Why Multiply Movement by Delta Time in Pixi.js

This article explains why multiplying movement values by delta time inside a Pixi.js update loop is essential for game development. You will learn how delta time ensures your animations and character movements remain consistent, smooth, and fair across different devices, monitors, and frame rates.

When building games or interactive applications in Pixi.js, you typically use an update loop (via app.ticker.add) to update the positions of your sprites on every frame. However, if you move a sprite by a fixed pixel value per frame—such as sprite.x += 5—the actual speed of the sprite becomes directly tied to the user’s frame rate.

The Problem with Frame-Rate Dependent Movement

In a frame-dependent setup, a game running on a standard 60 Hz monitor will render 60 frames per second (FPS), moving the sprite 300 pixels per second. If a user plays the same game on a 144 Hz gaming monitor, the update loop runs 144 times per second, causing the sprite to move at 720 pixels per second. Conversely, if a mobile device experiences lag and drops to 30 FPS, the sprite will crawl at only 150 pixels per second.

This inconsistency breaks gameplay physics, makes games unplayable on high-refresh-rate monitors, and punishes players with slower hardware.

How Delta Time Solves the Problem

Pixi.js provides a deltaTime value (often passed as a parameter in the ticker callback) to solve this issue. Delta time represents the fractional amount of time that has elapsed since the last frame, relative to a target frame rate (usually 60 FPS).

By multiplying your movement values by delta time, you transition from frame-dependent movement to time-dependent movement:

app.ticker.add((ticker) => {
    const speed = 5; // Base speed per frame at 60 FPS
    sprite.x += speed * ticker.deltaTime;
});

Why This is Crucial for Your Project

  1. Consistent Speed Across Hardware: Whether a player is on a 30 FPS budget smartphone or a 240 Hz desktop monitor, the sprite will cover the exact same distance in real-time.
  2. Fair Gameplay: In multiplayer or physics-heavy games, consistent movement speeds ensure that players with faster computers do not gain a speed advantage over those with slower hardware.
  3. Smooth Performance During Lag Spikes: If the rendering engine temporarily stutters, the next frame’s deltaTime will be larger to compensate for the lost time, “catching up” the sprite to its correct physical position.