developmentseed
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@@ -0,0 +1,134 @@
+# Render imagery crossing the antimeridian by cutting tiles in two
+
+- **Date:** 2026-05-27
+- **Issues:** [#171](https://github.com/developmentseed/deck.gl-raster/issues/171), [#366](https://github.com/developmentseed/deck.gl-raster/issues/366)
+- **Status:** Proposed
+- **Prerequisite (merged):** [#517](https://github.com/developmentseed/deck.gl-raster/issues/517) / [#518](https://github.com/developmentseed/deck.gl-raster/pull/518) — multi-world-copy tile traversal
+- **Related:** [#182](https://github.com/developmentseed/deck.gl-raster/issues/182), [#351](https://github.com/developmentseed/deck.gl-raster/pull/351) (reprojector sub-domain / cutline), [`dev-docs/coordinate-systems.md`](../coordinate-systems.md), [`dev-docs/world-copies.md`](../world-copies.md)
+- **Informed by (not the basis):** [#353](https://github.com/developmentseed/deck.gl-raster/pull/353) (rejected: global proj4 `+over` hack), [#374](https://github.com/developmentseed/deck.gl-raster/pull/374) and [#269](https://github.com/developmentseed/deck.gl-raster/pull/269) (AI-generated unwrap attempts)
+
+## Problem
+
+A single raster whose source extent crosses ±180° longitude does not render correctly in a Web Mercator viewport. This covers:
+
+- A global EPSG:4326 COG whose bounds touch or slightly overhang ±180° (e.g. `[-180.0012, …, 179.9987, …]`), where the dateline-edge tile straddles the seam.
+- A genuine crossing scene whose source grid wraps past ±180° (stored with longitudes running e.g. 170° → 190°).
+
+"Antimeridian" decomposes into three problems: **A** — tile *selection* across world copies (#517, fixed in #518); **B** — global-COG mesh divergence (#366); **C** — true crossing imagery (#171). A is merged. This spec addresses **B + C**, which are the same underlying problem at different tile geometries: a tile whose source extent crosses ±180° needs a *continuous* projection to mesh and place correctly.
+
+## Why it's hard
+
+The Web Mercator render path projects each tile through
+[`raster-tileset-2d.ts`](../../packages/deck.gl-raster/src/raster-tileset/raster-tileset-2d.ts) `projectPosition`:
+
+```ts
+projectPosition = (x, y) => rescaleEPSG3857ToCommonSpace(descriptor.projectTo3857(x, y));
+```
+
+`projectTo3857` is proj4 (source CRS → 3857 m). proj4 normalizes longitude to (−180°, 180°], so a tile straddling the dateline has corners at +179° → 3857 x ≈ **+19.9 Mm** (common-x ≈ 510) and +181°/−179° → 3857 x ≈ **−19.9 Mm** (common-x ≈ 2). The `RasterReprojector` (Delatin) mesh triangle spanning those corners covers the whole world, and its pixel-space error never converges (#366: `error=43200` after 10 000 iterations).
+
+**Unwrapping in source-longitude space does not work:** proj4 re-normalizes any longitude you hand it (190° → −170°), re-introducing the jump (dcherian, [#269](https://github.com/developmentseed/deck.gl-raster/pull/269)). Any unwrap must therefore act at/after the transform output — which is what makes the "keep it as one tile" approaches fragile.
+
+## Approach: cut the tile in two
+
+Rather than keep the crossing tile as one mesh and fight proj4 to make its coordinates continuous (the **render-as-one** family: #374 output-space shift, #269 reprojector unwrap, #353 global `+over`), **split the tile at the antimeridian into a west piece and an east piece.** Each piece lies wholly on one side of the dateline, so:
+
+- The west piece is monotonic in 3857 (all +x → common-x up to 512); the east piece all −x → common-x from 0. **The discontinuity never exists within a piece.**
+- `projectTo3857` stays stock — no unwrap, no proj4 reconfiguration, no `+over`.
