Implement offscreen rendering with workers

Move the work of drawing the PDF onto the cavas to a worker thread
using OffscreenCanvas. This should free up the main thread a bit by
moving all of the CanvasGraphics operations to this "renderer" worker.

src/core/document.js

@@ -129,10 +129,11 @@ class Page {
     };
   }
 
-  #createPartialEvaluator(handler) {
+  #createPartialEvaluator(handler, rendererHandler) {
     return new PartialEvaluator({
       xref: this.xref,
       handler,
+      rendererHandler,
       pageIndex: this.pageIndex,
       idFactory: this._localIdFactory,
       fontCache: this.fontCache,
@@ -438,6 +439,7 @@ class Page {
 
   async getOperatorList({
     handler,
+    rendererHandler,
     sink,
     task,
     intent,
@@ -448,7 +450,10 @@ class Page {
     const contentStreamPromise = this.getContentStream();
     const resourcesPromise = this.loadResources(RESOURCES_KEYS_OPERATOR_LIST);
 
-    const partialEvaluator = this.#createPartialEvaluator(handler);
+    const partialEvaluator = this.#createPartialEvaluator(
+      handler,
+      rendererHandler
+    );
 
     const newAnnotsByPage = !this.xfaFactory
       ? getNewAnnotationsMap(annotationStorage)
@@ -1331,7 +1336,7 @@ class PDFDocument {
     this.xfaFactory.setImages(xfaImages);
   }
 
-  async #loadXfaFonts(handler, task) {
+  async #loadXfaFonts(handler, task, rendererHandler) {
     const acroForm = await this.pdfManager.ensureCatalog("acroForm");
     if (!acroForm) {
       return;
@@ -1357,6 +1362,7 @@ class PDFDocument {
     const partialEvaluator = new PartialEvaluator({
       xref: this.xref,
       handler,
+      rendererHandler,
       pageIndex: -1,
       idFactory: this._globalIdFactory,
       fontCache,
@@ -1469,9 +1475,9 @@ class PDFDocument {
     this.xfaFactory.appendFonts(pdfFonts, reallyMissingFonts);
   }
 
-  loadXfaResources(handler, task) {
+  loadXfaResources(handler, task, rendererHandler) {
     return Promise.all([
-      this.#loadXfaFonts(handler, task).catch(() => {
+      this.#loadXfaFonts(handler, task, rendererHandler).catch(() => {
         // Ignore errors, to allow the document to load.
       }),
       this.#loadXfaImages(),

src/core/evaluator.js

@@ -222,6 +222,7 @@ class PartialEvaluator {
   constructor({
     xref,
     handler,
+    rendererHandler,
     pageIndex,
     idFactory,
     fontCache,
@@ -234,6 +235,7 @@ class PartialEvaluator {
   }) {
     this.xref = xref;
     this.handler = handler;
+    this.rendererHandler = rendererHandler;
     this.pageIndex = pageIndex;
     this.idFactory = idFactory;
     this.fontCache = fontCache;
@@ -553,13 +555,19 @@ class PartialEvaluator {
     const transfers = imgData ? [imgData.bitmap || imgData.data.buffer] : null;
 
     if (this.parsingType3Font || cacheGlobally) {
-      return this.handler.send(
+      this.handler.send("commonobj", [objId, "Image", imgData], transfers);
+      return this.rendererHandler.send(
         "commonobj",
         [objId, "Image", imgData],
         transfers
       );
     }
-    return this.handler.send(
+    this.handler.send(
+      "obj",
+      [objId, this.pageIndex, "Image", imgData],
+      transfers
+    );
+    return this.rendererHandler.send(
       "obj",
       [objId, this.pageIndex, "Image", imgData],
       transfers
@@ -787,11 +795,10 @@ class PartialEvaluator {
       // globally, check if the image is still cached locally on the main-thread
       // to avoid having to re-parse the image (since that can be slow).
       if (w * h > 250000 || hasMask) {
-        const localLength = await this.handler.sendWithPromise("commonobj", [
+        const localLength = await this.rendererHandler.sendWithPromise(
-          objId,
+          "commonobj",
-          "CopyLocalImage",
+          [objId, "CopyLocalImage", { imageRef }]
-          { imageRef },
+        );
-        ]);
 
         if (localLength) {
           this.globalImageCache.setData(imageRef, globalCacheData);
@@ -1021,6 +1028,7 @@ class PartialEvaluator {
 
     state.font = translated.font;
     translated.send(this.handler);
+    translated.send(this.rendererHandler);
     return translated.loadedName;
   }
 
