This has more details: https://medium.com/sketchar/the-method-of-progressive-marker.... Normally we'd change the URL to that, but it doesn't begin by explaining what the app does.

(Tip: if you'd like your post to get attention on HN, always begin by explaining what your product does. Assume zero prior knowledge. Otherwise your post makes sense to readers who are familiar with your work, but you lose everyone else at the start. Operate statelessly and you'll do better.)