Breaking change with contour-density thresholds starting from v3.0.2

[d3]

Breaking change with contour-density thresholds starting from v3.0.2

[NataliaM5]

Hi, @Fil !
Thank you for the response. Faced the same issue.
After diving deep into d3-contour source code, I found that k depends on cellSize provided value:

d3-contour/src/density.js

Line 130 in 5e9ee8a

return k = Math.floor(Math.log(_) / Math.LN2), resize();

So, to get a correct results, seems like we need to use the following formula when using cellSize method:

k = Math.floor(Math.log(cellSizeValue) / Math.LN2)

const thresholds = [0, 0.001, 0.002, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009, 0.01]; // old style
const contours = d3_302
  .contourDensity()
  .cellSize(cellSizeValue)
  .thresholds(thresholds.map(d => d/ Math.pow(2, 2 * k))) // convert to new style
    (data)

[Fil]

This notebook: https://observablehq.com/@fil/density-thresholds-70

Suppose you wanted to pass the following thresholds in d3-contour 3.0.1:

const contours = d3301
  .contourDensity()
  .thresholds([0, 0.001, 0.002, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009, 0.01])
    (data)

this would return contours with values:

contours.map((d) => d.value) // [0, 0.0000625, 0.000125, 0.0001875, 0.00025, 0.0003125, 0.000375, 0.0004375, 0.0005, 0.0005625, 0.000625]

This was inconsistent (you ask for 1, you get 1/16). #57 fixes the inconsistency, so starting with 3.0.2, you would pass this instead:

const thresholds = [0, 0.0000625, 0.000125, 0.0001875, 0.00025, 0.0003125, 0.000375, 0.0004375, 0.0005, 0.0005625, 0.000625];
const contours = d3_301
  .contourDensity()
  .thresholds(thresholds)
    (data)

and get the same contours (with the same values).

The relationship between the two is this ratio 2**k with k the cellSize which defaults to 4; in other words you should find the same contours as before if you did:

const thresholds = [0, 0.001, 0.002, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009, 0.01]; // old style
const contours = d3_302
  .contourDensity()
  .thresholds(thresholds.map(d => d/16)) // convert to new style
    (data)