@@ -1040,7 +1048,7 @@ class PartialEvaluator {
         PartialEvaluator.buildFontPaths(
           font,
           glyphs,
-          this.handler,
+          this.rendererHandler,
           this.options
         );
       }
@@ -1518,8 +1526,15 @@ class PartialEvaluator {
 
     if (this.parsingType3Font) {
       this.handler.send("commonobj", [id, "Pattern", patternIR]);
+      this.rendererHandler.send("commonobj", [id, "Pattern", patternIR]);
     } else {
       this.handler.send("obj", [id, this.pageIndex, "Pattern", patternIR]);
+      this.rendererHandler.send("obj", [
+        id,
+        this.pageIndex,
+        "Pattern",
+        patternIR,
+      ]);
     }
     return id;
   }

src/core/worker.js

@@ -82,6 +82,8 @@ class WorkerMessageHandler {
 
   static setup(handler, port) {
     let testMessageProcessed = false;
+    let rendererHandler = null;
+
     handler.on("test", data => {
       if (testMessageProcessed) {
         return; // we already processed 'test' message once
@@ -94,12 +96,19 @@ class WorkerMessageHandler {
 
     handler.on("configure", data => {
       setVerbosityLevel(data.verbosity);
+      rendererHandler = new MessageHandler(
+        "worker-channel",
+        "renderer-channel",
+        data.channelPort
+      );
     });
 
-    handler.on("GetDocRequest", data => this.createDocumentHandler(data, port));
+    handler.on("GetDocRequest", data =>
+      this.createDocumentHandler(data, port, rendererHandler)
+    );
   }
 
-  static createDocumentHandler(docParams, port) {
+  static createDocumentHandler(docParams, port, rendererHandler) {
     // This context is actually holds references on pdfManager and handler,
     // until the latter is destroyed.
     let pdfManager;
@@ -173,7 +182,11 @@ class WorkerMessageHandler {
         const task = new WorkerTask("loadXfaResources");
         startWorkerTask(task);
 
-        await pdfManager.ensureDoc("loadXfaResources", [handler, task]);
+        await pdfManager.ensureDoc("loadXfaResources", [
+          handler,
+          task,
+          rendererHandler,
+        ]);
         finishWorkerTask(task);
       }
 
@@ -776,6 +789,7 @@ class WorkerMessageHandler {
         page
           .getOperatorList({
             handler,
+            rendererHandler,
             sink,
             task,
             intent: data.intent,
@@ -859,8 +873,8 @@ class WorkerMessageHandler {
         .then(page => pdfManager.ensure(page, "getStructTree"));
     });
 
-    handler.on("FontFallback", function (data) {
+    rendererHandler.on("FontFallback", function (data) {
-      return pdfManager.fontFallback(data.id, handler);
+      return pdfManager.fontFallback(data.id, rendererHandler);
     });
 
     handler.on("Cleanup", function (data) {

src/display/api.js

@@ -41,6 +41,7 @@ import {
   isDataScheme,
   isValidFetchUrl,
   PageViewport,
+  PDFObjects,
   RenderingCancelledException,
   StatTimer,
 } from "./display_utils.js";
@@ -75,7 +76,6 @@ import { PDFDataTransportStream } from "./transport_stream.js";
 import { PDFFetchStream } from "display-fetch_stream";
 import { PDFNetworkStream } from "display-network";
 import { PDFNodeStream } from "display-node_stream";
-import { PDFObjects } from "./pdf_objects.js";
 import { TextLayer } from "./text_layer.js";
 import { XfaText } from "./xfa_text.js";
 
@@ -390,16 +390,23 @@ function getDocument(src = {}) {
         : new WasmFactory({ baseUrl: wasmUrl }),
   };
 
