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Improve Shader preprocessor documentation (#6563)

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Hugo Locurcio
2023-01-13 21:33:07 +01:00
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415
tutorials/shaders/shader_reference/shader_preprocessor.rst
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.. _doc_shader_preprocessor:
Shader Preprocessor
Shader preprocessor
===================
The shader preprocessor is an optional step before shader compilation of text shaders in Godot.
If you don't need it, you may ignore it, but it contains some useful macros which may speed up your productivity.
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 :ref:`visual shaders <doc_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.
- Preprocessor directives can span several lines by ending each line with a
blackslash (``\``). The first line break *not* featuring a backslash will end
the preprocessor statement.
#define
^^^^^^^
\ **Syntax:** `#define identifier <replacement_code>`.
\ **Syntax:** ``#define <identifier> [replacement_code]``.
Defines the identifier after that directive as a macro, and replaces all successive occurrence of it with the replacement code given in the shader.
If the replacement code is not defined, it may only be used within `#ifdef` or `#ifndef` directives.
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 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.
Compared to constants (``const CONSTANT = value;``), ``#define``s can be used
anywhere within the shader. ``#define``s can also be used to insert arbitrary
shader code at any location, while constants can't do that.
.. code-block:: glsl
shader_type spatial;
// Notice the lack of semicolon at the end of the line, as the replacement text
// shouldn't insert a semicolon on its own.
#define USE_MY_COLOR
#define MY_COLOR vec3(1, 0, 0)
// Replacement with arguments.
// All arguments are required (no default values can be provided).
#define BRIGHTEN_COLOR(r, g, b) vec3(r + 0.5, g + 0.5, b + 0.5)
// Multiline replacement using backslashes for continuation:
#define SAMPLE(param1, param2, param3, param4) long_function_call( \
param1, \
param2, \
param3, \
param4 \
)
void fragment() {
#ifdef USE_MY_COLOR
ALBEDO = MY_COLOR;
#endif
}
#if
^^^
\ **Syntax:** `#if condition`.
The `#if` directive checks the condition and if it evaluates to a non-zero value, the code block is included, otherwise it is skipped.
In order to evaluate, the condition must be an expression giving a simple floating-point, integer or boolean result. There may be multiple condition blocks connected by `||` or `&&` operators.
It may be continued by a `#else` block, but must be ended with the `#endif` directive.
.. code-block:: glsl
#define VAR 3
#define USE_LIGHT 0 // evaluates to false
#define USE_COLOR 1 // evaluates to true
#if VAR == 3 && (USE_LIGHT || USE_COLOR)
#endif
#ifdef
^^^^^^
\ **Syntax:** `#ifdef identifier`.
Checks whether the passed identifier is defined by `#define` before that directive. 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.
.. code-block:: glsl
#define USE_LIGHT
#ifdef USE_LIGHT
#endif
#ifndef
^^^^^^^
\ **Syntax:** `#ifndef identifier`.
Similar to `#ifdef` but checks whether the passed identifier is not defined by `#define` before that directive.
#else
^^^^^
\ **Syntax:** `#else`.
Defines the optional block which is included when the previously defined `#if`, `#ifdef` or `#ifndef` directive evaluates to false.
.. code-block:: glsl
shader_type spatial;
#define MY_COLOR vec3(1, 0, 0)
void fragment() {
#ifndef MY_COLOR
ALBEDO = MY_COLOR;
#else
ALBEDO = vec3(0, 0, 1);
#endif
}
#endif
^^^^^^
\ **Syntax:** `#endif`.
Used as terminator for the `#if`, `#ifdef`, `#ifndef` or subsequent `#else` directives.
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 the previously defined `#define` directive:
**Syntax:** ``#undef identifier``
The ``#undef`` directive may be used to cancel a previously defined ``#define`` directive:
.. code-block:: glsl
@@ -112,18 +111,274 @@ The `#undef` directive may be used to cancel the previously defined `#define` di
return MY_COLOR;
}
Without `#undef` in that case there will be a macro redefinition error.
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.
.. code-block:: glsl
#define VAR 3
#define USE_LIGHT 0 // Evaluates to `false`.
#define USE_COLOR 1 // Evaluates to `true`.
#if VAR == 3 && (USE_LIGHT || USE_COLOR)
// Condition is `true`. Include this portion in the final shader.
#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.
.. code-block:: glsl
#define USE_LIGHT
#define USE_COLOR
// Correct syntax:
#if defined(USE_LIGHT) || defined(USE_COLOR) || !defined(USE_REFRACTION)
// Condition is `true`. Include this portion in the final shader.
#endif
Be careful, as ``defined()`` must only wrap a single identifier within parentheses, never more:
.. code-block:: glsl
// Incorrect syntax (parentheses are not placed where they should be):
#if defined(USE_LIGHT || USE_COLOR || !USE_REFRACTION)
// This will cause an error or not behave as expected.
