diff --git a/examples/raster.js b/examples/raster.js
index feeb037888..d86e9d83c3 100644
--- a/examples/raster.js
+++ b/examples/raster.js
@@ -5,7 +5,7 @@ import XYZ from '../src/ol/source/XYZ.js';
 import {Image as ImageLayer, Tile as TileLayer} from '../src/ol/layer.js';
 
 const minVgi = 0;
-const maxVgi = 0.25;
+const maxVgi = 0.5;
 const bins = 10;
 
 /**
@@ -87,7 +87,7 @@ const raster = new RasterSource({
     summarize: summarize,
   },
 });
-raster.set('threshold', 0.1);
+raster.set('threshold', 0.25);
 
 function createCounts(min, max, num) {
   const values = new Array(num);
diff --git a/src/ol/source/Raster.js b/src/ol/source/Raster.js
index 22898169ca..3528ea3b51 100644
--- a/src/ol/source/Raster.js
+++ b/src/ol/source/Raster.js
@@ -46,12 +46,21 @@ export function newImageData(data, width, height) {
   return imageData;
 }
 
+/**
+ * @typedef {Object} MinionData
+ * @property {Array<ArrayBuffer>} buffers Array of buffers.
+ * @property {Object} meta Operation metadata.
+ * @property {boolean} imageOps The operation is an image operation.
+ * @property {number} width The width of the image.
+ * @property {number} height The height of the image.
+ */
+
 /* istanbul ignore next */
 /**
  * Create a function for running operations.  This function is serialized for
  * use in a worker.
  * @param {function(Array, Object):*} operation The operation.
- * @return {function(Object):ArrayBuffer} A function that takes an object with
+ * @return {function(MinionData):ArrayBuffer} A function that takes an object with
  * buffers, meta, imageOps, width, and height properties and returns an array
  * buffer.
  */
@@ -81,40 +90,40 @@ function createMinion(operation) {
 
     const numBuffers = buffers.length;
     const numBytes = buffers[0].byteLength;
-    let output, b;
 
     if (imageOps) {
       const images = new Array(numBuffers);
-      for (b = 0; b < numBuffers; ++b) {
+      for (let b = 0; b < numBuffers; ++b) {
         images[b] = newWorkerImageData(
           new Uint8ClampedArray(buffers[b]),
           width,
           height
         );
       }
-      output = operation(images, meta).data;
-    } else {
-      output = new Uint8ClampedArray(numBytes);
-      const arrays = new Array(numBuffers);
-      const pixels = new Array(numBuffers);
-      for (b = 0; b < numBuffers; ++b) {
-        arrays[b] = new Uint8ClampedArray(buffers[b]);
-        pixels[b] = [0, 0, 0, 0];
-      }
-      for (let i = 0; i < numBytes; i += 4) {
-        for (let j = 0; j < numBuffers; ++j) {
-          const array = arrays[j];
-          pixels[j][0] = array[i];
-          pixels[j][1] = array[i + 1];
-          pixels[j][2] = array[i + 2];
-          pixels[j][3] = array[i + 3];
-        }
-        const pixel = operation(pixels, meta);
-        output[i] = pixel[0];
-        output[i + 1] = pixel[1];
-        output[i + 2] = pixel[2];
-        output[i + 3] = pixel[3];
+      const output = operation(images, meta).data;
+      return output.buffer;
+    }
+
+    const output = new Uint8ClampedArray(numBytes);
+    const arrays = new Array(numBuffers);
+    const pixels = new Array(numBuffers);
+    for (let b = 0; b < numBuffers; ++b) {
+      arrays[b] = new Uint8ClampedArray(buffers[b]);
+      pixels[b] = [0, 0, 0, 0];
+    }
+    for (let i = 0; i < numBytes; i += 4) {
+      for (let j = 0; j < numBuffers; ++j) {
+        const array = arrays[j];
+        pixels[j][0] = array[i];
+        pixels[j][1] = array[i + 1];
+        pixels[j][2] = array[i + 2];
+        pixels[j][3] = array[i + 3];
       }
+      const pixel = operation(pixels, meta);
+      output[i] = pixel[0];
+      output[i + 1] = pixel[1];
+      output[i + 2] = pixel[2];
+      output[i + 3] = pixel[3];
     }
     return output.buffer;
   };
@@ -122,7 +131,7 @@ function createMinion(operation) {
 
 /**
  * Create a worker for running operations.
- * @param {Object} config Configuration.
+ * @param {ProcessorOptions} config Processor options.
  * @param {function(MessageEvent): void} onMessage Called with a message event.
  * @return {Worker} The worker.
  */
@@ -177,11 +186,22 @@ function createFauxWorker(config, onMessage) {
   };
 }
 
