Version: 2021.2
Shader variants
Using shader keywords with C# scripts

Shader keywords

You can create shaders that share some common code, but have different functionality when a given keyword is enabled or disabled.

Internally, shader keywords work by creating shader variants. Before you use shader keywords, it is important to understand how shader variants work, and the potential impact on performance and workflow. For information on this topic, see Shader variants.

Declaring shader keywords

You declare shader keywords in sets. A set contains mutually exclusive keywords.

For example, the following set contains three keywords:

  • COLOR_RED
  • COLOR_GREEN
  • COLOR_BLUE

Internally, shader keywords use #define preprocessor directives. When you declare a set of shader keywords, Unity compiles shader variants with matching #define directives.

Note: In Shader Graph, the terminology is different: a set of keywords is called a Keyword, and the keywords in a set are called states. Internally, the functionality is the same: Unity compiles them in the same way, you work with them the same way with C# scripts, and so on.

The number of keywords that you declare and the way that you declare these keywords has a significant impact on the number of shader variants that Unity compiles, which in turn can have a significant effect on the performance of your project and application. For more information, see Shader variants.

To declare keywords in hand-coded shaders, see Declaring and using shader keywords in HLSL. To declare keywords in Shader Graph shaders, see Keywords.

Declaring keywords with local or global scope

When you declare a set of keywords, you choose whether the keywords in the set have local or global scope. This affects Unity’s behavior when you enable and disable shader keywords. For more information on enabling and disabling shader keywords, see Using shader keywords with C# scripts.

To set this value in hand-coded shaders, see Declaring and using shader keywords in HLSL. To set this value in Shader Graph shaders, see Keywords.

Note: If a keyword with the same name exists in a shader source file and its dependencies, the scope of the keyword in the source file overrides the scope in the dependencies. Dependencies comprise all Shaders that are included via the Fallback command, and Passes that are included via the UsePass command.

Definition type: “multi compile” or “shader feature”

When you declare a set of keywords, you choose the technique that Unity uses to define them. This affects the number of shader variants that Unity compiles.

  • “multi compile” declares a set of keywords.

    Unity compiles shader variants for all keywords in the set.
  • “shader feature” declares a set of keywords, and also instructs the compiler to compile variants where none of these keywords are enabled.

    Unity examines the state of your project at build time, and only compiles variants for keywords that are in use. A keyword is in use if a material that is included in the build has that keyword enabled.

Which option is best depends on how you use the keywords. If you use the keywords to configure materials in your project and do not change their value from C# scripts at runtime, then you should use “shader feature” to reduce the number of shader keywords and variants in your project. If you enable and disable keywords at runtime using C# scripts, then you should use “multi compile” to prevent variants being stripped in error. For more information on shader stripping, see Shader variant stripping.

To set this value in hand-coded shaders, see Declaring and using shader keywords in HLSL. To set this value in Shader Graph shaders, see Keywords.

Stage-specific keywords

By default, Unity generates keyword variants for each stage of your shader. For example, if your shader contains a vertex stage and a fragment stage, Unity generates variants for every keyword combination for both the vertex and the fragment shader programs. If a set of keywords is only used in one of those stages, this results in identical variants for the other stage. Unity automatically identifies and deduplicates identical variants so that they do not increase build size, but they still result in wasted compilation time, increased shader loading times, and increased runtime memory usage.

To avoid this problem, when you declare a set of keywords in a hand-coded shader, you can instruct Unity to compile them only for a given shader stage.

Note: You are responsible for ensuring that the keywords are only used in the specified shader stages.

Graphics API support

Unity does not fully support the use of stage-specific keyword directives for the following graphics APIs:

  • OpenGL and Vulkan: At compile time, Unity automatically converts all stage-specific keyword directives to regular keyword directives.
  • Metal: any keyword targeting vertex stages also affects tessellation stages, and vice versa.

To set this value in hand-coded shaders, see Declaring and using shader keywords in HLSL. You cannot change this value in Shader Graph; by default, all keywords affect all stages.

Conditional compilation using shader keywords

You can mark sections of your shader source file so that Unity only includes that functionality in variants that are used when a given keyword is enabled.

The way that you do this is different in hand-coded shaders and in Shader Graph. For instructions for hand-coded shaders, see Defining and using shader keywords. For instructions for Shader Graph, see Shader Graph: Keyword Node.

Enabling and disabling shader keywords

You can enable or disable shader keywords. When you enable or disable a shader keyword, Unity uses the appropriate shader variant for rendering.

There are two ways to enable and disable shader keywords:

Unity’s predefined shader keywords

Unity uses predefined sets of shader keywords to enable common functionality. It adds the following sets of shader keywords at compile time:

  • By default, Unity adds this set of keywords to all graphics shader programs: STEREO_INSTANCING_ON, STEREO_MULTIVIEW_ON, STEREO_CUBEMAP_RENDER_ON, UNITY_SINGLE_PASS_STEREO. You can strip these keywords using an Editor script. For more information, see Shader variant stripping.
  • By default, Unity adds this set of keywords to the Standard Shader: LIGHTMAP_ON, DIRLIGHTMAP_COMBINED, DYNAMICLIGHTMAP_ON, LIGHTMAP_SHADOW_MIXING, SHADOWS_SHADOWMASK. You can strip these keywords using the Graphics settings window.
  • In the Built-in Render Pipeline, if your project uses tier settings that differ from each other, Unity adds this set of keywords to all graphics shaders: UNITY_HARDWARE_TIER1, UNITY_HARDWARE_TIER2, UNITY_HARDWARE_TIER3. For more information, see Graphics tiers: Graphics tiers and shader variants.

Shader keyword limits

Unity can use up to 4,294,967,294 global shader keywords. Individual shaders and compute shaders can use up to 65,534 local shader keywords.

Every keyword declared in the shader source file and its dependencies count towards this limit. Dependencies include Passes that the shader includes using UsePass, and fallbacks.

If Unity encounters a shader keyword with the same name multiple times, it only counts towards the limit once.

If a shader uses more than 128 keywords in total, it incurs a small runtime performance penalty; therefore, it is best to keep the number of keywords low.

Shader variants
Using shader keywords with C# scripts