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expand the matrix on the edges (with blurred values), then contract t…
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…he contour back to the frame

closes #72
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@Fil
Fil committed Nov 26, 2024
1 parent 9fc65d5 commit 11ae1ab
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Showing 5 changed files with 155 additions and 73 deletions.
134 changes: 83 additions & 51 deletions src/contours.js
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Original file line number Diff line number Diff line change
@@ -1,12 +1,12 @@
import {extent, nice, thresholdSturges, ticks} from "d3-array";
import {blur, extent, nice, thresholdSturges, ticks} from "d3-array";
import {slice} from "./array.js";
import ascending from "./ascending.js";
import area from "./area.js";
import constant from "./constant.js";
import contains from "./contains.js";
import noop from "./noop.js";

var cases = [
const cases = [
[],
[[[1.0, 1.5], [0.5, 1.0]]],
[[[1.5, 1.0], [1.0, 1.5]]],
Expand All @@ -25,14 +25,20 @@ var cases = [
[]
];

const blurEdges = 0.5;

function clamp(x, lo, hi) {
return x < lo ? lo : x > hi ? hi : x;
}

export default function() {
var dx = 1,
dy = 1,
threshold = thresholdSturges,
smooth = smoothLinear;
let dx = 1;
let dy = 1;
let threshold = thresholdSturges;
let smooth = smoothLinear;

function contours(values) {
var tz = threshold(values);
let tz = threshold(values);

// Convert number of thresholds into uniform thresholds.
if (!Array.isArray(tz)) {
Expand All @@ -53,17 +59,45 @@ export default function() {
const v = value == null ? NaN : +value;
if (isNaN(v)) throw new Error(`invalid value: ${value}`);

var polygons = [],
holes = [];
// Don’t round the corners by clamping values on the edge.
const bottom = values.slice(0, dx);
const top = values.slice(-dx);
const left = Array.from({length: dy}, (_, i) => values[i * dx]);
const right = Array.from({length: dy}, (_, i) => values[i * dx + dx - 1]);
blur(bottom, blurEdges);
blur(top, blurEdges);
blur(left, blurEdges);
blur(right, blurEdges);

function get(x, y) {
const x0 = clamp(x, 0, dx - 1);
const y0 = clamp(y, 0, dy - 1);
if (y < 0) return bottom[x0];
if (y >= dy) return top[x0];
if (x < 0) return left[y0];
if (x >= dx) return right[y0];
return values[x0 + y0 * dx];
}

isorings(values, v, function(ring) {
smooth(ring, values, v);
if (area(ring) > 0) polygons.push([ring]);
else holes.push(ring);
const polygons = [];
const holes = [];

isorings(get, value, function(ring) {
smooth(ring, get, value);
const r = [];
let x0, y0;
for (const point of ring) {
const x = clamp(point[0], 0, dx);
const y = clamp(point[1], 0, dy);
if (x !== x0 || y !== y0) r.push([(x0 = x), (y0 = y)]);
}
const a = area(r);
if (a > 0) polygons.push([r]);
else if (a < 0) holes.push(r);
});

holes.forEach(function(hole) {
for (var i = 0, n = polygons.length, polygon; i < n; ++i) {
for (let i = 0, n = polygons.length, polygon; i < n; ++i) {
if (contains((polygon = polygons[i])[0], hole) !== -1) {
polygon.push(hole);
return;
Expand All @@ -80,51 +114,52 @@ export default function() {

// Marching squares with isolines stitched into rings.
// Based on https://github.com/topojson/topojson-client/blob/v3.0.0/src/stitch.js
function isorings(values, value, callback) {
var fragmentByStart = new Array,
fragmentByEnd = new Array,
x, y, t0, t1, t2, t3;
function isorings(get, value, callback) {
const test = (x, y) => above(get(x, y), value);
const fragmentByStart = new Array;
const fragmentByEnd = new Array;
let x, y, t0, t1, t2, t3;

// Special case for the first row (y = -1, t2 = t3 = 0).
x = y = -1;
t1 = above(values[0], value);
x = y = -2;
t1 = test(-1, -1);
cases[t1 << 1].forEach(stitch);
while (++x < dx - 1) {
t0 = t1, t1 = above(values[x + 1], value);
while (++x < dx) {
t0 = t1, t1 = test(x + 1, -1);
cases[t0 | t1 << 1].forEach(stitch);
}
cases[t1 << 0].forEach(stitch);

// General case for the intermediate rows.
while (++y < dy - 1) {
x = -1;
t1 = above(values[y * dx + dx], value);
t2 = above(values[y * dx], value);
while (++y < dy) {
x = -2;
t1 = test(x + 1, y + 1);
t2 = test(x + 1, y);
cases[t1 << 1 | t2 << 2].forEach(stitch);
while (++x < dx - 1) {
t0 = t1, t1 = above(values[y * dx + dx + x + 1], value);
t3 = t2, t2 = above(values[y * dx + x + 1], value);
while (++x < dx) {
t0 = t1, t1 = test(x + 1, y + 1);
t3 = t2, t2 = test(x + 1, y);
cases[t0 | t1 << 1 | t2 << 2 | t3 << 3].forEach(stitch);
}
cases[t1 | t2 << 3].forEach(stitch);
}