+/**
+ * @typedef {function(Error, ImageData, (Object|Array<Object>)): void} JobCallback
+ */
+
+/**
+ * @typedef {Object} Job
+ * @property {Object} meta Job metadata.
+ * @property {Array<ImageData>} inputs Array of input data.
+ * @property {JobCallback} callback Called when the job is complete.
+ */
+
 /**
  * @typedef {Object} ProcessorOptions
  * @property {number} threads Number of workers to spawn.
- * @property {function(Array, Object):*} operation The operation.
- * @property {Object} [lib] Functions that will be made available to operations run in a worker.
+ * @property {Operation} operation The operation.
+ * @property {Object<string, Function>} [lib] Functions that will be made available to operations run in a worker.
  * @property {number} queue The number of queued jobs to allow.
  * @property {boolean} [imageOps=false] Pass all the image data to the operation instead of a single pixel.
  */
@@ -206,7 +226,11 @@ export class Processor extends Disposable {
     } else {
       threads = config.threads || 1;
     }
-    const workers = [];
+
+    /**
+     * @type {Array<Worker>}
+     */
+    const workers = new Array(threads);
     if (threads) {
       for (let i = 0; i < threads; ++i) {
         workers[i] = createWorker(config, this._onWorkerMessage.bind(this, i));
@@ -218,17 +242,32 @@ export class Processor extends Disposable {
       );
     }
     this._workers = workers;
+
+    /**
+     * @type {Array<Job>}
+     * @private
+     */
     this._queue = [];
+
     this._maxQueueLength = config.queue || Infinity;
     this._running = 0;
+
+    /**
+     * @type {Object<number, any>}
+     * @private
+     */
     this._dataLookup = {};
+
+    /**
+     * @type {Job}
+     * @private
+     */
     this._job = null;
   }
 
   /**
    * Run operation on input data.
-   * @param {Array<Array|ImageData>} inputs Array of pixels or image data
-   *     (depending on the operation type).
+   * @param {Array<ImageData>} inputs Array of image data.
    * @param {Object} meta A user data object.  This is passed to all operations
    *     and must be serializable.
    * @param {function(Error, ImageData, Object): void} callback Called when work
@@ -246,7 +285,7 @@ export class Processor extends Disposable {
 
   /**
    * Add a job to the queue.
-   * @param {Object} job The job.
+   * @param {Job} job The job.
    */
   _enqueue(job) {
     this._queue.push(job);
@@ -259,48 +298,51 @@ export class Processor extends Disposable {
    * Dispatch a job.
    */
   _dispatch() {
-    if (this._running === 0 && this._queue.length > 0) {
-      const job = this._queue.shift();
-      this._job = job;
-      const width = job.inputs[0].width;
-      const height = job.inputs[0].height;
-      const buffers = job.inputs.map(function (input) {
-        return input.data.buffer;
-      });
-      const threads = this._workers.length;
-      this._running = threads;
-      if (threads === 1) {
-        this._workers[0].postMessage(
-          {
-            buffers: buffers,
-            meta: job.meta,
-            imageOps: this._imageOps,
-            width: width,
-            height: height,
-          },
-          buffers
-        );
-      } else {
-        const length = job.inputs[0].data.length;
-        const segmentLength = 4 * Math.ceil(length / 4 / threads);
-        for (let i = 0; i < threads; ++i) {
-          const offset = i * segmentLength;
-          const slices = [];
-          for (let j = 0, jj = buffers.length; j < jj; ++j) {
-            slices.push(buffers[j].slice(offset, offset + segmentLength));
-          }
-          this._workers[i].postMessage(
-            {
-              buffers: slices,
-              meta: job.meta,
-              imageOps: this._imageOps,
-              width: width,
-              height: height,
-            },
-            slices
-          );
-        }
+    if (this._running || this._queue.length === 0) {
+      return;
+    }
+
+    const job = this._queue.shift();
+    this._job = job;
+    const width = job.inputs[0].width;
+    const height = job.inputs[0].height;
+    const buffers = job.inputs.map(function (input) {
+      return input.data.buffer;
+    });
+    const threads = this._workers.length;
+    this._running = threads;
+    if (threads === 1) {
+      this._workers[0].postMessage(
+        {
+          buffers: buffers,
+          meta: job.meta,
+          imageOps: this._imageOps,
+          width: width,
+          height: height,
+        },
+        buffers
+      );
+      return;
+    }
+
+    const length = job.inputs[0].data.length;
+    const segmentLength = 4 * Math.ceil(length / 4 / threads);
+    for (let i = 0; i < threads; ++i) {
+      const offset = i * segmentLength;
+      const slices = [];
+      for (let j = 0, jj = buffers.length; j < jj; ++j) {
+        slices.push(buffers[j].slice(offset, offset + segmentLength));
       }
+      this._workers[i].postMessage(
+        {
+          buffers: slices,
+          meta: job.meta,
+          imageOps: this._imageOps,
+          width: width,
+          height: height,
+        },
+        slices
+      );
     }
   }
 
