What It Means & How to Fix It in UE5

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🧠 Why You’re Seeing This Error

This error means the shader compiler tried to use a variable or function name that hasn’t been defined or declared in your shader file (or included files).

In Unreal Engine 5, this usually happens when:

• You misspelled a variable or function name
• A needed uniform, buffer, or macro is missing
• You forgot to include a .usf or .ush file with the needed definitions
• A preprocessor directive (#if, #define, etc.) blocked something from being declared

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💥 Example Error Message

error X3004: undeclared identifier 'LightDirection'

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🛠️ Common Shader Scenarios That Cause This

❌ Typo or Misspelled Variable

hCopyEditfloat3 result = LightDirektion; // ← typo: should be LightDirection

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❌ Missing Include File

hCopyEdit// You're using a function like TransformWorldToClip()
TransformWorldToClip(VertexPosition); // ❌ But Common.usf isn't included

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❌ Variable Is Inside a Disabled #if Block

#if USE_LIGHTING
    float3 LightColor = float3(1,1,1);
#endif

float3 Output = LightColor * Albedo; // ❌ LightColor may not exist if USE_LIGHTING is 0

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✅ How to Fix X3004 in UE5 – Step-by-Step

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✔️ 1. Check for Typos or Case-Sensitive Mistakes

VariableName ≠ variablename ≠ VARIABLEname

HLSL is case-sensitive, unlike Blueprint variable names.

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✔️ 2. Verify That the Variable or Function Is Defined

Search the file or included files for the identifier. If it’s not found:

- Declare it manually
- Or include the proper `.ush` / `.usf` file that contains it

Example:

#include "/Engine/Private/Common.ush"

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✔️ 3. Make Sure Preprocessor Conditions Don’t Block It

Check if the variable is inside a conditional block like:

#if USE_SHADOWS
    float ShadowAmount;
#endif

If USE_SHADOWS is undefined or 0, then ShadowAmount is never declared.

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✔️ 4. Review Included Files and Their Order

If you’re using custom shaders:

- Ensure required files are included (Common.ush, ScreenPass.ush, etc.)
- Avoid shadowing variable names with redefinitions

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✅ Summary: How to Fix “error X3004: undeclared identifier” in UE5

Cause	Fix Example
Variable/function was never declared	Define it, or include a file where it is declared
Misspelled name	Correct the spelling and casing
Macro or preprocessor condition excluded it	Check for missing #define or #if blocks
Missing include file	Add #include for needed shader utility files