Seamark

The page shows a chart. You need the numbers. Seamark reads them off the drawing.

A seamark is a fixed thing you can see from the water and steer by. This library works the same way. It looks at what a page draws, not what it stores, and tells you the values behind it. For when the data never shows up as text: SVG charts, third-party embeds, visual checks, and browser agents that have to act on what they see.

What it does

• Keeps coordinates straight. A browser graphic is measured in four rulers at once: SVG units, page pixels, the frame it sits in, and your screenshot. Mix two and you get a plausible wrong number. Every conversion here is explicit.
• Reads curves. Sample a drawn path, find where it crosses a level, fit a circle from a partial arc, and tell linear from quadratic from exponential.
• Compares formulas. x(x+8) and x²+8x look nothing alike and agree at exactly the point you would test first, so it compares them at several.
• Handles widgets. Picks the iframe you can actually see, pins labels to the shapes they sit on, plans the drags a dot plot needs, and clicks in a way frameworks accept.

Use

import { samplePath, crossings, fitCircle, family, makeDataSpace } from 'seamark';

const space = makeDataSpace(originInPagePx, oneUnitInPagePx);
const points = samplePath(document.querySelector('svg path'), space.toData);

crossings(points);   // where the curve meets y = 0
fitCircle(points);   // { cx, cy, r }
family(points);      // 'linear' | 'quadratic' | 'exponential' | null

npm test         # 16 tests
npm run build    # bundles src/ into public/seamark.js for the demo

Apps

The same demo everywhere, in English, Spanish, French, German, Japanese and Chinese. Each app carries its own port of the engine, with tests beside it. Nothing depends on a web view.

• Web: https://seamark.heyitsmejosh.com. Draw a curve and watch it get measured.
• iPhone, iPad and Mac: SwiftUI app in ios/. Engine.swift is the Swift port.
• Android, Windows and Linux: Compose Multiplatform app in kmp/. Engine.kt is the Kotlin port. Desktop packages as MSI, DEB or DMG.

cd ios && xcodegen generate                       # then open Seamark.xcodeproj
swiftc -o /tmp/smcheck ios/App/Engine.swift ios/Checks/main.swift && /tmp/smcheck

cd kmp && export JAVA_HOME=$(brew --prefix openjdk@17)/libexec/openjdk.jdk/Contents/Home  # D8 rejects newer JDKs
./gradlew :shared:jvmTest                         # Kotlin engine tests
./gradlew :composeApp:assembleDebug               # Android APK
./gradlew :composeApp:packageDistributionForCurrentOS

MIT 2026 Joshua Trommel

Whitepaper

Technical whitepaper