+- The `RasterReprojector` needs zero antimeridian awareness — Delatin converges normally on each piece.
+- Mesh vertices stay within `[0, 512]`, so the fp64 high-zoom precision scheme ([`coordinate-systems.md`](../coordinate-systems.md)) is untouched.
+- Each piece is a normal tile that the merged world-copy traversal (#518) selects and draws across copies.
+
+The antimeridian becomes *a tile boundary*, which the pipeline already handles, instead of a coordinate-space discontinuity.
+
+### Why not render-as-one
+
+Render-as-one is simpler at the render layer (one mesh, one draw, no internal seam) and more CRS-general (it unwraps the output value, indifferent to source pixel geometry). But it re-attempts the exact unwrap that has failed three times: detection needs phase-unwrapping (a full-world continuous tile must not be mistaken for a crossing tile), mesh vertices leave `[0, 512]` (precision risk), and forward+inverse must stay consistent. We choose cut-in-two as the primary mechanism and keep **render-as-one as the documented fallback for curved-meridian CRS** (see Scope), where cut-in-two degrades.
+
+## Locating the cut
+
+Compute the cut generally by **inverse-projecting the antimeridian into source space**: sample `(180°, lat)` for `lat ∈ [−90, 90]`, run each point through `descriptor.projectFrom4326` (WGS84 → source CRS) then the inverse geotransform → a polyline in source pixel / UV space. This is robust to rotated geotransforms and arbitrary CRS (it does not assume the cut is the `lng = 180°` pixel column).
+
+The cut's **shape** determines feasibility:
+
+- **Straight cut** (axis-aligned EPSG:4326 → vertical; rotated geotransform → slanted): a straight line splits the unit square into two **convex** pieces.
+- **Curved cut** (curved-meridian CRS): at least one piece is **concave**.
+
+The MVP handles **any straight cut** — vertical (axis-aligned EPSG:4326) *and* slanted (rotated geotransform) — since both yield convex pieces that delaunator triangulates exactly. It **errors clearly** only when the inverse-projected meridian is *curved* (concave pieces; curved-meridian CRS), which is deferred.
+
+## Architecture
+
+The split lives in **one place** — the per-tile sublayer factory — and every other component keeps its single-mesh contract.
+
+```
+RasterTileLayer._renderSubLayers (per tile)        ← the only split point
+  ├─ normal tile   → 1 RasterLayer  → 1 RasterReprojector → 1 mesh → 1 MeshTextureLayer
+  └─ crossing tile → 2 RasterLayers → (each) 1 reprojector → 1 mesh → 1 MeshTextureLayer
+```
+
+- **`RasterReprojector`** ([`delatin.ts`](../../packages/raster-reproject/src/delatin.ts)) — one mesh, always. Gains an optional **initial-triangulation seed** `{ uvs, triangles, halfedges }` (delaunator's shape), defaulting to today's unit-square 2-triangle seed. The refinement core (`_step`, `_legalize`, `_findReprojectionCandidate`, the error queue) is already seed-agnostic; only the constructor's hardcoded init changes. Refinement only ever *splits existing triangles*, so a seed covering `[0, u_cut]×[0,1]` keeps the whole mesh in that sub-region. `width`/`height` stay the full image, so sub-domain UVs index the full texture — no texture re-windowing.
+- **Seed building (no shipped wrapper)** — `raster-reproject` exposes only the `InitialTriangulation` type, not a builder. Wrapping delaunator is a one-liner, so its docstring documents the pattern instead (`uvs`/`triangles`/`halfedges` = delaunator's `coords`/`triangles`/`halfedges`). delaunator is a **dev/test dependency** of `raster-reproject` — used by tests to validate winding compatibility — *not* a runtime dependency, so nothing is shipped to consumers. The deck.gl-raster cut builder (follow-up) constructs each convex-piece seed at runtime; whether that uses delaunator (a runtime dep there) or a hand-rolled convex-fan triangulation is decided in the integration plan.
