text-overlap with symbol-placement: line

[lhapaipai]

Hi 👋,
I'm trying to place labels on a line with overlap, but I can't manage to find the right configuration.
I want the label to follow the line. If there is a collision, I want to force the label to still appear on the map.

I've tried several combinations to illustrate my questions 👇

🔗 Link to reproduction : https://jsbin.com/zojepikite/edit?html,js,output

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Case 1

• symbol-placement: point
• text-overlap: never

We see some collision boxes in red, and only the labels that don't collide are shown → ✅ OK.

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Case 2

• symbol-placement: point
• text-overlap: always

we see collision boxes in blue, and all the labels are shown → ✅ OK.

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Case 3 / 4

• symbol-placement: line
• text-overlap: never / always (❗ no difference between the 2 options)

At a low zoom level, nothing appears. What is causing not even a single label to appear?
Can we adjust the zoom threshold when the change happens?

At a medium zoom level, some labels appear. We always see collision boxes in blue, and I wonder why all labels don't appear?

Main Questions ❓

• I wonder if the tile boundaries can affect the placement. In this example, the labels that should appear in the middle are not shown, and we can see the vertical red line of the tile boundary. But then why is symbol-placement: point not affected?
• Is there an option to force all labels to be displayed, like with points?
• Is the collision method for line placement different? the collision boxes seems to be always blue.
• Am I missing some configuration option? for this example, I'm using the default value of text-padding.

Additional Experiments 🧪

• symbol-placement: line-center
• text-overlap: never / always

At a low zoom level, nothing appears (same as above).
At a medium zoom level, everything works as expected ✅.

At a high zoom level, it seems the tile boundaries prevent the option from working correctly, as if they acted like the end of the line.

line-center: The label is placed at the center of the line of the geometry. Can only be used on LineString and Polygon geometries. Note that a single feature in a vector tile may contain multiple line geometries. Documentation

✨ Conclusion
I'm scratching my head a bit 🤯. It feels like labels overlap behave differently if placement is point or line. Any hint, trick would be greatly appreciated. Thank you 🙏

[HarelM]

There is probably different logic for different placement, and there might be bugs in each implementation.
Try adding "text-ignore-placement" and se if it helps...

[lhapaipai]

As described earlier, collision management seemed to work correctly (the collision boxes turned red) when symbol-placement was set to point, and also for line-center at medium and high zoom levels (> 6).

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When using text-ignore-placement: true, the property works as expected for the cases where collision management seemed to work correctly.

However, at low zoom levels (< 6) with symbol-placement set to line or line-center, no labels appear (regardless of the value of text-ignore-placement).

At zoom levels > 6 with symbol-placement set to line, only a few labels appear, but in a completely unpredictable way (adding text-ignore-placement doesn’t change anything, text-overlap doesn't change anything).

[HarelM]

Thanks for the info! Feel free to open an issue, even better if you can open a PR to fix this. 🙏🏻