Mapbox GL style converter skips step expressions

[1ec5]

What is the bug or the crash?

When opening an MVT layer with a Mapbox or MapLibre stylesheet, QGIS attempts to convert the stylesheet to QGIS symbology, but this converter lacks support for the very common step expression operator, causing significant glitches in the resulting layer.

Steps to reproduce the issue

1. Follow these instructions to create a vector tile layer that has many symbols and labels based on an OpenHistoricalMap style.
2. Note the banner warning that “Style could not be completely converted”. Click the Details button.

Many of the layers include properties or filters set to step expressions, which result in the following warnings:

• roads_motorway: Skipping unsupported expression
• roads_motorway_construction: Skipping unsupported expression
• roads_motorwaybridgetop: Skipping unsupported expression
• roads_motorwaybridgetop_construction: Skipping unsupported expression

Versions

(Yes, I know v3.36 is a couple releases behind, but it’s the most recent version currently available for macOS.)

QGIS version	3.36.3-Maidenhead	QGIS code revision	2df9655
Qt version	5.15.2
Python version	3.9.5
GDAL/OGR version	3.3.2
PROJ version	8.1.1
EPSG Registry database version	v10.028 (2021-07-07)
GEOS version	3.9.1-CAPI-1.14.2
SQLite version	3.35.2
PDAL version	2.3.0
PostgreSQL client version	unknown
SpatiaLite version	5.0.1
QWT version	6.1.6
QScintilla2 version	2.11.5
OS version	macOS 14.5
Active Python plugins
QuickOSM	1.16.0
nominatim_locator_filter	0.2.3
processing	2.12.99
grassprovider	2.12.99
db_manager	0.1.20
MetaSearch	0.3.6

Supported QGIS version

• I'm running a supported QGIS version according to the roadmap.

New profile

• I tried with a new QGIS profile

Additional context

The warnings come from here:

QGIS/src/core/vectortile/qgsmapboxglstyleconverter.cpp

Line 3110 in 5e53e18

context.pushWarning( QObject::tr( "%1: Skipping unsupported expression" ).arg( context.layerId() ) );

The converter has some support for the interpolate operator, so step should be handled similarly.

[pathmapper]

Here is a style which isn't working with master:

https://krzn.ldproxy.net/lika/styles/vw-tn-flst-geb?f=mbs

[pathmapper]

Here a simplified version vw-tn-flst-geb.json:

"text-field" : [ "step", [ "zoom" ], "default", 16, "test" ]