fix: handle antimeridian-crossing tiles in Web Mercator reprojection

Tiles near ±180° longitude cause RasterReprojector mesh refinement to
diverge because forwardTo3857 maps nearby source coordinates on opposite
sides of the antimeridian to EPSG:3857 x-values ~40M meters apart.

Detect these tiles by checking if corner x-values in EPSG:3857 span
more than half the world circumference, then wrap the projection
functions to make the coordinate space continuous.

Closes #366

Author: james-willis

packages/deck.gl-geotiff/src/cog-layer.ts

@@ -53,6 +53,12 @@ const TILE_SIZE = 512;
  */
 const WEB_MERCATOR_METER_CIRCUMFERENCE = 40075016.686;
 
+/**
+ * Half the Web Mercator circumference in meters. Tiles whose corner x-values
+ * span more than this in EPSG:3857 are assumed to cross the antimeridian.
+ */
+const HALF_CIRCUMFERENCE = WEB_MERCATOR_METER_CIRCUMFERENCE / 2;
+
 /**
  * Scale factor for converting EPSG:3857 meters into deck.gl world units
  * (512×512).
@@ -463,11 +469,55 @@ export class COGLayer<
         };
         deckProjectionProps = {};
       } else {
+        // Detect antimeridian-crossing tiles: project the four tile corners to
+        // EPSG:3857 and check whether their x-values span more than half the
+        // globe. When a tile straddles ±180° longitude, nearby source
+        // coordinates on opposite sides of the antimeridian map to EPSG:3857
+        // x-values ~40 million meters apart (+20M vs -20M). This discontinuity
+        // causes RasterReprojector mesh refinement to diverge.
+        //
+        // The fix: shift negative x-values by +circumference so the tile's
+        // EPSG:3857 coordinates are continuous (all positive). The inverse
+        // function unwraps by subtracting the circumference for values beyond
+        // the half-circumference.
+        let tileForwardTo3857 = forwardTo3857;
+        let tileInverseFrom3857 = inverseFrom3857;
+
+        const corners = [
+          [0, 0],
+          [width, 0],
+          [width, height],
+          [0, height],
+        ] as const;
+        const cornerXs = corners.map(([cx, cy]) => {
+          const [sx, sy] = forwardTransform(cx, cy);
+          return forwardTo3857(sx, sy)[0];
+        });
+        const xMin = Math.min(...cornerXs);
+        const xMax = Math.max(...cornerXs);
+
+        if (xMax - xMin > HALF_CIRCUMFERENCE) {
+          tileForwardTo3857 = (x: number, y: number): [number, number] => {
+            const [px, py] = forwardTo3857(x, y);
+            return [
+              px < 0 ? px + WEB_MERCATOR_METER_CIRCUMFERENCE : px,
+              py,
+            ];
+          };
+          tileInverseFrom3857 = (x: number, y: number): [number, number] => {
+            const unwrapped =
+              x > HALF_CIRCUMFERENCE
+                ? x - WEB_MERCATOR_METER_CIRCUMFERENCE
+                : x;
+            return inverseFrom3857(unwrapped, y);
+          };
+        }
+
         reprojectionFns = {
           forwardTransform,
           inverseTransform,
-          forwardReproject: forwardTo3857,
-          inverseReproject: inverseFrom3857,
+          forwardReproject: tileForwardTo3857,
+          inverseReproject: tileInverseFrom3857,
         };
         // Scale 3857 meters → deck.gl world units (512×512).
         //