Unity compiles your shader source files into individual shader programs. Each compiled shader program has one or more variants. Variants are versions of the shader program that work with different combinations of shader keywords. At runtime, when Unity renders geometry, it uses the variant that matches the current requirements. For more information on how Unity loads and uses shader variants, see Shader loading.
Shader variants can improve performance, because they result in shader programs that contain only the code that is needed for the current material, at the current time. Common things to optimize in this way are texture reads, vertex inputs, interpolators, or complex code such as loops. Additionally, the shader programs themselves are smaller.
Shader variants can also improve your workflow, because they allow you to use the same shader source file in different ways. For example, you can configure settings for different materials, define functionality for different hardware, and dynamically change the behaviour of shaders at runtime.
However, there are potential downsides. It’s easy to create a very large number of variants, and this can result in the following:
In larger projects, these issues can lead to significant problems with performance and workflow. It is therefore very important to understand how shader variants work, and how to exclude (“strip”) unneeded variants from compilation. For more information on shader stripping, see Shader variant stripping.
Shaders with a very large number of variants are called “mega shaders” or “uber shaders”. Unity’s Standard Shader is an example of such a shader.
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.
To view and edit the list of graphics APIs for your current build target, use the Player Settings window, or the PlayerSettings API.
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 affect it. 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 keywords:
This set contains four keywords:
A shader program affected by those 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 keywords. For example, consider a fairly typical use case, where a shader has a number of sets of 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.