+  const workerChannel = new MessageChannel();
+
   if (!worker) {
     // Worker was not provided -- creating and owning our own. If message port
     // is specified in global worker options, using it.
     worker = PDFWorker.create({
       verbosity,
       port: GlobalWorkerOptions.workerPort,
+      channelPort: workerChannel.port1,
     });
     task._worker = worker;
   }
 
+  const renderer = new RendererWorker(workerChannel.port2, enableHWA);
+  const rendererHandler = renderer.handler;
+  task.renderer = renderer;
+
   const docParams = {
     docId,
     apiVersion:
@@ -504,10 +511,12 @@ function getDocument(src = {}) {
           networkStream,
           transportParams,
           transportFactory,
-          enableHWA
+          enableHWA,
+          rendererHandler
         );
         task._transport = transport;
         messageHandler.send("Ready", null);
+        rendererHandler.send("Ready", null);
       });
     })
     .catch(task._capability.reject);
@@ -544,6 +553,8 @@ class PDFDocumentLoadingTask {
    */
   _worker = null;
 
+  #renderer = null;
+
   /**
    * Unique identifier for the document loading task.
    * @type {string}
@@ -603,6 +614,8 @@ class PDFDocumentLoadingTask {
 
     this._worker?.destroy();
     this._worker = null;
+    this.#renderer.destroy();
+    this.#renderer = null;
   }
 
   /**
@@ -614,6 +627,17 @@ class PDFDocumentLoadingTask {
   async getData() {
     return this._transport.getData();
   }
+
+  get renderer() {
+    return this.#renderer;
+  }
+
+  set renderer(renderer) {
+    if (this.#renderer) {
+      throw new Error("PDFDocumentLoadingTask.renderer: already set.");
+    }
+    this.#renderer = renderer;
+  }
 }
 
 /**
@@ -1574,7 +1598,7 @@ class PDFPageProxy {
       useRequestAnimationFrame: !intentPrint,
       pdfBug: this._pdfBug,
       pageColors,
-      enableHWA: this._transport.enableHWA,
+      rendererHandler: this._transport.rendererHandler,
       operationsFilter,
     });
 
@@ -1992,6 +2016,12 @@ class PDFPageProxy {
   get stats() {
     return this._stats;
   }
+
+  resetCanvas(taskID) {
+    this._transport.rendererHandler.send("resetCanvas", {
+      taskID,
+    });
+  }
 }
 
 /**
@@ -2000,6 +2030,8 @@ class PDFPageProxy {
  * @property {Worker} [port] - The `workerPort` object.
  * @property {number} [verbosity] - Controls the logging level;
  *   the constants from {@link VerbosityLevel} should be used.
+ * @property {MessagePort} [channelPort] - The channel port to use for
+ *   communication with the renderer thread.
  */
 
 /**
@@ -2081,10 +2113,12 @@ class PDFWorker {
     name = null,
     port = null,
     verbosity = getVerbosityLevel(),
+    channelPort = null,
   } = {}) {
     this.name = name;
     this.destroyed = false;
     this.verbosity = verbosity;
+    this.channelPort = channelPort;
 
     if (port) {
       if (PDFWorker.#workerPorts.has(port)) {
@@ -2117,9 +2151,14 @@ class PDFWorker {
   #resolve() {
     this.#capability.resolve();
     // Send global setting, e.g. verbosity level.
-    this.#messageHandler.send("configure", {
+    this.#messageHandler.send(
-      verbosity: this.verbosity,
+      "configure",
-    });
+      {
+        verbosity: this.verbosity,
+        channelPort: this.channelPort,
+      },
+      [this.channelPort]
+    );
   }
 