+- **Cut builder** (deck.gl-raster) — computes the cut (inverse-project the antimeridian) → 1 or 2 sub-domain seeds. Lives in the tileset's `getTileMetadata` and is stored on tile metadata (per the "tile state on the tile" convention), so it is computed once and shared by both the render and the bounding volume.
+- **`RasterLayer`** ([`raster-layer.ts`](../../packages/deck.gl-raster/src/raster-layer.ts)) — one mesh, one `MeshTextureLayer`, unchanged except a new `initialTriangulation` prop (default: full square) passed to its reprojector.
+- **`RasterTileLayer._renderSubLayers`** — reads the tile's cut info and emits 1 or 2 `RasterLayer`s. Both crossing sub-layers share the **same** `reprojectionFns` (the tile's `_projectPosition`); they differ only in `initialTriangulation` and sublayer id (`…-raster-west` / `…-raster-east`).
+- **Traversal** — a **two-box bounding volume** for a crossing tile (west ≈ `[510,512]`, east ≈ `[0,2]`), each a normal `[0,512]` box, mapping 1:1 to the two `RasterLayer`s and composing with the world-copy traversal's per-offset selection (a crossing tile natively occupies two world bands at offset 0).
+
+## Transparency to end users
+
+The split is entirely below the tile-data boundary:
+
+- **`getTileData` is unchanged.** A crossing tile is one tile index `(x, y, z)` and a single *contiguous* source-pixel fetch — the discontinuity appears only when projecting to 3857, after fetch. Any data source (COG, zarr, user-supplied) needs zero antimeridian awareness, and the tile is decoded once (both pieces sample the one texture).
+- **`_renderSubLayers` is library-internal** — standard `COGLayer` / `RasterTileLayer` users never write it.
+
+Caveat: a user who *subclasses* and overrides `_renderSubLayers` would bypass the split.
+
+## Unification
+
+The initial-triangulation seed subsumes several pending needs into one primitive — *the caller hands the reprojector a seed*:
+
+- Normal tile → full unit square → 1 layer (unchanged behavior).
+- Antimeridian crossing → west + east seeds → 2 layers.
+- Pole clamp (#182) / `uvBounds` (#351) → one clamped-rectangle seed → 1 layer (data beyond ±85.051° is not meshed).
+- Collar cutline → one inset-domain seed → 1 layer.
+
+## Scope
+
+**In scope:**
+- Web Mercator viewport.
+- Straight cut (convex pieces): axis-aligned EPSG:4326 (vertical) *and* rotated geotransforms (slanted).
+- Test datasets:
+  - **Primary, deterministic:** the [`antimeridian.tif`](https://github.com/developmentseed/geotiff-test-data/blob/3c7ceb9ec2ed23b0ba71c2222ac4d5e6f31db0ec/rasterio_generated/fixtures/antimeridian.tif) fixture, already vendored via the `fixtures/geotiff-test-data` submodule (`fixtures/geotiff-test-data/rasterio_generated/fixtures/antimeridian.tif`). 42×42, EPSG:4326, bbox (−204, −18, −162, 24) → crosses −180° with a clean vertical cut at pixel column 24 (lng −204 ≡ +156 wrapped).
+  - **Global / edge-overhang variant:** a global EPSG:4326 COG that triggers #366 — e.g. WorldPop `ppp_2020_1km_Aggregated.tif` (from the issue) or the GEDTM30 global DEM (from #353).
+
+**Out of scope (deferred):**
+- Globe view (separate prototype).
+- Curved-meridian / polar CRS (concave pieces). delaunator fills the convex hull, so a concave piece would gain triangles across the seam; handling needs centroid-filtering or constrained Delaunay, or the render-as-one fallback. The MVP errors on a non-straight cut.
+
+## Edge cases & risks
+
+- **Degenerate slivers:** the half-pixel-overhang case (`−180.0012°`) splits into a sub-pixel sliver + a main piece. Skip pieces below an ε UV width so we don't emit a degenerate mesh.
