diff --git a/package.json b/package.json
index cc35511c6..b362696e3 100644
--- a/package.json
+++ b/package.json
@@ -53,6 +53,7 @@
     "lodash.defaultsdeep": "4.6.0",
     "minilog": "3.1.0",
     "nets": "3.2.0",
+    "pngjs": "^3.3.2",
     "promise": "8.0.1",
     "scratch-audio": "latest",
     "scratch-blocks": "latest",
diff --git a/src/extensions/scratch3_video_sensing/index.js b/src/extensions/scratch3_video_sensing/index.js
index a59daf489..941ba9904 100644
--- a/src/extensions/scratch3_video_sensing/index.js
+++ b/src/extensions/scratch3_video_sensing/index.js
@@ -81,15 +81,18 @@ class Scratch3VideoSensingBlocks {
          */
         this._sampleContext = null;
 
-        // Clear target motion state values when the project starts.
-        this.runtime.on(Runtime.PROJECT_RUN_START, this.reset.bind(this));
+        if (this.runtime.ioDevices) {
+            // Clear target motion state values when the project starts.
+            this.runtime.on(Runtime.PROJECT_RUN_START, this.reset.bind(this));
 
-        // Boot up the video, canvas to down/up sample the video stream, the
-        // preview skin and drawable, and kick off looping the analysis logic.
-        this._setupVideo();
-        this._setupSampleCanvas();
-        this._setupPreview();
-        this._loop();
+            // Boot up the video, canvas to down/up sample the video stream, the
+            // preview skin and drawable, and kick off looping the analysis
+            // logic.
+            this._setupVideo();
+            this._setupSampleCanvas();
+            this._setupPreview();
+            this._loop();
+        }
     }
 
     /**
@@ -322,6 +325,14 @@ class Scratch3VideoSensingBlocks {
         ];
     }
 
+    static get MOTION () {
+        return 1;
+    }
+
+    static get DIRECTION () {
+        return 2;
+    }
+
     /**
      * An array of info about each drum.
      * @type {object[]} an array of objects.
@@ -341,6 +352,14 @@ class Scratch3VideoSensingBlocks {
         ];
     }
 
+    static get STAGE () {
+        return 1;
+    }
+
+    static get SPRITE () {
+        return 2;
+    }
+
     /**
      * @returns {object} metadata for this extension and its blocks.
      */
@@ -359,12 +378,12 @@ class Scratch3VideoSensingBlocks {
                         MOTION_DIRECTION: {
                             type: ArgumentType.NUMBER,
                             menu: 'MOTION_DIRECTION',
-                            defaultValue: 1
+                            defaultValue: Scratch3VideoSensingBlocks.MOTION
                         },
                         STAGE_SPRITE: {
                             type: ArgumentType.NUMBER,
                             menu: 'STAGE_SPRITE',
-                            defaultValue: 1
+                            defaultValue: Scratch3VideoSensingBlocks.STAGE
                         }
                     }
                 },
@@ -413,11 +432,11 @@ class Scratch3VideoSensingBlocks {
         this.detect.analyzeFrame();
 
         let state = this.detect;
-        if (Number(args.STAGE_SPRITE) === 2) {
+        if (Number(args.STAGE_SPRITE) === Scratch3VideoSensingBlocks.SPRITE) {
             state = this._analyzeLocalMotion(util.target);
         }
 
