Vellum

How it works

Vellum comes in two pieces. Vellum Bridge exports, Vellum explores.

In the game: Vellum Bridge	On your desktop: Vellum
A Cities: Skylines mod that writes your city to a .vellummap file when you ask it to.	A map viewer for Windows, macOS and Linux that opens that file.
Acts only on your keypress and never touches the network.	Runs locally. No account, no upload, no server between you and it.

1. Subscribe to Vellum Bridge on the Steam Workshop and enable it in the game's Content Manager.
2. Load your city and press Ctrl+Shift+E. Bridge writes a .vellummap to Vellum Bridge/<your city>/, inside Documents on Windows and your home folder on macOS and Linux.
3. Drop that file onto Vellum, or open it with Ctrl/Cmd+O.

A .vellummap carries what older formats cannot: street names, district population and jobs, renamed and unique buildings, and stations that group every stop they serve.

[!TIP] Already have exports from CSL Map View? Vellum still opens .cslmap files and keeps its three classic styles, so your existing maps keep working.

Install

Platform	Where	Notes
Windows	Microsoft Store	Recommended. The Store keeps Vellum up to date.
Windows	GitHub Releases	Standalone .exe installer, or .msi for managed setups.
macOS	GitHub Releases	.dmg. Not notarized by Apple: see the notes below.
Linux	GitHub Releases	.AppImage, plus .deb and .rpm packages.

• Windows (standalone): the .exe and .msi installers are not Authenticode-signed and may show an unknown-publisher warning; check each release's signing-evidence.md. The .msi offers an opt-in .cslmap file association. The Store package is signed by Microsoft.
• macOS: open the .dmg and move Vellum to Applications. Vellum is not signed or notarized, so if Gatekeeper blocks it, clear quarantine once with xattr -cr /Applications/Vellum.app.
• Linux: make the .AppImage executable and run it.

What each platform gets and what its signing state means: Packaging and installers · Empaquetado e instaladores.

What you can do

Read the transit network	…or as a schematic diagram
Inspect districts and streets	Export a print-ready plate

• Explore seven independent layers (terrain, basemap and water, roads, transit, buildings, forests and districts) with Shift+1–7, down to street names and real road widths.
• Read the transit as a geographic map or as an octilinear or orthoradial schematic, with stations collapsed into single nodes.
• Inspect a district, park or building in a side card: population, homes, jobs and specializations.
• Make it yours with five built-in themes, or load your own .vellumstyle. Press H for clean mode.
• Export the current view as PNG (1×–4×) or editable SVG.
• Stay oriented with the minimap, precise zoom and a shortcuts sheet (?), in English or Spanish.

[!NOTE] Your city stays yours. Vellum works offline on your machine and needs no account. The manifesto explains why that matters.

Where it's heading

Vellum 1.0 is the map I wanted to build. Some ideas being explored for later, with no dates attached:

• a timelapse of how a city grew, built from successive exports;
• 3D terrain and building heights;
• a nautical chart style for the sea around your city.

Have a city that breaks Vellum, or an idea that would help you read yours? Open an issue.

Contributing

Vellum is open source under the MIT license, and contributions of every size are welcome: rendering quality, parser resilience, themes, documentation, or a well-described issue. Please read CONTRIBUTING.md before opening a pull request. For security issues, use SECURITY.md; for community expectations, see CODE_OF_CONDUCT.md.

Try it from source

You do not need Cities: Skylines or your own save to try the development build. The repository includes real city fixtures.

git clone https://github.com/sebas-tcotd/vellum.git
cd vellum
pnpm install
pnpm dev

Drop altavento.cslmap or aurelia-del-delta.cslmap onto the app window.

Tool	Version
Node.js	20
pnpm	10.33.0
Rust	1.96.0 from rust-toolchain.toml
Tauri CLI	2.x

Before pnpm install or pnpm dev, install the Tauri 2 prerequisites for your operating system.

pnpm dev
pnpm build
pnpm lint
pnpm check:architecture
pnpm test
pnpm test:e2e
pnpm format:check
pnpm rust:fmt
pnpm rust:lint
pnpm rust:test
pnpm release:verify

Run a focused package test with:

pnpm --filter @vellum/renderer-webgl test
pnpm --filter @vellum/ui test -- MapLibreRoot.test.tsx

Path	Purpose
apps/desktop	Tauri shell, native commands and composition root
packages/core	Domain types and IPC contract
packages/parser-cslmap	.cslmap and .vellummap readers
packages/renderer-webgl	Active MapLibre renderer
packages/theme-engine	Theme loading, validation and style parameters
packages/ui	React components and interaction layer
tools/vellum-bridge	Cities: Skylines mod that exports .vellummap
apps/landing	Project website on GitHub Pages
docs	Technical documentation and design references
docs/adr	Accepted architecture decision records

The engineering story

Vellum began with a Canvas 2D renderer. As maps grew, CPU rendering hit a ceiling, and a focused spike proved that MapLibre GL JS and WebGL could deliver the experience. The pivot was possible because the domain model was separated from rendering from the start; the Canvas renderer was retired in ADR-0001. The package graph is one-directional, and pnpm check:architecture enforces it.

flowchart LR
  G["Cities: Skylines + Vellum Bridge"] --> V[".vellummap"]
  V --> B["Rust parser"]
  A[".cslmap export"] --> B
  B --> C["Immutable CityData"]
  C --> D["MapLibre renderer"]
  D --> E["React UI in Tauri"]
  T["Theme engine"] --> D

graph TD
  desktop["apps/desktop<br/>(composition root)"] --> ui
  desktop --> parser
  desktop --> renderer
  desktop --> themes
  desktop --> core
  ui --> renderer
  ui --> themes
  ui --> core
  parser --> core
  renderer --> core
  themes --> core

@vellum/core is the dependency-free domain and IPC layer. apps/desktop is the only composition root. The active MapLibreRoot still uses MapLibre-specific APIs at the UI boundary, so renderer interchangeability is a deliberate architectural direction rather than a claim that every edge is already abstracted.

.vellummap is a versioned, schema-validated city document that does not depend on the program that wrote it. Its contract lives in docs/es/vellummap-format.md.

License

Vellum is released under the MIT License.

Acknowledgements

Built for the Cities: Skylines community, compatible with the .cslmap format from CSL Map View, and powered by Tauri, Rust, React, MapLibre GL JS and Turborepo.

Cities shown in this README: Costa Tijuca by MatBarbosa and Pepper Lake by takkerue, from the Steam Workshop.

Created and maintained by Sebastian Vargas, with an emphasis on understanding before building, coherent systems and reducing friction without hiding complexity.