Client-side rendering engine

[pnorman]

The four maintained browser-based client-side renderers are

• Leaflet.VectorGrid
• OpenLayers3 VectorTile
• Tangram
• Mapbox GL JS

VectorGrid and VectorTile both are styled by writing Javascript. There is work to allow VectorGrid to read Mapbox GL JSON, but it's not yet done. Javascript is not a style language, and we want one when dealing with something as complex as a basemap.

This leaves Tangram from Mapzen and Mapbox GL JS. They are similar in many ways:

• Both are essentially single-company projects with a low bus factor
• Both use WebGL and inherit a lot of their data models from it
• both read Mapbox Vector Tiles
• both have native versions for mobile apps which can also be run on the server to generate images

The big differences I've found are the styling languages, company practices, and multi-lingual names.

Styling languages

Mapbox GL JS is styled with Mapbox GL JSON, a well documented language. Because there aren't typically linebreaks in it, the merge conflicts of SQL in JSON won't occur here but it has no facility for comments. This means we'd need to do something like a yaml2json step that osm-carto used to have.

Mapbox GL JSON has no inheritance, unlike Carto*CSS. This makes writing styles very verbose. It is also designed to primarily be edited with a closed-source SaaS editor, and not by hand. This is a problem for a complex multi-contributor style.

Tangram is styled with Tangram scene files, which are YAML. The documentation is reasonable, although the language is more rapidly evolving than Mapbox GL JSON. There is inheritance - or at least it doesn't feel like I have to duplicate everything many times like with Mapbox GL JSON.

Having written both, I find Tangram scene files a lot more fun to write. We know from the old Mapnik XML stylesheets that having a language that people like is a huge part of attracting contributors.

Non-latin language support

Neither Mapbox GL JS nor Tangram fully support placing a label of any language on a curve, but for different reasons. Mapbox GL uses a custom glyph placement engine, and only supports some languages. Other languages cannot be labelled at all. Tangram uses the browser's placement, but doesn't handle curving the labels for all languages. This is under work (tangrams/tangram#555) and most common languages are supported.

I see Tangram supporting all used languages before Mapbox GL JS.

Other factors are

• Mapbox GL JSON has more people using it, although probably not writing it directly
• Tangram is still evolving rapidly
• Mapbox's release management practices for rendering software have been difficult to work with in the past
• Mapbox GL is a bigger name

[pnorman]

I am strongly inclined towards Tangram, primarily because the styling language is better suited to multiple-editor projects with code review.

[pnorman]

Going with tangram, mainly due to the language advantage, and the many closed-source parts needed to have a reasonable editing experience with Mapbox GL JSON.