Shader preprocessor

Why use a shader preprocessor?

In programming languages, a preprocessor allows changing the code before the compiler reads it. Unlike the compiler, the preprocessor does not care about whether the syntax of the preprocessed code is valid. The preprocessor always performs what the directives tell it to do. A directive is a statement starting with a hash symbol (#). It is not a keyword of the shader language (such as if or for), but a special kind of token within the language.

From Godot 4.0 onwards, you can use a shader preprocessor within text-based shaders. The syntax is similar to what most GLSL shader compilers support (which in turn is similar to the C/C++ preprocessor).

Note

The shader preprocessor is not available in visual shaders. If you need to introduce preprocessor statements to a visual shader, you can convert it to a text-based shader using the Convert to Shader option in the VisualShader inspector resource dropdown. This conversion is a one-way operation; text shaders cannot be converted back to visual shaders.

Directives

General syntax

  • Preprocessor directives do not use brackets ({}), but can use parentheses.

  • Preprocessor directives never end with semicolons (with the exception of #define, where this is allowed but potentially dangerous).

  • Preprocessor directives can span several lines by ending each line with a backslash (\). The first line break not featuring a backslash will end the preprocessor statement.

#define

Syntax: #define <identifier> [replacement_code].

Defines the identifier after that directive as a macro, and replaces all successive occurrences of it with the replacement code given in the shader. Replacement is performed on a “whole words” basis, which means no replacement is performed if the string is part of another string (without any spaces or operators separating it).

Defines with replacements may also have one or more arguments, which can then be passed when referencing the define (similar to a function call).

If the replacement code is not defined, the identifier may only be used with #ifdef or #ifndef directives.

If the concatenation symbol (##) is present in the replacement code then it will be removed upon macro insertion, together with any space surrounding it, and join the surrounding words and arguments into a new token.

  1. uniform sampler2D material0;
  3. #define SAMPLE(N) vec4 tex##N = texture(material##N, UV)
  5. void fragment() {
  6.     SAMPLE(0);
  7.     ALBEDO = tex0.rgb;
  8. }

Compared to constants (const CONSTANT = value;), #define can be used anywhere within the shader (including in uniform hints). #define can also be used to insert arbitrary shader code at any location, while constants can’t do that.

  1. shader_type spatial;
  3. // Notice the lack of semicolon at the end of the line, as the replacement text
  4. // shouldn't insert a semicolon on its own.
  5. // If the directive ends with a semicolon, the semicolon is inserted in every usage
  6. // of the directive, even when this causes a syntax error.
  7. #define USE_MY_COLOR
  8. #define MY_COLOR vec3(1, 0, 0)
  10. // Replacement with arguments.
  11. // All arguments are required (no default values can be provided).
  12. #define BRIGHTEN_COLOR(r, g, b) vec3(r + 0.5, g + 0.5, b + 0.5)
  14. // Multiline replacement using backslashes for continuation:
  15. #define SAMPLE(param1, param2, param3, param4) long_function_call( \
  16.         param1, \
  17.         param2, \
  18.         param3, \
  19.         param4 \
  20. )
  22. void fragment() {
  23. #ifdef USE_MY_COLOR
  24.     ALBEDO = MY_COLOR;
  25. #endif
  26. }

Defining a #define for an identifier that is already defined results in an error. To prevent this, use #undef <identifier>.

#undef

Syntax: #undef identifier

The #undef directive may be used to cancel a previously defined #define directive:

  1. #define MY_COLOR vec3(1, 0, 0)
  3. vec3 get_red_color() {
  4.     return MY_COLOR;
  5. }
  7. #undef MY_COLOR
  8. #define MY_COLOR vec3(0, 1, 0)
  10. vec3 get_green_color() {
  11.     return MY_COLOR;
  12. }
  14. // Like in most preprocessors, undefining a define that was not previously defined is allowed
  15. // (and won't print any warning or error).
  16. #undef THIS_DOES_NOT_EXIST

Without #undef in the above example, there would be a macro redefinition error.

#if

Syntax: #if <condition>

The #if directive checks whether the condition passed. If it evaluates to a non-zero value, the code block is included, otherwise it is skipped.

