Some contour polygon is reverse.

[hall]

I'm using d3-contour to draw beautiful chart recently. Most of the time it's fine.
But in this case: https://observablehq.com/@choukou/untitled
In hundreds of polygons, one is special

It's counterclockwise. That cause the whole screen green.
Would you give me some help?

[Philippe Rivière]

It seems that you are computing the contours on the spherical coordinates, then project the resulting polygons. But this creates issues with the holes. If you changed the order of operations (first project, then compute the contours), it would work. I'm sending you a suggestion.

See also https://observablehq.com/@d3/cloud-contours and https://observablehq.com/@fil/netcdf

[hall]

It seems that you are computing the contours on the spherical coordinates, then project the resulting polygons. But this creates issues with the holes. If you changed the order of operations (first project, then compute the contours), it would work. I'm sending you a suggestion.

See also https://observablehq.com/@d3/cloud-contours and https://observablehq.com/@fil/netcdf

Thanks very much for your speedy and professional advice, I'm reading your code now, may get back to you if any further help needed.

Wish you a nice weekend ahead.

[hall]

In the data source(contours.json), masked area(land) is set -1e20.
When I change -1e20 to -999, the chart becomes perfect.
I don't know why this value affect the contour polygons.

[Philippe Rivière]

Ah, true:

contours = d3.contours()
                  .size([data.width, data.height])
                  .thresholds(thresholds)(varData.values.map(d => Math.max(-1e13, d)))

is correct, but with 1e-14 it's broken:

contours = d3.contours()
                  .size([data.width, data.height])
                  .thresholds(thresholds)(varData.values.map(d => Math.max(-1e14, d)))

[Mike Bostock]

I suspect that this is related to numerical instability and I think it’s unlikely that we would implement a fully robust algorithm. Maybe we can at least guarantee consistency with d3.polygonArea though.