Rainbows Created with Processing

import java.util.ArrayList;

ArrayList<TrailPoint> trail = new ArrayList<TrailPoint>();
float hue = 0;

class TrailPoint {
  float x, y;
  color c;
  float size;
  
  TrailPoint(float x, float y, color c, float size) {
    this.x = x;
    this.y = y;
    this.c = c;
    this.size = size;
  }
}

void setup() {
  size(800, 600);
  colorMode(HSB, 360, 100, 100); // Using HSB for easier color cycling
  noStroke();
  background(360); // White background in HSB
}

void draw() {
  background(360); // Clear previous frame
  
  // Create new trail point with current position and color
  trail.add(new TrailPoint(mouseX, mouseY, color(hue, 80, 100), 30));
  
  // Remove oldest point if trail gets too long
  if(trail.size() > 50) {
    trail.remove(0);
  }
  
  // Draw all points in the trail
  for(int i = 0; i < trail.size(); i++) {
    TrailPoint p = trail.get(i);
    float alpha = map(i, 0, trail.size(), 30, 100); // Fade older points
    float size = map(i, 0, trail.size(), 10, p.size); // Shrink older points
    
    fill(p.c, alpha);
    ellipse(p.x, p.y, size, size);
  }
  
  // Cycle through hues
  hue = (hue + 1) % 360;
}

void mouseMoved() {
  // Add extra points when moving fast for smoother trail
  if(dist(mouseX, mouseY, pmouseX, pmouseY) > 5) {
    trail.add(new TrailPoint(mouseX, mouseY, color(hue, 80, 100), 20));
  }
}

Key features of this version:

  1. Persistent trail using an ArrayList to store circle positions
  2. Rainbow color effect using HSB color mode
  3. Dynamic trail properties:
    • Older circles fade and shrink
    • Trail length limited to 50 circles
    • Automatic color cycling through all hues
  4. Smooth trail even with fast mouse movements
  5. White background that refreshes each frame

How to use:

  1. Run the sketch
  2. Move your mouse around the window
  3. Watch the rainbow trail follow your cursor
  4. The faster you move, the more spaced-out the circles become

The trail will maintain 50 circles at most, with the oldest ones automatically disappearing as new ones are created. The color continuously cycles through the rainbow spectrum, and the circles gradually shrink and fade as they age.

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