feedback.frag

uniform sampler2D texture;
uniform vec2 resolution;

void main(void){
	vec2 uv = gl_FragCoord.xy / resolution.xy;
	vec3 tex = texture2D(texture, uv).rgb;
	vec3 offset = vec3(-0.01, -0.01, -0.005);
	tex += offset;
    gl_FragColor = vec4(tex, 1.);
}

ShaderFeedbackDemo.pde

PShader feedbackShader;
float t;

void setup() {
  size(1000, 1000, P2D);
  background(0);
  feedbackShader = loadShader("feedback.frag");
}

void draw() {
  t = radians(frameCount);
  translate(width / 2, height / 2);
  drawRotatingRectangle(0);
  filter(feedbackShader);
  if (frameCount > 360 && frameCount < 720) {
    //saveFrame("out2/###.jpg");
  }
}

void drawRotatingRectangle(int level) {
  noFill();
  stroke(255);
  strokeWeight(10);
  rotate(t * 0.05);
  rectMode(CENTER);
  float size = 800;
  rect(0, 0, size, size);
  scale(0.889);
  if (level < 30) {
    drawRotatingRectangle(level + 1);
  }
}

https://i.imgur.com/ltez9Hg.mp4