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Showing content from https://docs.unity3d.com/Manual/SL-MultipleProgramVariants-make-conditionals.html below:

Manual: Make shader behavior conditional on keywords

Toggle shader keywords in the Editor

Make shader behavior conditional on keywords

To mark parts of your shaderA program that runs on the GPU. More info See in Glossary code conditional based on whether you enable or disable a shader keyword, use an HLSL if statement.

For example:

#pragma multi_compile QUALITY_LOW QUALITY_MED QUALITY_HIGH

if (QUALITY_LOW)
{
    // code for low quality setting
}
else if (QUALITY_MED)
{
    // code for medium quality setting
}
else if (QUALITY_HIGH)
{
    // code for high quality setting
}

You can now enable and disable keywords using the Inspector or C# scripting.

Refer to How Unity compiles branching shaders for more information about when to use which shader directive.

Branch if a keyword exists

For each keyword in a set, Unity automatically adds a _KEYWORD_DECLARED keyword. For example, if you declare a QUALITY_LOW keyword, Unity adds a QUALITY_LOW_KEYWORD_DECLARED keyword.

You can use this to check if a keyword exists, regardless of whether itâs enabled or disabled.

For example:

#pragma multi_compile QUALITY_LOW QUALITY_MED QUALITY_HIGH

#if defined(QUALITY_LOW_KEYWORD_DECLARED)
{
    // The QUALITY_LOW keyword exists
}

Branch when all keywords in a set are disabled

To execute code when all keywords in a shader_feature or multi_compile set are disabled, Unity must create an additional shader variant for that state.

If you use shader_feature to create a single keyword, Unity automatically creates an additional shader variant. For example:

// Creates a variant for when FEATURE_1_ON is enabled, and another for when FEATURE_1_ON is disabled. 
#pragma shader_feature FEATURE_1_ON

If you use shader_feature or multi_compile to create a set of two or more keywords, add _ when you declare a keyword set. For example:

// Creates 5 shader variants, including one for when RED, GREEN, BLUE, and WHITE are all disabled.
#pragma shader_feature _ RED GREEN BLUE WHITE

// Creates 2 shader variants, including one for when FEATURE_3_ON is disabled.
#pragma multi_compile _ FEATURE_3_ON

If you use dynamic_branch, Unity doesnât need an additional uniform integer for when all keywords are disabled. But to indicate that you use that state in conditional code, you should add _ when you declare a keyword set. For example:

// Creates 3 uniform integers.
// The underscore doesn't create an additional uniform integer, but it indicates you use the additional state in conditional code.
#pragma dynamic_branch _ QUALITY_LOW QUALITY_MED QUALITY_HIGH

Use other statements to make shader behavior conditional

You can also use the following HLSL pre-processor directives to create conditional code with shader_feature or multi_compile:

Using these instead of if makes it more difficult to change the #pragma keyword directive later. For example, if you need to reduce the number of shader variants, itâs more difficult to change to dynamic_branch.

Additional resources

Toggle shader keywords in the Editor

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