   /**
@@ -2355,6 +2394,34 @@ class PDFWorker {
   }
 }
 
+class RendererWorker {
+  #worker;
+
+  #handler;
+
+  constructor(channelPort, enableHWA) {
+    this.#worker = new Worker("../src/display/renderer_worker.js", {
+      type: "module",
+    });
+    this.#handler = new MessageHandler("main", "renderer", this.#worker);
+    this.#handler.send("configure", { channelPort, enableHWA }, [channelPort]);
+    this.#handler.on("ready", () => {
+      // DO NOTHING
+    });
+  }
+
+  get handler() {
+    return this.#handler;
+  }
+
+  destroy() {
+    this.#worker.terminate();
+    this.#worker = null;
+    this.#handler.destroy();
+    this.#handler = null;
+  }
+}
+
 /**
  * For internal use only.
  * @ignore
@@ -2376,9 +2443,11 @@ class WorkerTransport {
     networkStream,
     params,
     factory,
-    enableHWA
+    enableHWA,
+    rendererHandler
   ) {
     this.messageHandler = messageHandler;
+    this.rendererHandler = rendererHandler;
     this.loadingTask = loadingTask;
     this.commonObjs = new PDFObjects();
     this.fontLoader = new FontLoader({
@@ -2544,8 +2613,13 @@ class WorkerTransport {
     const terminated = this.messageHandler.sendWithPromise("Terminate", null);
     waitOn.push(terminated);
 
+    const terminatedRenderer = this.rendererHandler.sendWithPromise(
+      "Terminate",
+      null
+    );
+    waitOn.push(terminatedRenderer);
+
     Promise.all(waitOn).then(() => {
-      this.commonObjs.clear();
       this.fontLoader.clear();
       this.#methodPromises.clear();
       this.filterFactory.destroy();
@@ -2564,7 +2638,13 @@ class WorkerTransport {
   }
 
   setupMessageHandler() {
-    const { messageHandler, loadingTask } = this;
+    const { messageHandler, loadingTask, rendererHandler } = this;
+
+    rendererHandler.on("continue", ({ taskID, arg }) => {
+      const continueFn = InternalRenderTask.continueFnMap.get(taskID);
+      assert(continueFn, `No continue function for taskID: ${taskID}`);
+      continueFn.call(arg);
+    });
 
     messageHandler.on("GetReader", (data, sink) => {
       assert(
@@ -3166,6 +3246,10 @@ class RenderTask {
       (separateAnnots.canvas && annotationCanvasMap?.size > 0)
     );
   }
+
+  get taskID() {
+    return this.#internalRenderTask.taskID;
+  }
 }
 
 /**
@@ -3177,6 +3261,10 @@ class InternalRenderTask {
 
   static #canvasInUse = new WeakSet();
 
+  static #taskCounter = 0n;
+
+  static continueFnMap = new Map();
+
   constructor({
     callback,
     params,
@@ -3190,9 +3278,10 @@ class InternalRenderTask {
     useRequestAnimationFrame = false,
     pdfBug = false,
     pageColors = null,
-    enableHWA = false,
+    rendererHandler,
     operationsFilter = null,
   }) {
+    this.taskID = InternalRenderTask.#taskCounter++;
     this.callback = callback;
     this.params = params;
     this.objs = objs;
@@ -3221,7 +3310,9 @@ class InternalRenderTask {
     this._nextBound = this._next.bind(this);
     this._canvas = params.canvas;
     this._canvasContext = params.canvas ? null : params.canvasContext;
-    this._enableHWA = enableHWA;
+    this._renderInWorker = this._canvasContext === null;
+    this.rendererHandler = rendererHandler;
+    InternalRenderTask.continueFnMap.set(this.taskID, this._continueBound);
     this._dependencyTracker = params.dependencyTracker;
     this._operationsFilter = operationsFilter;
   }
@@ -3256,31 +3347,47 @@ class InternalRenderTask {
     const { viewport, transform, background, dependencyTracker } = this.params;
 
     // When printing in Firefox, we get a specific context in mozPrintCallback
-    // which cannot be created from the canvas itself.
+    // which cannot be created from the canvas itself. In this case, we don't
-    const canvasContext =
+    // render in the worker and use the context directly.
-      this._canvasContext ||
+    if (this._renderInWorker) {
-      this._canvas.getContext("2d", {
+      const offscreen = this._canvas.transferControlToOffscreen();
-        alpha: false,
+      this.rendererHandler.send(
-        willReadFrequently: !this._enableHWA,
+        "init",
+        {
+          pageIndex: this._pageIndex,
+          canvas: offscreen,
+          map: this.annotationCanvasMap,
+          colors: this.pageColors,
+          taskID: this.taskID,
+          transform,
+          viewport,
+          transparency,
+          background,
+          optionalContentConfig,
+          dependencyTracker,
+        },
+        [offscreen]
+      );
+    } else {
+      this.gfx = new CanvasGraphics(
+        this._canvasContext,
+        this.commonObjs,
+        this.objs,
+        this.canvasFactory,
+        this.filterFactory,
+        { optionalContentConfig },
+        this.annotationCanvasMap,
+        this.pageColors,
+        dependencyTracker
+      );
+      this.gfx.beginDrawing({
+        transform,
+        viewport,
+        transparency,
+        background,
       });
+    }
 
