Add coverage LOD strategy for tile layers

Author: charlieforward9

docs/api-reference/geo-layers/tile-layer.md

@@ -317,6 +317,19 @@ Apps may define a custom `refinementStrategy` by supplying its own callback func
 
 When called, the function receives an array of [Tile](#tile) instances representing every tile that is currently in the cache. It is an opportunity to manipulate `tile.isVisible` before sub layers are rendered. `isVisible` is initially set to the value of `isSelected` (equivalent to `refinementStrategy: 'never'`).
 
+#### `lodStrategy` (string, optional) {#lodstrategy}
+
+Controls whether the layer prefetches lower-resolution tiles to provide instant coverage during viewport transitions (panning, zooming, or camera flights).
+
+* `'none'`: Only request tiles at the current zoom level. No additional lower-resolution coverage tiles are fetched.
+* `'coverage'`: In addition to the current zoom tiles, prefetch ancestor tiles at progressively lower zoom levels. These low-resolution tiles load quickly and act as fallback coverage while higher-resolution tiles are still loading, reducing blank areas during navigation.
+
+Use `'coverage'` when your application involves frequent camera movement (e.g. animated transitions, user exploration) and visual continuity matters more than minimizing network requests. Use `'none'` when bandwidth is constrained, tiles are expensive to fetch, or the viewport is mostly static.
+
+Note: The `'coverage'` strategy is designed for geospatial views (e.g. `MapView`) where tiles follow a power-of-2 hierarchy. It is not recommended for non-geospatial views (e.g. `FirstPersonView`, `OrthographicView`) where viewport movement patterns differ and ancestor tiles may not provide meaningful coverage.
+
+- Default: `'none'`
+
 #### `maxRequests` (number, optional) {#maxrequests}
 
 The maximum number of concurrent `getTileData` calls.

modules/geo-layers/src/terrain-layer/terrain-layer.ts

@@ -355,7 +355,8 @@ export default class TerrainLayer<ExtraPropsT extends {} = {}> extends Composite
       onTileError,
       maxCacheSize,
       maxCacheByteSize,
-      refinementStrategy
+      refinementStrategy,
+      lodStrategy
     } = this.props;
 
     if (this.state.isTiled) {
@@ -387,7 +388,8 @@ export default class TerrainLayer<ExtraPropsT extends {} = {}> extends Composite
           onTileError,
           maxCacheSize,
           maxCacheByteSize,
-          refinementStrategy
+          refinementStrategy,
+          lodStrategy
         }
       );
     }

modules/geo-layers/src/tile-layer/tile-layer.ts

@@ -22,6 +22,7 @@ import {
   Tileset2D,
   Tile2DHeader,
   RefinementStrategy,
+  LODStrategy,
   STRATEGY_DEFAULT,
   Tileset2DProps
 } from '../tileset-2d/index';
@@ -47,6 +48,7 @@ const defaultProps: DefaultProps<TileLayerProps> = {
   maxCacheSize: null,
   maxCacheByteSize: null,
   refinementStrategy: STRATEGY_DEFAULT,
+  lodStrategy: 'none',
   zRange: null,
   maxRequests: 6,
   debounceTime: 0,
@@ -126,6 +128,13 @@ type _TileLayerProps<DataT> = {
    */
   refinementStrategy?: RefinementStrategy;
 
+  /**
+   * How the tile layer prefetches lower resolution coverage for smooth transitions.
+   *
+   * @default 'none'
+   */
+  lodStrategy?: LODStrategy;
+
   /** Range of minimum and maximum heights in the tile. */
   zRange?: ZRange | null;
 
@@ -261,6 +270,7 @@ export default class TileLayer<DataT = any, ExtraPropsT extends {} = {}> extends
       maxCacheSize,
       maxCacheByteSize,
       refinementStrategy,
+      lodStrategy,
       extent,
       maxZoom,
       minZoom,
@@ -278,6 +288,7 @@ export default class TileLayer<DataT = any, ExtraPropsT extends {} = {}> extends
       minZoom,
       tileSize,
       refinementStrategy,
+      lodStrategy,
       extent,
       maxRequests,
       debounceTime,

modules/geo-layers/src/tileset-2d/index.ts

@@ -13,8 +13,8 @@ export type {
   TileBoundingBox
 } from './types';
 
-export type {Tileset2DProps, RefinementStrategy} from './tileset-2d';
-export {Tileset2D, STRATEGY_DEFAULT} from './tileset-2d';
+export type {Tileset2DProps, RefinementStrategy, LODStrategy} from './tileset-2d';
+export {Tileset2D, STRATEGY_DEFAULT, LOD_STRATEGY_COVERAGE, LOD_STRATEGY_NONE} from './tileset-2d';
 
 export {Tile2DHeader} from './tile-2d-header';
 

modules/geo-layers/src/tileset-2d/tile-2d-header.ts

@@ -10,6 +10,7 @@ import type {Layer} from '@deck.gl/core';
 export type TileLoadDataProps<DataT = any> = {
   requestScheduler: RequestScheduler;
   getData: (props: TileLoadProps) => Promise<DataT>;
+  getPriority: (tile: Tile2DHeader<DataT>) => number;
   onLoad: (tile: Tile2DHeader<DataT>) => void;
   onError: (error: any, tile: Tile2DHeader<DataT>) => void;
 };
@@ -18,6 +19,7 @@ export class Tile2DHeader<DataT = any> {
   index: TileIndex;
   isVisible: boolean;
   isSelected: boolean;
+  isPrefetch: boolean;
   parent: Tile2DHeader | null;
   children: Tile2DHeader[] | null;
   content: DataT | null;
@@ -41,6 +43,7 @@ export class Tile2DHeader<DataT = any> {
     this.index = index;
     this.isVisible = false;
     this.isSelected = false;
+    this.isPrefetch = false;
     this.parent = null;
     this.children = [];
 
@@ -106,6 +109,7 @@ export class Tile2DHeader<DataT = any> {
   /* eslint-disable max-statements */
   private async _loadData({
     getData,
+    getPriority,
     requestScheduler,
     onLoad,
     onError
@@ -116,10 +120,8 @@ export class Tile2DHeader<DataT = any> {
     this._abortController = new AbortController();
     const {signal} = this._abortController;
 
-    // @ts-expect-error (2345) Argument of type '(tile: any) => 1 | -1' is not assignable ...
-    const requestToken = await requestScheduler.scheduleRequest(this, tile => {
-      return tile.isSelected ? 1 : -1;
-    });
+    // @ts-expect-error (2345) loaders.gl's RequestScheduler callback type is too narrow.
+    const requestToken = await requestScheduler.scheduleRequest(this, getPriority);
 
     if (!requestToken) {
       this._isCancelled = true;