Shader variants, also sometimes called shader permutations, are one way of introducing conditional behavior into shader code.
Unity compiles shader source files into shader programs. Each compiled shader program has one or more variants: different versions of the shader program for different conditions. At runtime, Unity uses the variant that matches the current requirements. You configure variants using shader keywords.
The main advantage of shader variants is that they allow you to use runtime conditionals in your shader programs, without the GPU performance impact of dynamic branching. The main disadvantage of shader variants is that a large number of them can lead to both build time and runtime performance issues.
When Unity creates shader variants, it uses static branching to create multiple small, specialized shader programs. At runtime, Unity uses the shader program that matches the conditions. This means that you can use shader variants for code that would likely result in reduced GPU performance in a dynamic branch, without suffering a GPU performance penalty.
However, a large number of variants can result in increased build times, file sizes, runtime memory usage, and loading times. It also leads to greater complexity when manually preloading (“prewarming”) shaders. When a project contains a very large number of shader variants, these issues can lead to significant problems with performance and workflow.
Warning: It is easy to inadvertently create an excessively large number of shader variants, which can lead to significant performance problems. It is therefore very important to understand how Unity determines the number of shader variants, how to exclude (“strip”) unneeded variants from compilation, and when to use other types of conditionals in shaders.
At build time, Unity compiles one set of shader variants for each graphics API for the current build target. The number of variants for each combination of graphics API and build target depends on your shader source files, and your use of shader keywords.
Unity compiles one set of shader variants for each graphics API in the list for the current build target. The shaders differ for each combination of build target and graphics API; for example, Unity compiles different shaders for Metal on iOS than for Metal on macOS.
Some shader programs or keywords might only target a given graphics API or a given build target, so the total number of variants for each combination of graphics API and build target can differ; however, the process for compiling these variants is the same.
Unity must determine how many shader programs to compile for the current combination of build target and graphics API.
For each shader source file that is included in your build, Unity determines how many unique shader programs it defines:
Note: A shader source file is included in a build if it is referenced in a scene in that build, referenced by something in the Resources folder, or included in the Always-included shaders section of the Graphics Settings window.
When Unity has determined how many shader programs it must compile for the current build target and graphics API, it then determines how many shader variants it must compile for each shader program.
For each shader program, Unity determines the combination of shader keywords that result in different variants. This comprises:
The number of shader variants that Unity compiles for a shader program is the product of the keyword sets; that is to say, Unity compiles one variant for every combination that includes one element from each set.
For example, this set contains three shader variant keywords:
This set contains four shader variant keywords:
A shader program affected by those shader variant keywords will result in the following twelve variants:
The number of variants that Unity compiles can grow very rapidly as you add more sets of shader variant keywords. The term for this very rapid growth is combinatorial explosion.
For example, consider a fairly typical use case, where a shader has a number of sets of shader variant keywords that contain two keywords each (
<feature name>_ON and
<feature name>_OFF). If the shader has two such sets of keywords, this results in four variants. If the shader has ten such sets of keywords, this results in 1024 variants.
After compilation, Unity automatically identifies identical variants within the same Pass, and ensures that these identical variants point to the same bytecode. This is called deduplication.
Deduplication prevents identical variants in the same Pass from increasing file size; however, identical variants still result in wasted work during compilation, and increased memory usage and shader loading times at runtime. With this in mind, it is always best to strip unneeded variants.