To evaluate correctly, the condition must be an expression giving a simple floating-point, integer or boolean result. There may be multiple condition blocks connected by && (AND) or || (OR) operators. It may be continued by a #else block, but must be ended with the #endif directive.

  1. #define VAR 3
  2. #define USE_LIGHT 0 // Evaluates to `false`.
  3. #define USE_COLOR 1 // Evaluates to `true`.
  5. #if VAR == 3 && (USE_LIGHT || USE_COLOR)
  6. // Condition is `true`. Include this portion in the final shader.
  7. #endif

Using the defined() preprocessor function, you can check whether the passed identifier is defined a by #define placed above that directive. This is useful for creating multiple shader versions in the same file. It may be continued by a #else block, but must be ended with the #endif directive.

The defined() function’s result can be negated by using the ! (boolean NOT) symbol in front of it. This can be used to check whether a define is not set.

  1. #define USE_LIGHT
  2. #define USE_COLOR
  4. // Correct syntax:
  5. #if defined(USE_LIGHT) || defined(USE_COLOR) || !defined(USE_REFRACTION)
  6. // Condition is `true`. Include this portion in the final shader.
  7. #endif

Be careful, as defined() must only wrap a single identifier within parentheses, never more:

  1. // Incorrect syntax (parentheses are not placed where they should be):
  2. #if defined(USE_LIGHT || USE_COLOR || !USE_REFRACTION)
  3. // This will cause an error or not behave as expected.
  4. #endif

Tip

In the shader editor, preprocessor branches that evaluate to false (and are therefore excluded from the final compiled shader) will appear grayed out. This does not apply to run-time if statements.

#if preprocessor versus if statement: Performance caveats

The shading language supports run-time if statements:

  1. uniform bool USE_LIGHT = true;
  3. if (USE_LIGHT) {
  4.     // This part is included in the compiled shader, and always run.
  5. } else {
  6.     // This part is included in the compiled shader, but never run.
  7. }

If the uniform is never changed, this behaves identical to the following usage of the #if preprocessor statement:

  1. #define USE_LIGHT
  3. #if defined(USE_LIGHT)
  4. // This part is included in the compiled shader, and always run.
  5. #else
  6. // This part is *not* included in the compiled shader (and therefore never run).
  7. #endif

However, the #if variant can be faster in certain scenarios. This is because all run-time branches in a shader are still compiled and variables within those branches may still take up register space, even if they are never run in practice.

Modern GPUs are quite effective at performing “static” branching. “Static” branching refers to if statements where all pixels/vertices evaluate to the same result in a given shader invocation. However, high amounts of VGPRs (which can be caused by having too many branches) can still slow down shader execution significantly.

#elif

The #elif directive stands for “else if” and checks the condition passed if the above #if evaluated to false. #elif can only be used within an #if block. It is possible to use several #elif statements after an #if statement.

  1. #define VAR 2
  3. #if VAR == 0
  4. // Not included.
  5. #elif VAR == 1
  6. // Not included.
  7. #elif VAR == 2
  8. // Condition is `true`. Include this portion in the final shader.
  9. #else
  10. // Not included.
  11. #endif

Like with #if, the defined() preprocessor function can be used:

  1. #define SHADOW_QUALITY_MEDIUM
  3. #if defined(SHADOW_QUALITY_HIGH)
  4. // High shadow quality.
  5. #elif defined(SHADOW_QUALITY_MEDIUM)
  6. // Medium shadow quality.
  7. #else
  8. // Low shadow quality.
  9. #endif

#ifdef

Syntax: #ifdef <identifier>

This is a shorthand for #if defined(...). Checks whether the passed identifier is defined by #define placed above that directive. This is useful for creating multiple shader versions in the same file. It may be continued by a #else block, but must be ended with the #endif directive.

