Different line-offset values for start and end of line segments

[dellisd]

Motivation

The main purpose of this feature would be to enable smooth transitions between line segments with different line-offset values. By using a separate line segment that could have different offset values for the start and end of the line, the offset of the separate line segment could be interpolated from the start and end values along the length of the line to join the line segments that have different line-offset values.

This would be very useful for mapping transit networks where different routes/lines overlap and diverge throughout the network.

Design

A line-offset-start and line-offset-end paint property could be provided. If used, it could just override the value being supplied through line-offset (if any).

Mock-Up

From a developer perspective, it would ideally look just like a normal line-offset

const paint = {
  "line-offset-start": 5,
  "line-offset-end": 0
}

Using the above example^, to an end user it would look like a smooth transition between two lines with different offsets. compared to now where there would have to be an abrupt jump.

Implementation

Polygons may be a special case since line-offsets behave as inset/outsets.

How the offset is interpolated may affect how this appears to end users.
For example:

[gar3434]

Yes please! I am trying to build a transit map in Mapbox. Having the ability to gradually shift the offset of a linestring along its length would be much better than my current abrupt jumps.

Thank you @dellisd and @ansis for opening this issue (and confirming that this functionality can't currently be done in Mapbox)

[gar3434]

I suggested this feature to Mapbox technical support and they responded with this workaround. I haven't tried it yet but below is their suggestion:

It is tricky with lines segmented like that. The lines would have to be physically set apart and optimized for a specific zoom level, which in your case is a high zoom level. To fix this, you would have to use the Dataset editor to draw the ends of the lines close together so they have clean connections from group to group. As you can see from my replication, at lower zoom levels the transit lines connect fine, but at higher zoom levels they are disjointed.
Zoom level 14:

Zoom level 18:
​

Then when I went to the Dataset Editor, zoomed in, and manually adjusted the lines at higher zoom level 18, the lines aligned just right in the style. You can see the comparison below with the lines edited at zoom level 18 (purple) and not edited (red). In the image please note the purple lines are directly on top of the bottom red lines:
​

I have not come across tools that can automate this task by "joining" lines while also keeping them separate. You could perhaps prepare your data in a GIS like QGIS or ArcGIS to somewhat automate this by joining all the lines and then manually making the splits (at a high zoom level) with a splitting tool. However, this is also time consuming. As an editing tip in Mapbox Dataset Editor, you can press Shift and select multiple lines to edit them at the same time.

[alexwohlbruck]

+1 on this issue. this is essential for building transit maps and without it we have to resort to weird workarounds in a preprocessing pipeline for the transit data. a feature like this would be huge