Fix IOS 120Hz Stutter In PixiJS: A Deep Dive

The Problem: iOS ProMotion and PixiJS Touch Jank

Have you ever experienced that frustrating stutter or "jank" while interacting with a PixiJS application on an iPhone with a ProMotion display (120Hz)? It's a common issue, and you're not alone! Many developers have grappled with this problem, especially when aiming for smooth, responsive user experiences. This article delves into the root causes of the touch-related stuttering, investigates current workarounds, and explores potential solutions to make your PixiJS applications shine on iOS devices. The core issue revolves around how iOS handles its ProMotion displays, specifically the transition between different refresh rates, and how this interacts with the rendering pipeline of PixiJS. When the screen is idle, it often runs at 60Hz to save power. However, when the user interacts, such as by touching the screen, the display attempts to ramp up to 120Hz. This transition, unfortunately, can cause a noticeable stutter, which significantly degrades the user experience. This is what we refer to as "touch jank". This is particularly apparent in PixiJS applications, where even seemingly simple interactions can trigger this stutter. Let's dig deeper into the problem's origins and what can be done to address it, exploring the workarounds attempted and their effectiveness, along with strategies to minimize or eliminate this visual hiccup.

The Scenario and Its Impact

Consider the scenario: you've built a beautiful, interactive game or application using PixiJS. It runs perfectly on other devices, providing a fluid and engaging experience. However, when you test it on an iPhone with a ProMotion display (like the iPhone 13 Pro or 14 Pro), you notice that every touch triggers a noticeable stutter. The animation feels jerky, and the responsiveness is compromised. This is far from ideal. This "jank" ruins the perceived smoothness of the application and undermines user engagement. This stutter effect happens because the operating system is attempting to optimize power consumption. So, when the screen is idle, it typically runs at a lower refresh rate, such as 60Hz. When the screen is touched, the refresh rate increases to 120Hz to improve responsiveness. This shift can cause a delay in rendering and introduce visible stuttering, especially if the application's rendering is not perfectly in sync with this frequency change. The user will be left with a negative feeling towards the app.

Understanding the Root Cause: ProMotion's Refresh Rate Transitions

At the heart of the matter lies iOS's ProMotion technology, a dynamic refresh rate feature. ProMotion intelligently adjusts the display's refresh rate (the number of times the screen updates per second) to optimize for both visual smoothness and battery life. This is great in theory, but as the user has already experienced, in practice it sometimes leads to problems. When the display is idle, it often operates at a lower refresh rate, such as 60Hz, to conserve power. However, the system must quickly ramp up to 120Hz when user interaction occurs, like a touch event. The problem is this transition. This "ramp-up" process isn't always seamless. It can lead to frame drops and visual stutters. When the refresh rate changes, there's a slight delay as the hardware catches up, and that delay can manifest as a jarring visual experience. Also, the app is running at 60 FPS, and on touch, the device switches to 120Hz mode. The discrepancy between the frame rate and the refresh rate causes the jank that users have been experiencing. It's not just a matter of the display switching modes. It's the synchronization between the app's rendering pipeline (how PixiJS draws things on the screen) and the display's refresh rate that is off. When these two aren't in harmony, the user sees a stutter. This is the challenge that developers, including those using PixiJS, face. Many standard techniques for forcing a higher refresh rate, like using CSS tricks, don't always work as expected on modern iOS versions. This is because Apple's power-saving optimizations can override these attempts. The user has to wait for a solution to solve the problem.

The Silent Audio Hack and Other Failed Workarounds

Developers have attempted various strategies to prevent the refresh rate switching, but few have been consistently successful. Some of the most common approaches include:

• will-change: transform with a translateZ(0) animation in CSS: This CSS property signals to the browser that an element's appearance may change, potentially forcing the browser to optimize its rendering. The translateZ(0) can create a slight visual effect, but it is supposed to improve performance by moving it to the GPU. This strategy has been a popular choice. However, it doesn't always work. The iOS WebKit might ignore it.
• The silent AudioContext loop hack: The idea behind this is that by playing a silent audio track in the background, you can keep the device active and prevent it from dropping the refresh rate. This might work, but it's not a foolproof solution. The audio approach can sometimes prevent the device from going into a low-power mode, keeping the refresh rate stable at 120Hz. However, it can also consume extra power and it's not a guaranteed solution.

Unfortunately, neither of these methods nor others have provided a consistent fix. Many developers find that these techniques are ineffective on modern iOS versions, as Apple has become more aggressive with power-saving measures in recent updates. The device has to be ready to change the frame rate. These workarounds are not very effective.

Potential Solutions and Future Directions for PixiJS

Finding a definitive, universal solution remains a challenge, but here are some strategies and ideas to explore:

Optimization Techniques Within PixiJS

• Optimize PixiJS Rendering: Ensure the PixiJS application is as efficient as possible. This involves optimizing the rendering process to reduce the load on the CPU and GPU. This includes using texture atlases, batching draw calls, and avoiding unnecessary updates. The more efficiently your application renders, the less likely it is to be affected by refresh rate changes. It also involves reducing the number of draw calls, which can cause significant performance bottlenecks.
• Frame Rate Limiting: Consider using PixiJS's built-in frame rate limiting to cap the application's frame rate at a stable level (e.g., 60 FPS). While this might not eliminate the stutter entirely, it can make it less noticeable. By keeping the frame rate consistent, you can reduce the frequency of jarring visual changes.
• Investigate Hardware-Accelerated Rendering: Make sure PixiJS is using hardware acceleration effectively. This will offload the rendering work to the device's GPU, leading to smoother performance. Check your PixiJS configuration to ensure that WebGL or WebGPU rendering is enabled and optimized.

Exploring Alternative Approaches

• WebKit-Specific Hacks: Investigate any WebKit-specific techniques or experimental features that might influence the refresh rate behavior. WebKit is the rendering engine used by Safari and other browsers on iOS, so any targeted solutions may be beneficial.
• Monitor iOS Updates: Keep an eye on iOS updates. Apple may address the issue directly in future releases or introduce new APIs that give developers more control over display refresh rates. Staying current with iOS updates is critical. Apple often provides improvements and fixes that could resolve or mitigate the issue.

Conclusion: Navigating the iOS ProMotion Challenge

Dealing with the iOS ProMotion stutter in PixiJS is a persistent problem for many. The dynamic refresh rate on ProMotion devices can introduce noticeable "touch jank" when rendering PixiJS applications. While there isn't a single guaranteed fix, a combination of optimization techniques within PixiJS, exploration of platform-specific features, and staying informed about iOS updates can mitigate the issue. Developers should prioritize efficient rendering, consider frame rate limiting, and investigate hardware acceleration. Staying abreast of Apple's updates and any related changes to the WebKit browser engine is also important. As iOS technology evolves, so will the solutions. Continuous monitoring of updates and experimenting with new strategies will be important to improve the user experience. By being proactive and adaptable, you can create smooth and engaging PixiJS applications that perform well on ProMotion devices.

For further reading and resources, check out:

• Apple's Developer Documentation on Display Technologies: https://developer.apple.com/documentation/metal/working_with_display_refresh_rates – This can provide a deeper understanding of the inner workings of ProMotion and its implications for developers.