A guide to optimizing Unity UI

Checked with version: 5.3


Difficulty: Advanced

Optimizing a user interface driven by Unity UI is an art. Hard-and-fast rules are rare; instead, each situation must be carefully evaluated with the system’s behavior in mind. The core tension when optimizing any Unity UI is the balancing of draw calls with batching costs. While some common-sense techniques can be used to reduce one or the other, complex UIs must make trade-offs.

However, as is best practice elsewhere, attempts to optimize Unity UIs should begin with profiling. The primary task before attempting to optimize a Unity UI system is to locate the precise reason for an observed performance problem. There are four common classes of problem encountered by users of Unity UI:

  • Excessive GPU fragment shader utilization (i.e. fill-rate overutilization)
  • Excessive CPU time spent rebuilding a Canvas batch
  • Excessive numbers of rebuilds of Canvas batches (over-dirtying)
  • Excessive CPU time spent generating vertices (usually from text)

It is, in principle, possible to create a Unity UI whose performance is constrained by the sheer number of draw calls being sent to the GPU. However, in practice, any project overloading the GPU with draw calls is more likely to be bound by fill-rate overutilization.

This guide will discuss the fundamental concepts, algorithms and code underlying Unity UI as well as discussing common problems and solutions. It is broken into five chapters:

  1. The Fundamentals of Unity UI chapter defines terminology specific to Unity UI and discusses the details of many of the fundamental processes performed to render the UI, including the building of batched geometry. It is strongly recommended that readers begin with this chapter.
  2. The Unity UI profiling tools chapter discusses gathering profiling data with the various tools available to developers.
  3. The Fill-rate, Canvases and input chapter discusses ways to improve the performance of Unity UI's Canvas and Input Components.
  4. The UI controls chapter discusses UI Text, Scroll Views and other component-specific optimizations, along with some techniques that do not fit well elsewhere.
  5. The Other techniques and tips chapter discusses a handful of issues that do not fit elsewhere, including some basic tips and workarounds for "gotchas" in the UI system.

UI Source Code

Always remember that Unity UI’s Graphic and Layout components are entirely open source. Their source code can be found on Unity’s Bitbucket repository, under UI.