-    this.gfx = new CanvasGraphics(
-      canvasContext,
-      this.commonObjs,
-      this.objs,
-      this.canvasFactory,
-      this.filterFactory,
-      { optionalContentConfig },
-      this.annotationCanvasMap,
-      this.pageColors,
-      dependencyTracker
-    );
-    this.gfx.beginDrawing({
-      transform,
-      viewport,
-      transparency,
-      background,
-    });
     this.operatorListIdx = 0;
     this.graphicsReady = true;
     this.graphicsReadyCallback?.();
@@ -3289,7 +3396,12 @@ class InternalRenderTask {
   cancel(error = null, extraDelay = 0) {
     this.running = false;
     this.cancelled = true;
-    this.gfx?.endDrawing();
+    if (this._renderInWorker) {
+      this.rendererHandler.send("end", { taskID: this.taskID });
+    } else {
+      this.gfx.endDrawing();
+    }
+    InternalRenderTask.continueFnMap.delete(this.taskID);
     if (this.#rAF) {
       window.cancelAnimationFrame(this.#rAF);
       this.#rAF = null;
@@ -3348,19 +3460,35 @@ class InternalRenderTask {
     if (this.cancelled) {
       return;
     }
-    this.operatorListIdx = this.gfx.executeOperatorList(
+    const { operatorList, operatorListIdx, taskID } = this;
-      this.operatorList,
+    if (this._renderInWorker) {
-      this.operatorListIdx,
+      this.operatorListIdx = await this.rendererHandler.sendWithPromise(
-      this._continueBound,
+        "execute",
-      this.stepper,
+        {
-      this._operationsFilter
+          operatorList,
-    );
+          operatorListIdx,
+          taskID,
+          operationsFilter: this._operationsFilter,
+        }
+      );
+    } else {
+      this.operatorListIdx = this.gfx.executeOperatorList(
+        this.operatorList,
+        this.operatorListIdx,
+        this._continueBound,
+        this.stepper,
+        this._operationsFilter
+      );
+    }
     if (this.operatorListIdx === this.operatorList.argsArray.length) {
       this.running = false;
       if (this.operatorList.lastChunk) {
-        this.gfx.endDrawing();
+        if (this._renderInWorker) {
+          this.rendererHandler.send("end", { taskID });
+        } else {
+          this.gfx.endDrawing();
+        }
         InternalRenderTask.#canvasInUse.delete(this._canvas);
-
         this.callback();
       }
     }

src/display/canvas_factory.js

@@ -89,4 +89,14 @@ class DOMCanvasFactory extends BaseCanvasFactory {
   }
 }
 
-export { BaseCanvasFactory, DOMCanvasFactory };
+class OffscreenCanvasFactory extends BaseCanvasFactory {
+  constructor({ enableHWA = false }) {
+    super({ enableHWA });
+  }
+
+  _createCanvas(width, height) {
+    return new OffscreenCanvas(width, height);
+  }
+}
+
+export { BaseCanvasFactory, DOMCanvasFactory, OffscreenCanvasFactory };

src/display/display_utils.js

@@ -995,6 +995,115 @@ function renderRichText({ html, dir, className }, container) {
   container.append(fragment);
 }
 