-        if (Number(args.MOTION_DIRECTION) === 1) {
+        if (Number(args.MOTION_DIRECTION) === Scratch3VideoSensingBlocks.MOTION) {
             return state.motionAmount;
         }
         return state.motionDirection;
diff --git a/test/unit/extension_video_sensing.js b/test/unit/extension_video_sensing.js
new file mode 100644
index 000000000..35f0381fa
--- /dev/null
+++ b/test/unit/extension_video_sensing.js
@@ -0,0 +1,409 @@
+const {createReadStream} = require('fs');
+const {join} = require('path');
+
+const {PNG} = require('pngjs');
+const {test} = require('tap');
+
+const {wrapClamp} = require('../../src/util/math-util');
+
+const VideoSensing = require('../../src/extensions/scratch3_video_sensing/index.js');
+const VideoMotion = require('../../src/extensions/scratch3_video_sensing/library.js');
+
+/**
+ * Prefix to the mock frame images used to test the video sensing extension.
+ * @type {string}
+ */
+const pngPrefix = 'extension_video_sensing_';
+
+/**
+ * Map of frame keys to the image filenames appended to the pngPrefix.
+ * @type {object}
+ */
+const framesMap = {
+    center: 'center',
+    left: 'left-5',
+    left2: 'left-10',
+    down: 'down-10'
+};
+
+/**
+ * Asynchronously read a png file and copy its pixel data into a typed array
+ * VideoMotion will accept.
+ * @param {string} name - partial filename to read
+ * @returns {Promise.<Uint32Array>} pixel data of the image
+ */
+const readPNG = name => (
+    new Promise((resolve, reject) => {
+        const png = new PNG();
+        createReadStream(join(__dirname, `${pngPrefix}${name}.png`))
+            .pipe(png)
+            .on('parsed', () => {
+                // Copy the RGBA pixel values into a separate typed array and
+                // cast the array to Uint32, the array format VideoMotion takes.
+                resolve(new Uint32Array(new Uint8ClampedArray(png.data).buffer));
+            })
+            .on('error', reject);
+    })
+);
+
+/**
+ * Read all the frames for testing asynchrnously and produce an object with
+ * keys following the keys in framesMap.
+ * @returns {object} mapping of keys in framesMap to image data read from disk
+ */
+const readFrames = (() => {
+    // Use this immediately invoking function expression (IIFE) to delay reading
+    // once to the first test that calls readFrames.
+    let _promise = null;
+
+    return () => {
+        if (_promise === null) {
+            _promise = Promise.all(Object.keys(framesMap).map(key => readPNG(framesMap[key])))
+                .then(pngs => (
+                    Object.keys(framesMap).reduce((frames, key, i) => {
+                        frames[key] = pngs[i];
+                        return frames;
+                    }, {})
+                ));
+        }
+        return _promise;
+    };
+})();
+
+/**
+ * Match if actual is within optMargin to expect. If actual is under -180,
+ * match if actual + 360 is near expect. If actual is over 180, match if actual
+ * - 360 is near expect.
+ * @param {number} actual - actual angle in degrees
+ * @param {number} expect - expected angle in degrees
+ * @param {number} optMargin - allowed margin between actual and expect in degrees
+ * @returns {boolean} true if actual is close to expect
+ */
+const isNearAngle = (actual, expect, optMargin = 10) => (
+    (wrapClamp(actual - expect, 0, 359) < optMargin) ||
+    (wrapClamp(actual - expect, 0, 359) > 360 - optMargin)
+);
+
+// A fake scratch-render drawable that will be used by VideoMotion to restrain
+// the area considered for motion detection in VideoMotion.getLocalMotion
+const fakeDrawable = {
+    getFastBounds () {
+        return {
+            left: -120,
+            top: 60,
+            right: 0,
+            bottom: -60
+        };
+    },
+
+    isTouching () {
+        return true;
+    }
+};
+
+// A fake MotionState used to test the stored values in
+// VideoMotion.getLocalMotion, VideoSensing.videoOn and
+// VideoSensing.whenMotionGreaterThan.
+const fakeMotionState = {
+    motionFrameNumber: -1,
+    motionAmount: -1,
+    motionDirection: -Infinity
+};
+
+// A fake target referring to the fake drawable and MotionState.
+const fakeTarget = {
+    drawableID: 0,
+
+    getCustomState () {
+        return fakeMotionState;
+    },
+    setCustomState () {}
+};
+
+const fakeRuntime = {
+    targets: [fakeTarget],
+
+    // Without defined devices, VideoSensing will not try to start sampling from
+    // a video source.
+    ioDevices: null,
+
+    renderer: {
+        _allDrawables: [
+            fakeDrawable
+        ]
+    }
+};
+
+const fakeBlockUtility = {
+    target: fakeTarget
+};
+
+test('detect motionAmount between frames', t => {
+    t.plan(6);
+
+    return readFrames()
+        .then(frames => {
+            const detect = new VideoMotion();
+
+            // Each of these pairs should have enough motion for the detector.
+            const framePairs = [
+                [frames.center, frames.left],