+- **Seam between pieces:** west's cut edge lands at common-x 512, east's at 0 ≡ 512 in the +1 world copy — they abut across the world-copy boundary. Encode the shared edge bit-identically (same discipline as adjacent tiles, [`coordinate-systems.md`](../coordinate-systems.md)).
+- **delaunator ↔ delatin orientation:** this repo's delatin works in UV (y-down). Verify winding/`inCircle` compatibility with a test (delaunator on the 4 unit-square corners → seed → delatin refines identically to the current hardcoded init).
+- **Texture upload:** both sublayers reference the same tile image; without a shared luma `Texture` it uploads twice. Negligible for the prototype (dateline tiles are a thin strip); optimize later if needed.
+
+## Test plan
+
+**Unit**
+- Reprojector seeded with a delaunator-built sub-rectangle (the documented pattern) converges and adds no vertices outside the seed domain; a delaunator unit-square seed refines validly (winding compatibility), equivalent to the current default.
+- Cut location: inverse-projecting the antimeridian yields the expected cut line — a vertical UV column for axis-aligned EPSG:4326 (the `antimeridian.tif` fixture cuts at column 24 / `u ≈ 0.571`), a slanted line for a rotated geotransform; a *curved* cut is detected and errors.
+- Two-box bounding volume for a crossing tile (west/east boxes; correct selection under the world-copy traversal).
+
+**Integration / visual (cog-basic)**
+- The `antimeridian.tif` fixture renders as a single contiguous image across ±180° (west piece near +180°, east piece near −180°), staying continuous when panning across the seam.
+- A global EPSG:4326 COG (WorldPop / GEDTM30) renders correctly at the dateline (no `error=43200` divergence; no mislocated rectangles).
+- Before/after comparison against current main.
+
+## Implementation stages (high level)
+
+1. `RasterReprojector` accepts an `InitialTriangulation` seed (default unchanged); `InitialTriangulation` docstring documents the delaunator pattern; delaunator added as a dev dependency; tests use a delaunator-built seed to validate winding + sub-domain confinement. **(Done.)**
+2. Cut location (inverse-project the antimeridian) + convexity check (error on a curved/concave cut), on tile metadata.
+3. Two-box bounding volume in traversal for crossing tiles.
+4. `RasterLayer` `initialTriangulation` prop; `_renderSubLayers` emits 1 or 2 `RasterLayer`s, each seeded from its cut sub-domain.
+5. Example wiring + visual validation in cog-basic.
+
+(Detailed task breakdown lives in the implementation plan, not here.)
@@ -7,7 +7,10 @@ import type {
 } from "@deck.gl/core";
 import { CompositeLayer } from "@deck.gl/core";
 import { PolygonLayer } from "@deck.gl/layers";
-import type { ReprojectionFns } from "@developmentseed/raster-reproject";
+import type {
+  InitialTriangulation,
+  ReprojectionFns,
+} from "@developmentseed/raster-reproject";
 import { RasterReprojector } from "@developmentseed/raster-reproject";
 import { splitFloat64Array } from "./fp64.js";
 import { buildUniformGridMesh } from "./globe-grid-mesh.js";
@@ -76,6 +79,14 @@ export interface RasterLayerProps extends CompositeLayerProps {
    */
   reprojectionFns: ReprojectionFns;
 
+  /**
+   * Optional seed triangulation for the reprojector — e.g. to clamp the mesh to
+   * a UV sub-region (such as the valid Web Mercator latitude band). Defaults to
+   * the full image. Must be reference-stable across renders to avoid
+   * regenerating the mesh every frame.
