Linearly bin fires so weak heat spots stay round

Dropping each detection whole into its nearest pixel snapped spots by up
to half a pixel and left weak lone fires as squares or plus signs once
thresholded. Share each detection's weight between the four nearest pixel
centres instead (linear binning, Wand 1994): spots sit at the true
position and blur round, at the same build cost.

docs/CURRENT-STATE.md

@@ -2184,7 +2184,9 @@ detections are under 20 MW and stay dim red. Glow sprites and fire-card
 accents read that ramp; confidence and brightness do not affect the color.
 
 At far zoom (camera above ~3,000 km) the layer drapes a Snap Map–style
-heatmap instead of individual markers: the shown detections are binned on a
+heatmap instead of individual markers: the shown detections are linearly binned
+(each detection's weight shared between the four nearest pixel centres, so spots
+sit at the true position and stay round) on a
 global 2048 × 1024 grid (≈20 km per pixel), each weighted by its FRP in megawatts (so heat is
 radiated power, and raising the FRP slider does not dim areas where strong fires
 burn), Gaussian-blurred by one pixel so each fire stays a distinct spot about

src/data/firmsHeatRaster.js

@@ -1,8 +1,9 @@
 /**
  * Snap Map–style heat raster for the Active Fires far-zoom bands: detections
- * are binned on an equirectangular grid (row 0 = north, column 0 = 180°W),
- * blurred, normalized on a log scale and mapped through HEAT_PALETTE. Pure:
- * no Cesium, no DOM, so the whole pipeline is unit-tested in node.
+ * are linearly binned on an equirectangular grid (row 0 = north, column
+ * 0 = 180°W), blurred, normalized on a log scale and mapped through
+ * HEAT_PALETTE. Pure: no Cesium, no DOM, so the whole pipeline is unit-tested
+ * in node.
  */
 
 /** Below this normalized heat nothing is painted: empty land stays empty. */
@@ -54,19 +55,35 @@ export const HEAT_MIN_WEIGHT_MW = 0.5;
 const defaultWeight = (fire) =>
   Math.max(HEAT_MIN_WEIGHT_MW, Number(fire.frp) || 0);
 
+/**
+ * Linear binning (Wand 1994): each detection's weight is shared between the
+ * four pixel centres around it in proportion to closeness, instead of
+ * dropping whole into one pixel. Spots then sit at the true position rather
+ * than snapping by up to half a pixel (~10 km), and a lone weak fire blurs
+ * into a round spot instead of a square or a plus sign. Longitude wraps;
+ * rows beyond the poles fold back onto the edge row.
+ */
 export function binFires(fires, { width, height, weight = defaultWeight }) {
   const grid = new Float32Array(width * height);
+  const row = (y) => Math.min(height - 1, Math.max(0, y)) * width;
+  const col = (x) => ((x % width) + width) % width;
   for (const fire of fires) {
     if (!Number.isFinite(fire.lat) || !Number.isFinite(fire.lon)) continue;
-    const x = Math.min(
-      width - 1,
-      Math.max(0, Math.floor(((fire.lon + 180) / 360) * width)),
-    );
-    const y = Math.min(
-      height - 1,
-      Math.max(0, Math.floor(((90 - fire.lat) / 180) * height)),
-    );
-    grid[y * width + x] += weight(fire);
+    const fx = ((fire.lon + 180) / 360) * width - 0.5;
+    const fy = ((90 - fire.lat) / 180) * height - 0.5;
+    const x0 = Math.floor(fx);
+    const y0 = Math.floor(fy);
+    const tx = fx - x0;
+    const ty = fy - y0;
+    const w = weight(fire);
+    const top = row(y0);
+    const bottom = row(y0 + 1);
+    const left = col(x0);
+    const right = col(x0 + 1);
+    grid[top + left] += w * (1 - tx) * (1 - ty);
+    grid[top + right] += w * tx * (1 - ty);
+    grid[bottom + left] += w * (1 - tx) * ty;
+    grid[bottom + right] += w * tx * ty;
   }
   return grid;
 }

src/data/firmsHeatRaster.test.mjs

@@ -28,22 +28,37 @@ test('palette runs ember to white-hot and starts fully transparent', () => {
   }
 });
 
-test('binFires drops each detection into its equirectangular pixel', () => {
-  const grid = binFires([fire(89.9, -179.9, 1), fire(-89.9, 179.9, 1)], {
+test('binFires puts a detection at a pixel centre wholly in that pixel', () => {
+  // 4×2 grid: pixel centres at lon −135/−45/45/135, lat ±45.
+  const grid = binFires([fire(45, -135, 1), fire(-45, 135, 1)], {
     width: 4,
     height: 2,
   });
-  assert.equal(grid[0], 1, 'north-west corner');
-  assert.equal(grid[7], 1, 'south-east corner');
+  assert.equal(grid[0], 1, 'north-west pixel');
+  assert.equal(grid[7], 1, 'south-east pixel');
   assert.equal(
     grid.reduce((a, b) => a + b, 0),
     2,
   );
 });
 
+test('binFires shares a detection between the pixels around it', () => {
+  const between = binFires([fire(45, 0, 1)], { width: 4, height: 2 });
+  assert.equal(between[1], 0.5, 'half to the pixel west of it');
+  assert.equal(between[2], 0.5, 'half to the pixel east of it');
+  const quarter = binFires([fire(45, -112.5, 1)], { width: 4, height: 2 });
+  assert.equal(quarter[0], 0.75, 'closer centre takes more');
+  assert.equal(quarter[1], 0.25);
+  const wrapped = binFires([fire(45, 180, 1)], { width: 4, height: 2 });
+  assert.equal(wrapped[3], 0.5, 'the antimeridian wraps');
+  assert.equal(wrapped[0], 0.5);
+  const polar = binFires([fire(90, -135, 1)], { width: 4, height: 2 });
+  assert.equal(polar[0], 1, 'beyond the last row folds back onto it');
+});
+
 test('binFires weights each detection by its radiative power', () => {
   const grid = binFires(
-    [fire(0.1, 0.1, 250), fire(0.2, 0.2, 0), fire(0.3, 0.3, Number.NaN)],
+    [fire(45, 45, 250), fire(45, 45, 0), fire(45, 45, Number.NaN)],
     { width: 4, height: 2 },
   );
   assert.equal(grid[2], 250 + 2 * HEAT_MIN_WEIGHT_MW);