stadiamaps / cartogrify Goto Github PK

This may be a core Tangram issue, but currently generated styles seem to have huge areas where labels are initially rendered but then disappear, presumably due to Tangram's label placement or collision constraints. Additionally, the precedence/priority does not seem to be quite right. This is probably an implementation error on my part.

Here is the source GL JSON style that exhibits the issue: https://tiles.stadiamaps.com/styles/alidade_smooth_dark.json (you can run a server on localhost to test; see https://docs.stadiamaps.com/ for auth info). Below is an example of such a "hole" at a particular zoom level in Germany.

Currently only the most basic decision expressions are supported. The remaining ones appear to be representable using Tangram functions.

• case
• coalesce
• match
• within

Mapbox has a simple format language for specifying a text label in terms of multiple fields (it's very much like a Python f-string). Currently we just ignore this unless the source is just a single field. We shouldn't ;)

Just FYI in case anyone comes across this, I built a similar thing in JavaScript a few years back: https://gitlab.com/stevage/mapbox2tangram

Currently, cartogrify does not generate a fonts section. However, with a bit of work, we could probably look up many fonts using the Google Fonts API.

First we need to investigate whether this is possible. If yes, we can implement it with the API key as a command line switch. A bit of parsing to separate font families and names out may also be necessary in the process.

Mapbox GL supports stops (for width, colors, etc.) that follow both a linear and exponential progression. Tangram does not appear to support this directly, but cartogrify could reasonably fake it by computing the intermediate stop values.

Current status:

• Support numeric interpolation (https://github.com/mapbox/mapbox-gl-js/blob/f0cc0159819617f59bbb5c3405a13a22f2f5c704/src/style-spec/util/interpolate.js#L5-L7)
• Support color interpolation (https://github.com/mapbox/mapbox-gl-js/blob/f0cc0159819617f59bbb5c3405a13a22f2f5c704/src/style-spec/util/interpolate.js#L9-L16}
• Support array interpolation (https://github.com/mapbox/mapbox-gl-js/blob/f0cc0159819617f59bbb5c3405a13a22f2f5c704/src/style-spec/util/interpolate.js#L18-L22)

Mapbox supports a few text effects directly that will likely require a bit more work to implement in Tangram.

MBGL allows for icons with a specific symbol placement etc. from a sprite definition file. Some further investigation is needed on my part, but I believe the following is a general rough outline of what needs to happen:

• Generate a sprites block from the JSON sprite file referenced in the MBGL JSON style
• Generate a points drawing layer for any MBGL symbol layers that have sprites
• Offset the text drawing layer as necessary
• Support complex icon-image expressions (not used in any Stadia Maps styles; can wait unless someone else wants this... see https://docs.mapbox.com/mapbox-gl-js/style-spec/types/#resolvedimage for specifics)