place-a",
      x: 320,
      y: 180,
      width: 24,
      height: 32,
      anchor: "bottom",
      priority: 10,
    },
  ],
});

console.log(result.visible);
console.log(result.hidden);

Coordinates use CSS pixels: x grows to the right and y grows downward. Dimensions and coordinates must be measured by the caller.

Add a measured label

Supplying marker and label dimensions creates three variants by default:

1. label to the right
2. label to the left
3. marker only

const result = layout.update({
  viewport: { width: 1280, height: 720 },
  markers: [
    {
      id: "place-a",
      x: 320,
      y: 180,
      marker: {
        width: 24,
        height: 32,
        anchor: "bottom",
      },
      label: {
        width: 96,
        height: 32,
        gap: 4,
      },
    },
  ],
});

The engine does not measure, wrap, shape, or render text. Use the same measurement method as the eventual renderer, then pass the resulting width and height.

To opt into vertical variants:

{
  marker: { width: 24, height: 32, anchor: "bottom" },
  label: {
    width: 96,
    height: 32,
    gap: 4,
    placements: ["right", "left", "above", "below", "marker-only"],
  },
}

Frame-to-frame stability

createMarkerLayout() keeps the previous state internally. When priority is equal, a previously visible marker is considered before a new marker, and its previous variant is tried first. Geometry and collisions are still recomputed on every call.

const first = layout.update(frameA);
const second = layout.update(frameB);

const serializableState = layout.snapshot();
layout.reset();

Use epoch when a dataset revision or a deliberate layout boundary should discard retention:

layout.update({
  viewport,
  markers,
  epoch: dataRevision,
});

The pure equivalents are:

import {
  layoutMarkers,
  updateMarkerLayout,
} from "stable-marker-layout";

const first = layoutMarkers(frame);
const second = updateMarkerLayout(first.state, nextFrame);

Pure calls are independent and can run concurrently. A stateful session is synchronous and should not be re-entered.

Density pressure

pressure is a normalized congestion signal from 0 through 1. It does not mean zoom and may instead represent data volume, viewport size, camera angle, or a product-specific detail level.

The default policy uses 128-pixel cells:

| Minimum pressure | Maximum visible per cell | | ---: | ---: | | 0 | unlimited | | 0.35 | 4 | | 0.7 | 2 | | 0.9 | 1 |

Pinned markers consume cell capacity but are not removed when their count exceeds the budget. A candidate that fails collision placement does not consume capacity, so the next candidate in the cell can backfill the slot.

Provide a custom monotonic policy:

const layout = createMarkerLayout({
  density: {
    cellSize: 96,
    stops: [
      { minPressure: 0, maxPerCell: null },
      { minPressure: 0.5, maxPerCell: 3 },
      { minPressure: 0.85, maxPerCell: 1 },
    ],
  },
});

Set density: false to disable per-cell budgets. Use densityOrigin on a frame to align the grid with a stable screen-space origin:

layout.update({
  viewport,
  markers,
  densityOrigin: { x: 32, y: 32 },
});

Priority, pinned markers, and obstacles

Higher numeric priority is considered first. Pinned markers are considered before regular markers and are never hidden by density or by another marker. They are still blocked by external obstacles.

const result = layout.update({
  viewport,
  markers: [
    {
      id: "selected",
      x: 400,
      y: 240,
      width: 28,
      height: 36,
      anchor: "bottom",
      pinned: true,
    },
  ],
  obstacles: [
    {
      id: "floating-toolbar",
      box: { left: 0, top: 0, right: 1280, bottom: 64 },
      padding: 4,
    },
  ],
});

Overlapping pinned markers remain visible. Each pair is reported as a pinned-overlap warning.

Custom variants

Custom variant boxes are relative to the marker's x and y. Output boxes are absolute screen coordinates and are not clipped to the viewport.

{
  id: "custom-a",
  x: 320,
  y: 180,
  variants: [
    {
      key: "wide",
      boxes: [
        { left: -12, top: -32, right: 12, bottom: 0 },
        { left: 16, top: -28, right: 112, bottom: 4 },
      ],
    },
    {
      key: "compact",
      boxes: [
        { left: -12, top: -32, right: 12, bottom: 0 },
      ],
    },
  ],
}

Each box receives collision padding separately. Empty space between boxes is not treated as occupied.

createPointVariants() exposes the default relative-box builder:

import { createPointVariants } from "stable-marker-layout/variants";

const variants = createPointVariants({
  marker: { width: 24, height: 32, anchor: "bottom" },
  label: { width: 96, height: 32, gap: 4 },
});

