pandemonium_engine_docs/usage/shaders/introduction_to_shaders.md

Introduction to shaders

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

Pandemonium 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 Pandemonium, shaders are made up of three main functions: vertex(), fragment(), and light().

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

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

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

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.

Shader types

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

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

    shader_type spatial;

Here are the available types:

• spatial ( doc_spatial_shader ) for 3D rendering.
• canvas_item ( doc_canvas_item_shader ) for 2D rendering.
• particles ( doc_particle_shader ) for particle systems.

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 Pandemonium 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.

Processor functions

Depending on the shader type, you can override different processor functions. For spatial and canvas_item, you have access to vertex(), fragment(), and light(). For particles, you only have access to vertex().

Vertex processor ^^^^^^^^^^^^^^^^

The vertex() processing function is called once for every vertex in spatial and canvas_item shaders. For particles shaders, it is called once for every particle.

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, Pandemonium 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 ( doc_spatial_shader ) for an example.

Fragment processor ^^^^^^^^^^^^^^^^^^

The fragment() processing function is used to set up the Pandemonium material parameters per pixel. This code runs on every visible pixel the object or primitive draws. It is only available in spatial and canvas_item 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 Pandemonium will optimize it away. For example, if you do not write a value to RIM, Pandemonium will not calculate rim lighting. During compilation, Pandemonium 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 ( doc_canvas_item_shader ) and Spatial shaders ( doc_spatial_shader ), respectively.