+                [frames.center, frames.left2],
+                [frames.left, frames.left2],
+                [frames.left, frames.center],
+                [frames.center, frames.down],
+                [frames.down, frames.center]
+            ];
+
+            // Add both frames of a pair and test for motion.
+            let index = 0;
+            for (const [frame1, frame2] of framePairs) {
+                detect.addFrame(frame1);
+                detect.addFrame(frame2);
+
+                detect.analyzeFrame();
+                t.ok(
+                    detect.motionAmount > 10,
+                    `frame pair ${index + 1} has motion ${detect.motionAmount} over threshold (10)`
+                );
+                index += 1;
+            }
+
+            t.end();
+        });
+});
+
+test('detect local motionAmount between frames', t => {
+    t.plan(6);
+
+    return readFrames()
+        .then(frames => {
+            const detect = new VideoMotion();
+
+            // Each of these pairs should have enough motion for the detector.
+            const framePairs = [
+                [frames.center, frames.left],
+                [frames.center, frames.left2],
+                [frames.left, frames.left2],
+                [frames.left, frames.center],
+                [frames.center, frames.down],
+                [frames.down, frames.center]
+            ];
+
+            // Add both frames of a pair and test for local motion.
+            let index = 0;
+            for (const [frame1, frame2] of framePairs) {
+                detect.addFrame(frame1);
+                detect.addFrame(frame2);
+
+                detect.analyzeFrame();
+                detect.getLocalMotion(fakeDrawable, fakeMotionState);
+                t.ok(
+                    fakeMotionState.motionAmount > 10,
+                    `frame pair ${index + 1} has motion ${fakeMotionState.motionAmount} over threshold (10)`
+                );
+                index += 1;
+            }
+
+            t.end();
+        });
+});
+
+test('detect motionDirection between frames', t => {
+    t.plan(6);
+
+    return readFrames()
+        .then(frames => {
+            const detect = new VideoMotion();
+
+            // Each of these pairs is moving in the given direction. Does the detector
+            // guess a value to that?
+            const directionMargin = 10;
+            const framePairs = [
+                {
+                    frames: [frames.center, frames.left],
+                    direction: -90
+                },
+                {
+                    frames: [frames.center, frames.left2],
+                    direction: -90
+                },
+                {
+                    frames: [frames.left, frames.left2],
+                    direction: -90
+                },
+                {
+                    frames: [frames.left, frames.center],
+                    direction: 90
+                },
+                {
+                    frames: [frames.center, frames.down],
+                    direction: 180
+                },
+                {
+                    frames: [frames.down, frames.center],
+                    direction: 0
+                }
+            ];
+
+            // Add both frames of a pair and check if the motionDirection is near the
+            // expected angle.
+            let index = 0;
+            for (const {frames: [frame1, frame2], direction} of framePairs) {
+                detect.addFrame(frame1);
+                detect.addFrame(frame2);
+
+                detect.analyzeFrame();
+                t.ok(
+                    isNearAngle(detect.motionDirection, direction, directionMargin),
+                    `frame pair ${index + 1} is ${detect.motionDirection.toFixed(0)} ` +
+                    `degrees and close to ${direction} degrees`
+                );
+                index += 1;
+            }
+
+            t.end();
+        });
+});
+
+test('detect local motionDirection between frames', t => {
+    t.plan(6);
+
+    return readFrames()
+        .then(frames => {
+            const detect = new VideoMotion();
+
+            // Each of these pairs is moving in the given direction. Does the detector
+            // guess a value to that?
+            const directionMargin = 10;
+            const framePairs = [
+                {
+                    frames: [frames.center, frames.left],
+                    direction: -90
+                },
+                {
+                    frames: [frames.center, frames.left2],
+                    direction: -90
+                },
+                {
+                    frames: [frames.left, frames.left2],
+                    direction: -90
+                },
+                {
+                    frames: [frames.left, frames.center],
+                    direction: 90
+                },
+                {
+                    frames: [frames.center, frames.down],