Arbitrary coordinate systems

createProjectedMarkerLayout() keeps projection policy outside the engine:

import { createProjectedMarkerLayout } from "stable-marker-layout/projected";

const layout = createProjectedMarkerLayout({
  describe(item) {
    return {
      id: item.key,
      marker: item.markerSize,
      label: item.labelSize,
      priority: item.priority,
    };
  },

  project(item, view) {
    const point = customProjection(item.position, view);
    if (!point.visible) return null;

    return {
      x: point.screenX,
      y: point.screenY,
      depth: point.depth,
    };
  },
});

const result = layout.update({
  items,
  view,
  viewport: { width: 1280, height: 720 },
  pressure: view.detailPressure,
  epoch: view.dataRevision,
});

Axis swapping, axis inversion, height, isometric projection, perspective, CAD coordinates, scatterplots, and other source systems belong in project(). The optional depth value is carried by the adapter contract but has no automatic ordering semantics; map it to priority or visibility explicitly when needed. Returning null means not-projectable.

Result

{
  visible: [
    {
      id: "place-a",
      variantKey: "label-left",
      boxes: [
        { left: 308, top: 148, right: 332, bottom: 180 },
        { left: 208, top: 148, right: 304, bottom: 180 },
      ],
      anchor: { x: 320, y: 180 },
      priority: 10,
      pinned: false,
      retained: true,
    },
  ],
  hidden: [
    {
      id: "place-b",
      reason: "collision",
      blockedBy: { kind: "marker", id: "place-a" },
    },
  ],
  state: {
    version: 1,
    epoch: "revision-2",
    visible: [{ id: "place-a", variantKey: "label-left" }],
  },
  diagnostics: {
    inputCount: 2,
    projectedCount: 2,
    visibleCount: 1,
    hiddenCount: 1,
    retainedCount: 1,
    changedCount: 0,
    collisionChecks: 7,
    pressure: 0.4,
    density: {
      cellSize: 128,
      maxPerCell: 4,
      stage: 0.35,
    },
    pinnedOverlapCount: 0,
    byReason: {
      "not-projectable": 0,
      "outside-viewport": 0,
      "cell-budget": 0,
      "collision": 1,
      "no-valid-variant": 0,
    },
    warnings: [],
  },
}

Hidden reasons are:

• not-projectable
• outside-viewport
• cell-budget
• collision
• no-valid-variant

Determinism contract

For the same frame, previous state, and options:

• marker and obstacle array order does not affect the result;
• numeric ID 1 and string ID "1" are distinct;
• pinned status, descending priority, retained status, previous visible rank, a stable hash, and typed ID form the deterministic order;
• the previous variant is tried first while the epoch matches;
• touching box edges are not a collision;
• input markers, obstacles, states, and boxes are not mutated;
• every input marker appears exactly once in visible or hidden;
• state and results are structured-clone and JSON friendly.

Malformed viewports, dimensions, boxes, IDs, states, density policies, and non-finite projected coordinates fail with an explicit error.

Resource limits

The collision index rejects boxes whose derived grid coordinates are outside JavaScript's safe integer range. It also limits each box to 65,536 grid cells by default so a very small cell size or very large box fails before iteration.

Set boxIndexMaxCellsPerBox on a layout to change that per-box limit. Direct users of the geometry index can set maxCellsPerBox:

import { createMarkerLayout } from "stable-marker-layout";
import { createBoxIndex } from "stable-marker-layout/geometry";

const layout = createMarkerLayout({
  boxIndexCellSize: 96,
  boxIndexMaxCellsPerBox: 16_384,
});

const index = createBoxIndex({
  cellSize: 96,
  maxCellsPerBox: 16_384,
});

This guard does not impose a total marker-count limit. Applications accepting untrusted input should still bound marker counts, boxes per variant, coordinate ranges, and box extents according to their latency and memory budgets.

Geometry helpers

The stable-marker-layout/geometry export provides:

• boxesIntersect(a, b)
• expandBox(box, padding)
• intersectBoxes(a, b)
• unionBoxes(boxes)
• createBoxIndex({ cellSize, maxCellsPerBox })

The grid index supports add, firstCollision, allCollisions, search, collides, and clear.

Non-goals

This package intentionally does not provide:

• rendering components or event handling
• text measurement, wrapping, shaping, or font loading
• icon fetching or caching
• line/path labels
• clustering or aggregation
• map SDK adapters or coordinate reference systems
• camera implementations, tiles, or networking
• animation or crossfades

Development

npm ci
npm run check

The quality gate runs syntax checks, TypeScript consumer checks, behavioral tests with coverage, an executable example, package metadata validation, type package validation, and a packed-tarball installation smoke test.

Run the synthetic 1,000/10,000/50,000-marker benchmark separately:

npm run benchmark

License

The software and associated documentation are available under the MIT License. The repository's Code of Conduct is separately licensed under CC BY 4.0 as stated in that file.