First project - moving a transparent circle over a semi opaque background with an image behind

Processing Beginners
7

Using Thomas approach with koogs’ suggestion. Calling filter() in draw() is expensive.

@dan850 How would you use blend() to get this effect?

This post is relevant: How to do this kind of gradient on shapes

Kf

final int RAD=80;

PImage img, img2, imgBlur;
PGraphics msk;

void setup() {
  size(600, 600);
  msk = createGraphics(width, height);
  background(100, 0, 0);
  noStroke();
  for ( int t = 0; t < 100; t++) {
    fill(random(200), random(200), random(200));
    rect( random(width-20), random(height-20), 20, 20);
  }
  img = get();
  
  imgBlur = img.get();
  imgBlur.filter(BLUR, 7);
}

void draw() {
  msk.beginDraw();
  msk.background(255);
  msk.fill(0);
  msk.ellipse(mouseX, mouseY, RAD,RAD);
  msk.endDraw();
  background(img);
  
  img2 = imgBlur.get();;
  img2.mask(msk);
  
  image( img2, 0, 0);
  noFill();
  stroke(0);
  ellipse(mouseX, mouseY, RAD,RAD);
}