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Shader-Guide.md

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Image Processing Shader Guide

This guide will explain how to create custom GLSL shaders to use with my image processing library.
It will only explain how to design fragment shaders.

Version

Use version 330 core : all the pre-built filters use it, and other versions may not work.

Inputs / Outputs

The input of all shaders is the coordinate of the currently processed pixel. It's a 2-dimensional vector (x, y) with 'x' and 'y' varying from 0.0 to 1.0. The point (0, 0) is the bottom-left image corner and the point (1, 1) is the top-right image corner. The vector is named uv.
The shader output is a 4-dimensional vector representing the color of the current pixel (r, g, b, a). You can name it whatever you want. I personally name it frag_col or fragColor.

Uniforms

A uniform is a global shader variable modifiable by the CPU, so you can change its value in your python code.
To access the processed texture, a uniform is always required : display.
You also have access to the texture dimensions (in pixels) with the res uniform.
You can set as many uniforms as you want to make your shader customizable by the python code.

Main function

The shader needs a main function to work : it's necessarily called main().

Example

#version 330 core

uniform sampler2D display; // The processed texture is always necessary
uniform vec2 res; // Optional but prefer not to write it when you don't use it.

in vec2 uv;
out vec4 frag_col;

void main()
{
    vec4 color = texture(display, uv);
    frag_col = vec4(color.rgb, 1.0);
}

Filters Uniforms

Some pre-buillt shaders have customizable uniforms :

  • RGBA : uniforms float r=1., float g=1., float b=1., float a=1. : original pixel color multipliers.
  • BOX_BLUR : int kernel=3 : the kernel size used to blur. Must be an odd number.
  • MOTION_BLUR : vec2 direction=vec2(0, 0) : vector that represents the motion effect direction, float intensity=0.1 : the intensity of the motion blur effect.
  • RADIAL_BLUR : vec2 center=vec2(0.5, 0.5) : the center of the radial blur effect, float intensity=0.1 : the intensity of the radial blur effect.
  • CONVOLUTION : mat3 kernel=mat3(0, 0, 0, 0, 1, 0, 0, 0, 0) : the factors matrix used in the convolution process.
  • BLOOM : float spread=1. : the bloom spread, float intensity=0.5 : the bloom intensity.
  • BARREL : float BarrelPower=1.5 : the barrel effect intensity.
  • ANTI_FISH_EYE : float power=0.4 : the intensity of the effect.
  • VIGNETTE : float intensity=0.3 : the vignette effect intensity, which controls the border size magnitude.