+                    direction: 180
+                },
+                {
+                    frames: [frames.down, frames.center],
+                    direction: 0
+                }
+            ];
+
+            // Add both frames of a pair and check if the local motionDirection is near
+            // the expected angle.
+            let index = 0;
+            for (const {frames: [frame1, frame2], direction} of framePairs) {
+                detect.addFrame(frame1);
+                detect.addFrame(frame2);
+
+                detect.analyzeFrame();
+                detect.getLocalMotion(fakeDrawable, fakeMotionState);
+                const motionDirection = fakeMotionState.motionDirection;
+                t.ok(
+                    isNearAngle(motionDirection, direction, directionMargin),
+                    `frame pair ${index + 1} is ${motionDirection.toFixed(0)} degrees and close to ${direction} degrees`
+                );
+                index += 1;
+            }
+
+            t.end();
+        });
+});
+
+test('videoOn returns value dependent on arguments', t => {
+    t.plan(4);
+
+    return readFrames()
+        .then(frames => {
+            const sensing = new VideoSensing(fakeRuntime);
+
+            // With these two frame test if we get expected values depending on the
+            // arguments to videoOn.
+            sensing.detect.addFrame(frames.center);
+            sensing.detect.addFrame(frames.left);
+
+            const motionAmount = sensing.videoOn({
+                MOTION_DIRECTION: VideoSensing.MOTION,
+                STAGE_SPRITE: VideoSensing.STAGE
+            }, fakeBlockUtility);
+            t.ok(
+                motionAmount > 10,
+                `stage motionAmount ${motionAmount} is over the threshold (10)`
+            );
+
+            const localMotionAmount = sensing.videoOn({
+                MOTION_DIRECTION: VideoSensing.MOTION,
+                STAGE_SPRITE: VideoSensing.SPRITE
+            }, fakeBlockUtility);
+            t.ok(
+                localMotionAmount > 10,
+                `sprite motionAmount ${localMotionAmount} is over the threshold (10)`
+            );
+
+            const motionDirection = sensing.videoOn({
+                MOTION_DIRECTION: VideoSensing.DIRECTION,
+                STAGE_SPRITE: VideoSensing.STAGE
+            }, fakeBlockUtility);
+            t.ok(
+                isNearAngle(motionDirection, -90),
+                `stage motionDirection ${motionDirection.toFixed(0)} degrees is close to ${90} degrees`
+            );
+
+            const localMotionDirection = sensing.videoOn({
+                MOTION_DIRECTION: VideoSensing.DIRECTION,
+                STAGE_SPRITE: VideoSensing.SPRITE
+            }, fakeBlockUtility);
+            t.ok(
+                isNearAngle(localMotionDirection, -90),
+                `sprite motionDirection ${localMotionDirection.toFixed(0)} degrees is close to ${90} degrees`
+            );
+
+            t.end();
+        });
+});
+
+test('whenMotionGreaterThan returns true if local motion meets target', t => {
+    t.plan(2);
+
+    return readFrames()
+        .then(frames => {
+            const sensing = new VideoSensing(fakeRuntime);
+
+            // With these two frame test if we get expected values depending on the
+            // arguments to whenMotionGreaterThan.
+            sensing.detect.addFrame(frames.center);
+            sensing.detect.addFrame(frames.left);
+
+            const over20 = sensing.whenMotionGreaterThan({
+                REFERENCE: 20
+            }, fakeBlockUtility);
+            t.ok(
+                over20,
+                `enough motion in drawable bounds to reach reference of 20`
+            );
+
+            const over80 = sensing.whenMotionGreaterThan({
+                REFERENCE: 80
+            }, fakeBlockUtility);
+            t.notOk(
+                over80,
+                `not enough motion in drawable bounds to reach reference of 80`
+            );
+
+            t.end();
+        });
+});
diff --git a/test/unit/extension_video_sensing_center.png b/test/unit/extension_video_sensing_center.png
new file mode 100644
index 000000000..eed3cb73e
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diff --git a/test/unit/extension_video_sensing_down-10.png b/test/unit/extension_video_sensing_down-10.png
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index 000000000..109d6b8ca
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diff --git a/test/unit/extension_video_sensing_left-10.png b/test/unit/extension_video_sensing_left-10.png
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index 000000000..8a42a119b
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diff --git a/test/unit/extension_video_sensing_left-5.png b/test/unit/extension_video_sensing_left-5.png
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index 000000000..5e84c3edf
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