gtk2/demos/gtk-demo/kaleidoscope.glsl

uniform float progress;
uniform sampler2D u_texture1;
uniform sampler2D u_texture2;

vec4 getFromColor (vec2 uv) {
  return GskTexture(u_texture1, uv);
}

vec4 getToColor (vec2 uv) {
  return GskTexture(u_texture2, uv);
}

// Source: https://gl-transitions.com/editor/kaleidoscope
// Author: nwoeanhinnogaehr
// License: MIT

const float speed = 1.0;
const float angle = 1.0;
const float power = 1.5;

vec4 transition(vec2 uv) {
  vec2 p = uv.xy / vec2(1.0).xy;
  vec2 q = p;
  float t = pow(progress, power)*speed;
  p = p -0.5;
  for (int i = 0; i < 7; i++) {
    p = vec2(sin(t)*p.x + cos(t)*p.y, sin(t)*p.y - cos(t)*p.x);
    t += angle;
    p = abs(mod(p, 2.0) - 1.0);
  }
  abs(mod(p, 1.0));
  return mix(
    mix(getFromColor(q), getToColor(q), progress),
    mix(getFromColor(p), getToColor(p), progress), 1.0 - 2.0*abs(progress - 0.5));
}


void mainImage(out vec4 fragColor, in vec2 fragCoord, in vec2 resolution, in vec2 uv)
{
  fragColor = transition(uv);
}