Polygon Outline Styling Support

[guthriealexander]

Hello, is Guthrie from Amazon with some cartographic feature requests:

In order to have a polygon with an outline you need to create two different features (a line and polygon) and layer them. This creates excess layers and can make styling polygons with consistent opacity and other values more difficult.

It would be great if polygon feature outlines could have the full styling capability of the line object.

[louwers]

Hey @guthriealexander thanks for the feature request.

Just to clarify, you are talking about using a fill layer with a line layer to achieve a polygon with an outline? If I am not mistaken you can already add a fill outline color which will create a single pixel outline. Adding additional styling capabilities to fill layers probably would need to be discussed in the MapLibre Style Spec repo although I think the current design is on purpose to simplify the internals.

cc @sjg-wdw

[sjg-wdw]

They're talking about more complex features. @guthriealexander if you could share your pictures, that'd be great.

[sjg-wdw]

I think I've heard this one before. With OpenGL it made sense because we could use pretty normal lines for this. With Metal and Vulkan we have to implement our own wide lines anyway, so why not allow for more features on those lines?

Well, WebGL would be the answer to that I suppose.

[giswqs]

The problem is that the fill-opacity also controls the opacity of the outline. Setting fill-opacity=0 will make the outline disappear, which is not ideal. Sometimes we just want to show the outline without fill. It would be great to add a fill-outline-opacity property.

I know the workaround is to add add the polygon as a line layer. However, users need to click along the lines to get the popup attributes. Unlike polygon layers, you can click inside anywhere within the polygon to get attributes.

[giswqs]

These two demos illstrute the problem.

Rendering the data as a polygon layer allows users to click anywhere within each polygon to get popup attributes. It is infeasbile to show the outline only unless we add a line type layer on top of the polygon layer.

polygon.mp4

Rendering the data as a line layer requires users to click along the line the get popup attributes. This can be quite confusing when two polygons share the boundary.

line.mp4

[HarelM]

Can't you render the polygon layer with a transparent color for the click and a line layer for the outline?