Feature request: Rounded corners

[shannonturner]

A frequently-requested feature is being able to add rounded corners to lines, to have the map feel less as though it's on a grid.

While it may look sleek, it would require either a careful rework of the line drawing algorithm, or potentially removing the grid, or both.

Worth thinking carefully about the best way to handle this while balancing the maintainability of everything else; it would likely require a mapDataVersion bump.

[shannonturner]

Also requested on Youtube in https://www.youtube.com/watch?v=VAbBlOTnVLQ&lc=UgylhKNf36p9W-8KLZN4AaABAg:

I spent hours trying to make the corners curve, with a radius I can choose or one that I am forced to use, but can’t find how. Is that a future feature? Is it too hard to add the feature?

... let's look at London Tube map, and I'm using Autumn 2024 but any recent one should be fine. You see District and Piccadilly lines going between Ealing Common and Acton Town? Piccadilly Line has a 90-degree turn with a small radius, and District Line has a 90-degree turn with a larger radius. Measure the pixels from where the curve begins & ends for District Line, and it's more pixels than Piccadilly. The entire curve is not a sequence of straight lines with rounded corners either — it's 25% of a circle with a fixed line thickness. But the radius changes. You can see this 25% of a circle happen three times at Earl's Court. Once you have that ability, then the other curves are easier too: Turnham Green has the 45-degree junction for District Line (and a 45-degree bend for the Overground line). A 45-degree bend just means we need 12.5% of a circle instead of 25% of a circle. The line thickness remains the same of course.

(See left hand side of map in https://content.tfl.gov.uk/standard-tube-map.pdf)