// Special case for the last row (y = dy - 1, t0 = t1 = 0).
x = -1;
t2 = values[y * dx] >= value;
x = -2;
t2 = test(x, y);
cases[t2 << 2].forEach(stitch);
while (++x < dx - 1) {
t3 = t2, t2 = above(values[y * dx + x + 1], value);
while (++x < dx) {
t3 = t2, t2 = test(x + 1, y);
cases[t2 << 2 | t3 << 3].forEach(stitch);
}
cases[t2 << 3].forEach(stitch);

function stitch(line) {
var start = [line[0][0] + x, line[0][1] + y],
end = [line[1][0] + x, line[1][1] + y],
startIndex = index(start),
endIndex = index(end),
f, g;
const start = [line[0][0] + x, line[0][1] + y];
const end = [line[1][0] + x, line[1][1] + y];
const startIndex = index(start);
const endIndex = index(end);
let f, g;
if (f = fragmentByEnd[startIndex]) {
if (g = fragmentByStart[endIndex]) {
delete fragmentByEnd[f.end];
Expand Down Expand Up @@ -165,27 +200,24 @@ export default function() {
return point[0] * 2 + point[1] * (dx + 1) * 4;
}

function smoothLinear(ring, values, value) {
function smoothLinear(ring, get, value) {
ring.forEach(function(point) {
var x = point[0],
y = point[1],
xt = x | 0,
yt = y | 0,
v1 = valid(values[yt * dx + xt]);
if (x > 0 && x < dx && xt === x) {
point[0] = smooth1(x, valid(values[yt * dx + xt - 1]), v1, value);
}
if (y > 0 && y < dy && yt === y) {
point[1] = smooth1(y, valid(values[(yt - 1) * dx + xt]), v1, value);
}
const x = point[0];
const y = point[1];
const xt = x | 0;
const yt = y | 0;
const v1 = valid(get(xt, yt));
if (x > 0 && x < dx && xt === x) point[0] = smooth1(x, valid(get(xt - 1, yt)), v1, value);
if (y > 0 && y < dy && yt === y) point[1] = smooth1(y, valid(get(xt, yt - 1)), v1, value);
});
}

contours.contour = contour;

contours.size = function(_) {
if (!arguments.length) return [dx, dy];
var _0 = Math.floor(_[0]), _1 = Math.floor(_[1]);
const _0 = Math.floor(_[0]);
const _1 = Math.floor(_[1]);
if (!(_0 >= 0 && _1 >= 0)) throw new Error("invalid size");
return dx = _0, dy = _1, contours;
};
Expand Down
2 changes: 1 addition & 1 deletion test/contours-test.js
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Original file line number Diff line number Diff line change
Expand Up @@ -252,7 +252,7 @@ it("contours(values) treats null, undefined, NaN and -Infinity as holes", () =>
1, 1, NaN, 1, 1, 1, 2, -Infinity, 2, 1,
1, 1, 1, 1, 1, 1, 2, 2, 2, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1
], 0), {"type":"MultiPolygon","value":0,"coordinates":[[[[10,9.5],[10,8.5],[10,7.5],[10,6.5],[10,5.5],[10,4.5],[10,3.5],[10,2.5],[10,1.5],[10,0.5],[9.5,0],[8.5,0],[7.5,0],[6.5,0],[5.5,0],[4.5,0],[3.5,0],[2.5,0],[1.5,0],[0.5,0],[0,0.5],[0,1.5],[0,2.5],[0,3.5],[0,4.5],[0,5.5],[0,6.5],[0,7.5],[0,8.5],[0,9.5],[0.5,10],[1.5,10],[2.5,10],[3.5,10],[4.5,10],[5.5,10],[6.5,10],[7.5,10],[8.5,10],[9.5,10],[10,9.5]],[[1.5,2.5],[0.5,1.5],[1.5,0.5],[2.5,1.5],[1.5,2.5]],[[3.5,5.5],[2.5,4.5],[3.5,3.5],[4.5,4.5],[3.5,5.5]],[[2.5,8.5],[1.5,7.5],[2.5,6.5],[3.5,7.5],[2.5,8.5]],[[7.5,8.5],[6.5,7.5],[7.5,6.5],[8.5,7.5],[7.5,8.5]]]]});
], 0), {"type":"MultiPolygon","value":0,"coordinates":[[[[10,10],[10,9.5],[10,8.5],[10,7.5],[10,6.5],[10,5.5],[10,4.5],[10,3.5],[10,2.5],[10,1.5],[10,0.5],[10,0],[9.5,0],[8.5,0],[7.5,0],[6.5,0],[5.5,0],[4.5,0],[3.5,0],[2.5,0],[1.5,0],[0.5,0],[0,0],[0,0.5],[0,1.5],[0,2.5],[0,3.5],[0,4.5],[0,5.5],[0,6.5],[0,7.5],[0,8.5],[0,9.5],[0,10],[0.5,10],[1.5,10],[2.5,10],[3.5,10],[4.5,10],[5.5,10],[6.5,10],[7.5,10],[8.5,10],[9.5,10],[10,10]],[[1.5,2.5],[0.5,1.5],[1.5,0.5],[2.5,1.5],[1.5,2.5]],[[3.5,5.5],[2.5,4.5],[3.5,3.5],[4.5,4.5],[3.5,5.5]],[[2.5,8.5],[1.5,7.5],[2.5,6.5],[3.5,7.5],[2.5,8.5]],[[7.5,8.5],[6.5,7.5],[7.5,6.5],[8.5,7.5],[7.5,8.5]]]]});
});

it("contours(values) returns the expected result for a +Infinity value", () => {
Expand Down
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