+const INITIAL_DATA = Symbol("INITIAL_DATA");
+
+/**
+ * A PDF document and page is built of many objects. E.g. there are objects for
+ * fonts, images, rendering code, etc. These objects may get processed inside of
+ * a worker. This class implements some basic methods to manage these objects.
+ */
+class PDFObjects {
+  #objs = Object.create(null);
+
+  /**
+   * Ensures there is an object defined for `objId`.
+   *
+   * @param {string} objId
+   * @returns {Object}
+   */
+  #ensureObj(objId) {
+    return (this.#objs[objId] ||= {
+      ...Promise.withResolvers(),
+      data: INITIAL_DATA,
+    });
+  }
+
+  /**
+   * If called *without* callback, this returns the data of `objId` but the
+   * object needs to be resolved. If it isn't, this method throws.
+   *
+   * If called *with* a callback, the callback is called with the data of the
+   * object once the object is resolved. That means, if you call this method
+   * and the object is already resolved, the callback gets called right away.
+   *
+   * @param {string} objId
+   * @param {function} [callback]
+   * @returns {any}
+   */
+  get(objId, callback = null) {
+    // If there is a callback, then the get can be async and the object is
+    // not required to be resolved right now.
+    if (callback) {
+      const obj = this.#ensureObj(objId);
+      obj.promise.then(() => callback(obj.data));
+      return null;
+    }
+    // If there isn't a callback, the user expects to get the resolved data
+    // directly.
+    const obj = this.#objs[objId];
+    // If there isn't an object yet or the object isn't resolved, then the
+    // data isn't ready yet!
+    if (!obj || obj.data === INITIAL_DATA) {
+      throw new Error(`Requesting object that isn't resolved yet ${objId}.`);
+    }
+    return obj.data;
+  }
+
+  /**
+   * @param {string} objId
+   * @returns {boolean}
+   */
+  has(objId) {
+    const obj = this.#objs[objId];
+    return !!obj && obj.data !== INITIAL_DATA;
+  }
+
+  /**
+   * @param {string} objId
+   * @returns {boolean}
+   */
+  delete(objId) {
+    const obj = this.#objs[objId];
+    if (!obj || obj.data === INITIAL_DATA) {
+      // Only allow removing the object *after* it's been resolved.
+      return false;
+    }
+    delete this.#objs[objId];
+    return true;
+  }
+
+  /**
+   * Resolves the object `objId` with optional `data`.
+   *
+   * @param {string} objId
+   * @param {any} [data]
+   */
+  resolve(objId, data = null) {
+    const obj = this.#ensureObj(objId);
+    obj.data = data;
+    obj.resolve();
+  }
+
+  clear() {
+    for (const objId in this.#objs) {
+      const { data } = this.#objs[objId];
+      data?.bitmap?.close(); // Release any `ImageBitmap` data.
+    }
+    this.#objs = Object.create(null);
+  }
+
+  *[Symbol.iterator]() {
+    for (const objId in this.#objs) {
+      const { data } = this.#objs[objId];
+
+      if (data === INITIAL_DATA) {
+        continue;
+      }
+      yield [objId, data];
+    }
+  }
+}
+
 export {
   applyOpacity,
   ColorScheme,
@@ -1016,6 +1125,7 @@ export {
   OutputScale,
   PageViewport,
   PDFDateString,
+  PDFObjects,
   PixelsPerInch,
   RenderingCancelledException,
   renderRichText,