@@ -334,7 +376,7 @@ export class Processor extends Disposable {
     } else {
       const length = job.inputs[0].data.length;
       data = new Uint8ClampedArray(length);
-      meta = new Array(length);
+      meta = new Array(threads);
       const segmentLength = 4 * Math.ceil(length / 4 / threads);
       for (let i = 0; i < threads; ++i) {
         const buffer = this._dataLookup[i]['buffer'];
@@ -388,14 +430,17 @@ export class Processor extends Disposable {
  */
 const RasterEventType = {
   /**
-   * Triggered before operations are run.
+   * Triggered before operations are run.  Listeners will receive an event object with
+   * a `data` property that can be used to make data available to operations.
    * @event module:ol/source/Raster.RasterSourceEvent#beforeoperations
    * @api
    */
   BEFOREOPERATIONS: 'beforeoperations',
 
   /**
-   * Triggered after operations are run.
+   * Triggered after operations are run.  Listeners will receive an event object with
+   * a `data` property.  If more than one thread is used, `data` will be an array of
+   * objects.  If a single thread is used, `data` will be a single object.
    * @event module:ol/source/Raster.RasterSourceEvent#afteroperations
    * @api
    */
@@ -424,7 +469,8 @@ export class RasterSourceEvent extends Event {
   /**
    * @param {string} type Type.
    * @param {import("../PluggableMap.js").FrameState} frameState The frame state.
-   * @param {Object} data An object made available to operations.
+   * @param {Object|Array<Object>} data An object made available to operations.  For "afteroperations" evenets
+   * this will be an array of objects if more than one thread is used.
    */
   constructor(type, frameState, data) {
     super(type);
@@ -776,7 +822,7 @@ class RasterSource extends ImageSource {
    * @param {import("../PluggableMap.js").FrameState} frameState The frame state.
    * @param {Error} err Any error during processing.
    * @param {ImageData} output The output image data.
-   * @param {Object} data The user data.
+   * @param {Object|Array<Object>} data The user data (or an array if more than one thread).
    * @private
    */
   onWorkerComplete_(frameState, err, output, data) {
diff --git a/test/browser/spec/ol/source/raster.test.js b/test/browser/spec/ol/source/raster.test.js
index cb871ab677..eb24be69dc 100644
--- a/test/browser/spec/ol/source/raster.test.js
+++ b/test/browser/spec/ol/source/raster.test.js
@@ -26,7 +26,7 @@ const green =
   'AABAAEAAAICRAEAOw==';
 
 where('Uint8ClampedArray').describe('ol.source.Raster', function () {
-  let map, target, redSource, greenSource, blueSource, raster;
+  let map, target, redSource, greenSource, blueSource, layer, raster;
 
   beforeEach(function () {
     target = document.createElement('div');
@@ -73,6 +73,10 @@ where('Uint8ClampedArray').describe('ol.source.Raster', function () {
       },
     });
 
+    layer = new ImageLayer({
+      source: raster,
+    });
+
     map = new Map({
       target: target,
       view: new View({
@@ -83,11 +87,7 @@ where('Uint8ClampedArray').describe('ol.source.Raster', function () {
           extent: extent,
         }),
       }),
-      layers: [
-        new ImageLayer({
-          source: raster,
-        }),
-      ],
+      layers: [layer],
     });
   });
 
@@ -366,6 +366,32 @@ where('Uint8ClampedArray').describe('ol.source.Raster', function () {
       view.setCenter([0, 0]);
       view.setZoom(0);
     });
+
+    it('is passed an array of data if more than one thread', function (done) {
+      const threads = 3;
+
+      raster = new RasterSource({
+        threads: threads,
+        sources: [redSource, greenSource, blueSource],
+        operation: function (inputs, data) {
+          data.prop = 'value';
+          return inputs[0];
+        },
+      });
+
+      layer.setSource(raster);
+
+      raster.once('afteroperations', function (event) {
+        expect(event.data).to.be.an(Array);
+        expect(event.data).to.have.length(threads);
+        expect(event.data[0].prop).to.equal('value');
+        done();
+      });
+
+      const view = map.getView();
+      view.setCenter([0, 0]);
+      view.setZoom(0);
+    });
   });
 
   describe('tile loading', function () {