+   */
+  initialTriangulation?: InitialTriangulation;
+
   /**
    * The image to display. Accepts any luma.gl `TextureSource` (e.g. a URL,
    * `HTMLImageElement`, `ImageData`, etc.). deck.gl manages the texture
@@ -187,7 +198,8 @@ export class RasterLayer extends CompositeLayer<RasterLayerProps> {
       props.width !== oldProps.width ||
       props.height !== oldProps.height ||
       reprojectionFnsChanged ||
-      props.maxError !== oldProps.maxError;
+      props.maxError !== oldProps.maxError ||
+      props.initialTriangulation !== oldProps.initialTriangulation;
 
     if (needsMeshUpdate) {
       this._generateMesh();
@@ -199,6 +211,7 @@ export class RasterLayer extends CompositeLayer<RasterLayerProps> {
       width,
       height,
       reprojectionFns,
+      initialTriangulation,
       maxError = DEFAULT_MAX_ERROR,
     } = this.props;
 
@@ -238,6 +251,7 @@ export class RasterLayer extends CompositeLayer<RasterLayerProps> {
       reprojectionFns,
       width + 1,
       height + 1,
+      { initialTriangulation },
     );
     reprojector.run(maxError);
     const { indices, positions64High, positions64Low, texCoords } =
 
@@ -443,6 +443,11 @@ export class RasterTileLayer<
         renderPipeline,
         maxError,
         reprojectionFns,
+        // Web Mercator: clamp the mesh to the valid latitude band for tiles
+        // past ±85.051°. Globe renders the full mesh (it shows the poles).
+        initialTriangulation: isGlobe
+          ? undefined
+          : tile._webMercatorInitialTriangulation,
         debug,
         debugOpacity,
         coordinateSystem,
 
@@ -14,6 +14,7 @@ import type {
 } from "@deck.gl/geo-layers";
 import { _Tileset2D as Tileset2D } from "@deck.gl/geo-layers";
 import { transformBounds } from "@developmentseed/proj";
+import type { InitialTriangulation } from "@developmentseed/raster-reproject";
 import type { Matrix4 } from "@math.gl/core";
 import { BoundingVolumeCache } from "./bounding-volume-cache.js";
 import {
@@ -31,6 +32,7 @@ import type {
   TileIndex,
   ZRange,
 } from "./types.js";
+import { createInitialWebMercatorTriangulation } from "./web-mercator-clamp.js";
 
 /** Type returned by {@link RasterTileset2D.getTileMetadata} */
 export type RasterTileMetadata = {
@@ -99,6 +101,14 @@ export type RasterTileMetadata = {
    * as {@link RasterTileMetadata._projectPosition}.
    */
   _unprojectPosition: ProjectionFunction;
+
+  /**
+   * Seed triangulation that clamps this tile's reprojection mesh to the valid
+   * Web Mercator latitude band (±85.051°), or `undefined` if no clamp is needed.
+   * Consumed only by the Web Mercator render path; the globe path renders the
+   * full mesh. See {@link createInitialWebMercatorTriangulation}.
+   */
+  _webMercatorInitialTriangulation?: InitialTriangulation;
 };
 
 /**
@@ -373,6 +383,19 @@ export class RasterTileset2D extends Tileset2D {
     const { forwardTransform, inverseTransform } =
       levelDescriptor.tileTransform(x, y);
 
+    // Clamp the reprojection mesh to the valid Web Mercator latitude band for
+    // tiles that extend past ±85.051° (e.g. a global EPSG:4326 image reaching
+    // ±90°). Computed once here so the reference is stable across renders.
+    const cornerLat = (corner: [number, number]) =>
+      this.descriptor.projectTo4326(corner[0], corner[1])[1];
+    const _webMercatorInitialTriangulation =
+      createInitialWebMercatorTriangulation({
+        topLeft: cornerLat(topLeft),
+        topRight: cornerLat(topRight),
+        bottomLeft: cornerLat(bottomLeft),
+        bottomRight: cornerLat(bottomRight),
+      });
+
     return {
       bbox: {
         west,
@@ -393,6 +416,7 @@ export class RasterTileset2D extends Tileset2D {
       inverseTransform,
       _projectPosition: this.projectPosition,
       _unprojectPosition: this.unprojectPosition,
+      _webMercatorInitialTriangulation,
     };
   }
 }