src/display/renderer_worker.js

@@ -0,0 +1,243 @@
+import { assert, warn } from "../shared/util.js";
+import { FontFaceObject, FontLoader } from "./font_loader.js";
+import { CanvasGraphics } from "./canvas.js";
+import { DOMFilterFactory } from "./filter_factory.js";
+import { MessageHandler } from "../shared/message_handler.js";
+import { OffscreenCanvasFactory } from "./canvas_factory.js";
+import { PDFObjects } from "./display_utils.js";
+
+class RendererMessageHandler {
+  static #commonObjs = new PDFObjects();
+
+  static #objsMap = new Map();
+
+  static #tasks = new Map();
+
+  static #fontLoader = new FontLoader({
+    ownerDocument: self,
+  });
+
+  static #canvasFactory;
+
+  static #filterFactory;
+
+  static #enableHWA = false;
+
+  static {
+    this.initializeFromPort(self);
+  }
+
+  static pageObjs(pageIndex) {
+    let objs = this.#objsMap.get(pageIndex);
+    if (!objs) {
+      objs = new PDFObjects();
+      this.#objsMap.set(pageIndex, objs);
+    }
+    return objs;
+  }
+
+  static initializeFromPort(port) {
+    let terminated = false;
+    let mainHandler = new MessageHandler("renderer", "main", port);
+    mainHandler.send("ready", null);
+    mainHandler.on("Ready", function () {
+      // DO NOTHING
+    });
+
+    mainHandler.on("configure", ({ channelPort, enableHWA }) => {
+      this.#enableHWA = enableHWA;
+      const workerHandler = new MessageHandler(
+        "renderer-channel",
+        "worker-channel",
+        channelPort
+      );
+      this.#canvasFactory = new OffscreenCanvasFactory({
+        enableHWA,
+      });
+      this.#filterFactory = new DOMFilterFactory({});
+
+      workerHandler.on("commonobj", ([id, type, data]) => {
+        if (terminated) {
+          throw new Error("Renderer worker has been terminated.");
+        }
+        this.handleCommonObj(id, type, data, workerHandler);
+      });
+
+      workerHandler.on("obj", ([id, pageIndex, type, data]) => {
+        if (terminated) {
+          throw new Error("Renderer worker has been terminated.");
+        }
+        this.handleObj(pageIndex, id, type, data);
+      });
+    });
+
+    mainHandler.on(
+      "init",
+      ({
+        pageIndex,
+        canvas,
+        map,
+        colors,
+        taskID,
+        transform,
+        viewport,
+        transparency,
+        background,
+        optionalContentConfig,
+      }) => {
+        assert(!this.#tasks.has(taskID), "Task already initialized");
+        const ctx = canvas.getContext("2d", {
+          alpha: false,
+          willReadFrequently: this.#enableHWA,
+        });
+        const objs = this.pageObjs(pageIndex);
+        const gfx = new CanvasGraphics(
+          ctx,
+          this.#commonObjs,
+          objs,
+          this.#canvasFactory,
+          this.#filterFactory,
+          { optionalContentConfig },
+          map,
+          colors
+        );
+        gfx.beginDrawing({ transform, viewport, transparency, background });
+        this.#tasks.set(taskID, { canvas, gfx });
+      }
+    );
+
+    mainHandler.on(
+      "execute",
+      async ({ operatorList, operatorListIdx, taskID }) => {
+        if (terminated) {
+          throw new Error("Renderer worker has been terminated.");
+        }
+        const task = this.#tasks.get(taskID);
+        assert(task !== undefined, "Task not initialized");
+        return task.gfx.executeOperatorList(
+          operatorList,
+          operatorListIdx,
+          arg => mainHandler.send("continue", { taskID, arg })
+        );
+      }
+    );
+
+    mainHandler.on("end", ({ taskID }) => {
+      if (terminated) {
+        throw new Error("Renderer worker has been terminated.");
+      }
+      const task = this.#tasks.get(taskID);
+      assert(task !== undefined, "Task not initialized");
+      task.gfx.endDrawing();
+    });
+
+    mainHandler.on("resetCanvas", ({ taskID }) => {
+      if (terminated) {
+        throw new Error("Renderer worker has been terminated.");
+      }
+      const task = this.#tasks.get(taskID);
+      assert(task !== undefined, "Task not initialized");
+      const canvas = task.canvas;
+      canvas.width = canvas.height = 0;
+    });
+
+    mainHandler.on("Terminate", async () => {
+      terminated = true;
+      this.#commonObjs.clear();
+      for (const objs of this.#objsMap.values()) {
+        objs.clear();
+      }
+      this.#objsMap.clear();
+      this.#tasks.clear();
+      this.#fontLoader.clear();
+      mainHandler.destroy();
+      mainHandler = null;
+    });
+  }
+
+  static handleCommonObj(id, type, exportedData, handler) {
+    if (this.#commonObjs.has(id)) {
+      return null;
+    }
+
+    switch (type) {
+      case "Font":
+        if ("error" in exportedData) {
+          const exportedError = exportedData.error;
+          warn(`Error during font loading: ${exportedError}`);
+          this.#commonObjs.resolve(id, exportedError);
+          break;
+        }
+
+        // TODO: Make FontInspector work again.
+        const inspectFont = null;
+        // this._params.pdfBug && globalThis.FontInspector?.enabled
+        //   ? (font, url) => globalThis.FontInspector.fontAdded(font, url)
+        // : null;
+        const font = new FontFaceObject(exportedData, inspectFont);
+
+        this.#fontLoader
+          .bind(font)
+          .catch(() => handler.sendWithPromise("FontFallback", { id }))
+          .finally(() => {
+            if (!font.fontExtraProperties && font.data) {
+              // Immediately release the `font.data` property once the font
+              // has been attached to the DOM, since it's no longer needed,
+              // rather than waiting for a `PDFDocumentProxy.cleanup` call.
+              // Since `font.data` could be very large, e.g. in some cases
+              // multiple megabytes, this will help reduce memory usage.
+              font.data = null;
+            }
+            this.#commonObjs.resolve(id, font);
+          });
+        break;
+      case "CopyLocalImage":
+        const { imageRef } = exportedData;
+        assert(imageRef, "The imageRef must be defined.");
+
+        for (const objs of this.#objsMap.values()) {
+          for (const [, data] of objs) {
+            if (data?.ref !== imageRef) {
+              continue;
+            }
+            if (!data.dataLen) {
+              return null;
+            }
+            this.#commonObjs.resolve(id, structuredClone(data));
+            return data.dataLen;
+          }
+        }
+        break;
+      case "FontPath":
+      case "Image":
+      case "Pattern":
+        this.#commonObjs.resolve(id, exportedData);
+        break;
+      default:
+        throw new Error(`Got unknown common object type ${type}`);
+    }
+
+    return null;
+  }
+
+  static handleObj(pageIndex, id, type, exportedData) {
+    const objs = this.pageObjs(pageIndex);
+
+    if (objs.has(id)) {
+      return;
+    }
+
+    switch (type) {
+      case "Image":
+      case "Pattern":
+        objs.resolve(id, exportedData);
+        break;
+      default:
+        throw new Error(
+          `Got unknown object type ${type} id ${id} for page ${pageIndex} data ${JSON.stringify(exportedData)}`
+        );
+    }
+  }
+}
+
+export { RendererMessageHandler };