#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 :ref:`shading language <doc_shading_language>` supports run-time ``if`` statements:
.. code-block:: glsl
uniform bool USE_LIGHT = true;
if (USE_LIGHT) {
// This part is included in the compiled shader, and always run.
} else {
// This part is included in the compiled shader, but never run.
}
If the uniform is never changed, this behaves identical to the following usage
of the ``#if`` preprocessor statement:
.. code-block:: glsl
#define USE_LIGHT
#if defined(USE_LIGHT)
// This part is included in the compiled shader, and always run.
#else
// This part is *not* included in the compiled shader (and therefore never run).
#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 <https://medium.com/@jasonbooth_86226/branching-on-a-gpu-18bfc83694f2>`__.
"Static" branching refers to ``if`` statements where *all* pixels/vertices
evaluate to the same result in a given shader invocation. However, high amounts
of :abbr:`VGPR (Vector General-Purpose Register)`s (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``s in the same ``#if`` statement.
.. code-block:: glsl
#define VAR 3
#define USE_LIGHT 0 // Evaluates to `false`.
#define USE_COLOR 1 // Evaluates to `true`.
#if VAR == 3 && (USE_LIGHT || USE_COLOR)
// Condition is `true`. Include this portion in the final shader.
#endif
Like with ``#if``, the ``defined()`` preprocessor function can be used:
.. code-block:: glsl
#define SHADOW_QUALITY_MEDIUM
#if defined(SHADOW_QUALITY_HIGH)
// High shadow quality.
#elif defined(SHADOW_QUALITY_MEDIUM)
// Medium shadow quality.
#else
// Low shadow quality.
#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.
.. code-block:: glsl
#define USE_LIGHT
#ifdef USE_LIGHT
#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:
.. code-block:: glsl
#define SHADOW_QUALITY_MEDIUM
#ifdef SHADOW_QUALITY_HIGH
// High shadow quality.
#else
#ifdef SHADOW_QUALITY_MEDIUM
// Medium shadow quality.
#else
// Low shadow quality.
#endif // This ends `SHADOW_QUALITY_MEDIUM`'s branch.
#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.
.. code-block:: glsl
#define USE_LIGHT
#ifndef USE_LIGHT
// Evaluates to `false`. This portion won't be included in the final shader.
#endif
#ifndef USE_COLOR
// Evaluates to `true`. This portion will be included in the final shader.
#endif
#else
^^^^^
**Syntax:** ``#else``
Defines the optional block which is included when the previously defined `#if`,
``#ifdef` or `#ifndef`` directive evaluates to false.
.. code-block:: glsl
shader_type spatial;
#define MY_COLOR vec3(1.0, 0, 0)
void fragment() {
#ifndef MY_COLOR
ALBEDO = MY_COLOR;
#else
ALBEDO = vec3(0, 0, 1.0);
#endif
}
#endif
^^^^^^
**Syntax:** ``#endif``
Used as terminator for the ``#if``, ``#`ifdef``, ``#ifndef`` or subsequent ``#else`` directives.
#include
^^^^^^^^
\ **Syntax:** `#include "path"`.
The `#include` directive includes the content of shader include to a shader. It may be used in any place, but is recommended at the beginning of the shader file,
after the `shader_type` to prevent possible errors. The shader include may be created by using a `File → Create Shader Include` menu option of the shader editor.
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.
This directive may be used in any place, but is recommended at
the beginning of the shader file, after the ``shader_type`` to prevent possible
errors. The shader include may be created by using a **File > Create Shader
Include** menu option of the shader editor.
``#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:
.. code-block:: glsl
#include "my_shader_inc.gdshaderinc"
// fancy_color.gdshaderinc
// While technically allowed, there is usually no `shader_type` declaration in include files.
vec3 get_fancy_color() {
return vec3(0.3, 0.6, 0.9);
}
Example base shader (using the include file we created above):
.. code-block:: glsl
// material.gdshader
shader_type spatial;
#include "res://fancy_color.gdshaderinc"
void fragment() {
// No error, as we've included a definition for `get_fancy_color()` via the shader include.
COLOR = get_fancy_color();
}
#pragma
^^^^^^^
@@ -131,9 +386,15 @@ after the `shader_type` to prevent possible errors. The shader include may be cr
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, and it will speed up the shader compilation by excluding the preprocessor step.
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.
.. code-block:: glsl
#pragma disable_preprocessor
#if USE_LIGHT
// This causes a shader compilation error, as the `#if USE_LIGHT` and `#endif`
// are included as-is in the final shader code.
#endif

2
tutorials/shaders/shader_reference/shading_language.rst
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@@ -512,7 +512,7 @@ Godot Shading language supports the most common types of flow control:
} while (cond);
Keep in mind that, in modern GPUs, an infinite loop can exist and can freeze
Keep in mind that in modern GPUs, an infinite loop can exist and can freeze
your application (including editor). Godot can't protect you from this, so be
careful not to make this mistake!