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Introduction to shaders

This page explains what shaders are and will give you an overview of how they work in Redot. For a detailed reference of the engine's shading language, see Shading language.

Shaders are a special kind of program that runs on Graphics Processing Units (GPUs). They were initially used to shade 3D scenes but can nowadays do much more. You can use them to control how the engine draws geometry and pixels on the screen, allowing you to achieve all sorts of effects.

Modern rendering engines like Redot draw everything with shaders: graphics cards can run thousands of instructions in parallel, leading to incredible rendering speed.

Because of their parallel nature, though, shaders don't process information the way a typical program does. Shader code runs on each vertex or pixel in isolation. You cannot store data between frames either. As a result, when working with shaders, you need to code and think differently from other programming languages.

Suppose you want to update all the pixels in a texture to a given color. In GDScript, your code would use for loops:

for x in range(width):
  for y in range(height):
    set_color(x, y, some_color)

Your code is already part of a loop in a shader, so the corresponding code would look like this.

void fragment() {
  COLOR = some_color;
}

Note

The graphics card calls the fragment() function once or more for each pixel it has to draw. More on that below.

Shaders in Redot

Redot provides a shading language based on the popular OpenGL Shading Language (GLSL) but simplified. The engine handles some of the lower-level initialization work for you, making it easier to write complex shaders.

In Redot, shaders are made up of main functions called "processor functions". Processor functions are the entry point for your shader into the program. There are seven different processor functions.

  1. The vertex() function runs over all the vertices in the mesh and sets their positions and some other per-vertex variables. Used in canvas_item shaders and spatial shaders.

  2. The fragment() function runs for every pixel covered by the mesh. It uses values output by the vertex() function, interpolated between the vertices. Used in canvas_item shaders and spatial shaders.

  3. The light() function runs for every pixel and for every light. It takes variables from the fragment() function and from its previous runs. Used in canvas_item shaders and spatial shaders.

  4. The start() function runs for every particle in a particle system once when the particle is first spawned. Used in particles shaders.

  5. The process() function runs for every particle in a particle system for each frame. Used in particles shaders.

  6. The sky() function runs for every pixel in the radiance cubemap when the radiance cubemap needs to be updated, and for every pixel on the current screen. Used in sky shaders.

  7. The fog() function runs for every froxel in the volumetric fog froxel buffer that intersects with the FogVolume. Used by fog shaders.

Warning

The light() function won't run if the vertex_lighting render mode is enabled, or if Rendering > Quality > Shading > Force Vertex Shading is enabled in the Project Settings. It's enabled by default on mobile platforms.

Note

Redot also exposes an API for users to write totally custom GLSL shaders. For more information see Using compute shaders.

Shader types

Instead of supplying a general-purpose configuration for all uses (2D, 3D, particles, sky, fog), you must specify the type of shader you're writing. Different types support different render modes, built-in variables, and processing functions.

In Redot, all shaders need to specify their type in the first line, like so:

shader_type spatial;

Here are the available types:

Render modes

Shaders have optional render modes you can specify on the second line, after the shader type, like so:

shader_type spatial;
render_mode unshaded, cull_disabled;

Render modes alter the way Redot applies the shader. For example, the unshaded mode makes the engine skip the built-in light processor function.

Each shader type has different render modes. See the reference for each shader type for a complete list of render modes.

Vertex processor

The vertex() processing function is called once for every vertex in spatial and canvas_item shaders.

Each vertex in your world's geometry has properties like a position and color. The function modifies those values and passes them to the fragment function. You can also use it to send extra data to the fragment function using varyings.

By default, Redot transforms your vertex information for you, which is necessary to project geometry onto the screen. You can use render modes to transform the data yourself; see the Spatial shader doc for an example.

Fragment processor

The fragment() processing function is used to set up the Redot material parameters per pixel. This code runs on every visible pixel the object or primitive draws. It is only available in spatial, canvas_item, and sky shaders.

The standard use of the fragment function is to set up material properties used to calculate lighting. For example, you would set values for ROUGHNESS, RIM, or TRANSMISSION, which would tell the light function how the lights respond to that fragment. This makes it possible to control a complex shading pipeline without the user having to write much code. If you don't need this built-in functionality, you can ignore it and write your own light processing function, and Redot will optimize it away. For example, if you do not write a value to RIM, Redot will not calculate rim lighting. During compilation, Redot checks to see if RIM is used; if not, it cuts all the corresponding code out. Therefore, you will not waste calculations on the effects that you do not use.

Light processor

The light() processor runs per pixel too, and it runs once for every light that affects the object. It does not run if no lights affect the object. It exists as a function called inside the fragment() processor and typically operates on the material properties setup inside the fragment() function.

The light() processor works differently in 2D than it does in 3D; for a description of how it works in each, see their documentation, CanvasItem shaders and Spatial shaders, respectively.