Add contour source

[msbarry]

Add a new source type that generates contour isolines from raster-dem sources.

Fixes #583 - see that issue for the full design and discussion.

Launch Checklist

• Confirm your changes do not include backports from Mapbox projects (unless with compliant license) - if you are not sure about this, please ask!
• Briefly describe the changes in this PR.
• Link to related issues.
• Include before/after visuals or gifs if this PR includes visual changes.
• Write tests for all new functionality.
• Document any changes to public APIs.
• Post benchmark scores.
• Add an entry to CHANGELOG.md under the ## main section.
• Major vs. index terminology?
• Use expression for interval and major/index that only support ["zoom"] as an argument
• should updating a raster-dem source used by a contour source cause remove/add on both sources?

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.54%. Comparing base (c755d44) to head (101e4fc).
Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #623      +/-   ##
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+ Coverage   92.23%   92.54%   +0.31%     
==========================================
  Files         100      105       +5     
  Lines        4196     4666     +470     
  Branches     1203     1316     +113     
==========================================
+ Hits         3870     4318     +448     
- Misses        326      348      +22     

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[msbarry]

I'll leave this open as a draft as I implement the gl-js side of this.

[HarelM]

This looks great overall, the only question left is the expression for the intervals and multiplier.
Thanks!!!

[zstadler]

please use `filter` rather than `filter

[HarelM]

@msbarry any updates on this? Is there something I can help with?

[msbarry]

I was planning to keep this open as I implement the maplibre-gl-js side of it in case I have to make any changes here. I got a bit sidetracked trying to get a planetiler release out with parquet/overture support but should have that wrapped up soon so I can get back to this.

My in-progress changes to maplibre-gl-js are here: https://github.com/msbarry/maplibre-gl-js/tree/contour-source - I have it working but it's pretty inefficient (overfetching a lot of tiles) and doesn't handle adding border pixels yet. Need to do some more thinking about how to handle those... It might make sense to refactor source_cache first to decouple staging the tile pyramid for a source consumer from fetching/caching tiles if you had any ideas there? I think if terrain and hillshading could read from the same source, then it should be ready for contour tiles to read from it as well.

[HarelM]

No rush, just wanted to make sure you haven't forgot about it 🙂
I don't have a good perspective about source cache ATM in order to retractor it, so you'll probably beat me to it...

[HarelM]

@msbarry where are we with this?
We are in the middle of 5.x version of maplibre, so this might be a good opportunity to break things if they are needed as part of this feature.

[msbarry]

I'm still hoping to do this eventually, but it's gotten bumped further down my todo list in the short term 😞 I had been thinking of this as a purely additive thing. The only change to existing functionality would be if I can fix source cache and remove the need to define the same DEM source twice to use across terrain and hillshading, but that's just a convention change, not breaking. Was there anything you were thinking?

[HarelM]

I'm not sure what we need to break if at all in order to implement this feature. I recall stuff that might be related to source cache, although source cache is not a public API, but still.
If the feature doesn't require any changes, we can introduce it later on, in a minor release.
But since you looked at the code to implement it, you might have more insights as to what needs to change in order to implement this smoothly.
In any case, I hope the following PR would allow for most of what you need:
maplibre/maplibre-gl-js#4779