src/shared/message_handler.js

@@ -90,6 +90,7 @@ class MessageHandler {
     comObj.addEventListener("message", this.#onMessage.bind(this), {
       signal: this.#messageAC.signal,
     });
+    comObj.start?.();
   }
 
   #onMessage({ data }) {

web/base_pdf_page_view.js

@@ -50,6 +50,8 @@ class BasePDFPageView {
 
   renderTask = null;
 
+  renderTaskID = null;
+
   resume = null;
 
   constructor(options) {
@@ -162,7 +164,7 @@ class BasePDFPageView {
 
       if (prevCanvas) {
         prevCanvas.replaceWith(canvas);
-        prevCanvas.width = prevCanvas.height = 0;
+        this.pdfPage.resetCanvas(this.renderTaskID);
       } else {
         onShow(canvas);
       }
@@ -190,7 +192,7 @@ class BasePDFPageView {
       return;
     }
     canvas.remove();
-    canvas.width = canvas.height = 0;
+    this.pdfPage.resetCanvas(this.renderTaskID);
     this.canvas = null;
     this.#resetTempCanvas();
   }
@@ -212,6 +214,8 @@ class BasePDFPageView {
       }
     };
 
+    this.renderTaskID = renderTask.taskID;
+
     let error = null;
     try {
       await renderTask.promise;

web/pdf_thumbnail_view.js

@@ -303,7 +303,7 @@ class PDFThumbnailView {
       await renderTask.promise;
     } catch (e) {
       if (e instanceof RenderingCancelledException) {
-        zeroCanvas(canvas);
+        pdfPage.resetCanvas(renderTask.taskID);
         return;
       }
       error = e;
@@ -318,7 +318,7 @@ class PDFThumbnailView {
     this.renderingState = RenderingStates.FINISHED;
 
     this.#convertCanvasToImage(canvas);
-    zeroCanvas(canvas);
+    pdfPage.resetCanvas(renderTask.taskID);
 
     this.eventBus.dispatch("thumbnailrendered", {
       source: this,