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Quiz: Touch Gestures, Mobile Sensors & Interaction

Test your understanding of Mobile touch events, touches array, multi-touch pinch/zoom, accelerometer, and gyroscope tilt with these review questions.

1. Which array stores the coordinates and identifiers of all active touch contact points on a touchscreen device?

  1. touches[]
  2. touchPoints[]
  3. fingers[]
  4. contactList[]
Show Answer

The correct answer is A. The built-in touches[] array contains an object for each finger currently touching the screen, with x, y, and id properties. Options B, C, and D are not p5.js system arrays.

Concept Tested: Touches Array

2. Which system variables report the device's physical rotational tilt around the X, Y, and Z axes?

  1. rotationX, rotationY, rotationZ
  2. tiltX, tiltY, tiltZ
  3. gyroX, gyroY, gyroZ
  4. angleX, angleY, angleZ
Show Answer

The correct answer is A. rotationX (pitch), rotationY (roll), and rotationZ (yaw) provide real-time angular orientation from the device's built-in gyroscope. Options B, C, and D are not p5.js system variables.

Concept Tested: Device Orientation Tilt

3. What mobile sensor measures linear acceleration forces in meters per second squared along the three spatial axes?

  1. Magnetometer compass
  2. Accelerometer (accelerationX, accelerationY, accelerationZ)
  3. Barometer pressure sensor
  4. Ambient light photometer
Show Answer

The correct answer is B. Accelerometer sensors measure dynamic changes in velocity and gravitational acceleration along X, Y, and Z axes. Options B, C, and D measure magnetic fields, air pressure, and light.

Concept Tested: Device Accelerometer Motion

4. How do you calculate the distance between two fingers in a two-touch pinch gesture?

  1. touches[0].dist(touches[1])
  2. dist(touches[0].x, touches[0].y, touches[1].x, touches[1].y)
  3. touches.getPinchDistance()
  4. abs(touches[0].x - touches[1].x)
Show Answer

The correct answer is B. Calling dist() with the coordinates of the first two touch objects in the touches[] array calculates the Euclidean distance between the two fingertips, enabling pinch-to-zoom scaling. Options B, C, and D are invalid syntax.

Concept Tested: Pinch Gesture Math

5. Why must web applications request explicit user permission before reading accelerometer or gyroscope sensors on modern iOS Safari devices?

  1. Because mobile browsers cannot process floating-point math without permission
  2. To protect user privacy and prevent unauthorized fingerprinting or covert motion tracking
  3. Because mobile sensors consume 100% of CPU power when enabled
  4. To allow the browser to download external sensor drivers
Show Answer

The correct answer is B. Modern mobile browsers enforce security policies requiring explicit user interaction (such as tapping a button that triggers DeviceOrientationEvent.requestPermission()) before exposing motion data. Options B, C, and D are false.

Concept Tested: Sensor Permission Model

6. Which event function is called when a touch point is dragged across the glass surface?

  1. touchEnded()
  2. touchStarted()
  3. touchMoved()
  4. touchDragged()
Show Answer

The correct answer is C. touchMoved() executes continuously as touch points move across the screen. touchStarted() runs on initial contact, and touchEnded() runs on finger lift. touchDragged() is not a standard p5.js function.

Concept Tested: Touch Moved Event

7. If a mobile sketch does not implement touch event handlers, how does p5.js handle single-finger touch interactions by default?

  1. It ignores all screen touches completely
  2. It crashes the sketch with an UnhandledTouchError
  3. It automatically emulates mouse events, mapping touch coordinates to mouseX and mouseY
  4. It opens the device settings menu
Show Answer

The correct answer is C. For backwards compatibility, p5.js automatically maps single-touch events to standard mouse variables (mouseX, mouseY, mouseIsPressed, mousePressed()), allowing simple sketches to work seamlessly on mobile. Options B, C, and D are incorrect.

Concept Tested: Touch Mouse Emulation

8. What is the purpose of pAccelerationX, pAccelerationY, and pAccelerationZ?

  1. They store predicted acceleration for physics prediction
  2. They store the peak maximum acceleration ever recorded
  3. They store the acceleration values from the previous frame to calculate sudden spikes (shakes)
  4. They record GPS positioning coordinates
Show Answer

The correct answer is C. Comparing current acceleration to pAcceleration (previous frame) enables detection of rapid threshold changes such as shaking or flicking the device. Options B, C, and D are incorrect.

Concept Tested: Previous Acceleration Shake

9. To make a ball roll naturally around the screen when tilting a phone in landscape or portrait, how should tilt angles be mapped to physics forces?

  1. Multiply velocity by the device battery percentage
  2. Set the ball's position directly to rotationX and rotationY coordinates
  3. Rotate the entire canvas with rotate(rotationZ)
  4. Map rotationX and rotationY to horizontal and vertical force vectors applied to the ball's acceleration
Show Answer

The correct answer is D. Treating device tilt angles (rotationY and rotationX) as gravitational force components ($F_x, F_y$) and accumulating them into the physics acceleration vector creates natural rolling physics. Options B, C, and D do not produce realistic motion.

Concept Tested: Tilt Physics Mapping

10. Why is returning false from touchMoved() especially crucial on mobile web browsers?

  1. It locks screen brightness at 100%
  2. It speeds up screen refresh rates to 120Hz
  3. It automatically saves a screenshot to the photo library
  4. It prevents the mobile browser from performing default page rubber-banding, scrolling, or pull-to-refresh gestures
Show Answer

The correct answer is D. Returning false cancels default touch actions like browser viewport scrolling or pull-to-refresh, keeping the interactive canvas stable under the user's fingers. Options B, C, and D are false.

Concept Tested: Touch Move Page Lock