  1. #define USE_LIGHT
  3. #ifdef USE_LIGHT
  4. // USE_LIGHT is defined. Include this portion in the final shader.
  5. #endif

The processor does not support #elifdef as a shortcut for #elif defined(...). Instead, use the following series of #ifdef and #else when you need more than two branches:

  1. #define SHADOW_QUALITY_MEDIUM
  3. #ifdef SHADOW_QUALITY_HIGH
  4. // High shadow quality.
  5. #else
  6. #ifdef SHADOW_QUALITY_MEDIUM
  7. // Medium shadow quality.
  8. #else
  9. // Low shadow quality.
  10. #endif // This ends `SHADOW_QUALITY_MEDIUM`'s branch.
  11. #endif // This ends `SHADOW_QUALITY_HIGH`'s branch.

#ifndef

Syntax: #ifndef <identifier>

This is a shorthand for #if !defined(...). Similar to #ifdef, but checks whether the passed identifier is not defined by #define before that directive.

This is the exact opposite of #ifdef; it will always match in situations where #ifdef would never match, and vice versa.

  1. #define USE_LIGHT
  3. #ifndef USE_LIGHT
  4. // Evaluates to `false`. This portion won't be included in the final shader.
  5. #endif
  7. #ifndef USE_COLOR
  8. // Evaluates to `true`. This portion will be included in the final shader.
  9. #endif

#else

Syntax: #else

Defines the optional block which is included when the previously defined #if, #elif, #ifdef or #ifndef directive evaluates to false.

  1. shader_type spatial;
  3. #define MY_COLOR vec3(1.0, 0, 0)
  5. void fragment() {
  6. #ifdef MY_COLOR
  7.     ALBEDO = MY_COLOR;
  8. #else
  9.     ALBEDO = vec3(0, 0, 1.0);
  10. #endif
  11. }

#endif

Syntax: #endif

Used as terminator for the #if, #ifdef, #ifndef or subsequent #else directives.

#include

Syntax: #include "path"

The #include directive includes the entire content of a shader include file in a shader. "path" can be an absolute res:// path or relative to the current shader file. Relative paths are only allowed in shaders that are saved to .gdshader or .gdshaderinc files, while absolute paths can be used in shaders that are built into a scene/resource file.

You can create new shader includes by using the File > Create Shader Include menu option of the shader editor, or by creating a new ShaderInclude resource in the FileSystem dock.

Shader includes can be included from within any shader, or other shader include, at any point in the file.

When including shader includes in the global scope of a shader, it is recommended to do this after the initial shader_type statement.

You can also include shader includes from within the body a function. Please note that the shader editor is likely going to report errors for your shader include’s code, as it may not be valid outside of the context that it was written for. You can either choose to ignore these errors (the shader will still compile fine), or you can wrap the include in an #ifdef block that checks for a define from your shader.

#include is useful for creating libraries of helper functions (or macros) and reducing code duplication. When using #include, be careful about naming collisions, as redefining functions or macros is not allowed.

#include is subject to several restrictions:

  • Only shader include resources (ending with .gdshaderinc) can be included. .gdshader files cannot be included by another shader, but a .gdshaderinc file can include other .gdshaderinc files.

  • Cyclic dependencies are not allowed and will result in an error.

  • To avoid infinite recursion, include depth is limited to 25 steps.

Example shader include file:

  1. // fancy_color.gdshaderinc
  3. // While technically allowed, there is usually no `shader_type` declaration in include files.
  5. vec3 get_fancy_color() {
  6.     return vec3(0.3, 0.6, 0.9);
  7. }

Example base shader (using the include file we created above):

  1. // material.gdshader
  3. shader_type spatial;
  5. #include "res://fancy_color.gdshaderinc"
  7. void fragment() {
  8.     // No error, as we've included a definition for `get_fancy_color()` via the shader include.
  9.     COLOR = get_fancy_color();
  10. }

#pragma

Syntax: #pragma value

The #pragma directive provides additional information to the preprocessor or compiler.

Currently, it may have only one value: disable_preprocessor. If you don’t need the preprocessor, use that directive to speed up shader compilation by excluding the preprocessor step.

  1. #pragma disable_preprocessor
  3. #if USE_LIGHT
  4. // This causes a shader compilation error, as the `#if USE_LIGHT` and `#endif`
  5. // are included as-is in the final shader code.
  6. #endif

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