String [] letters = {
  "A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z"
};
PFont f;
int ypos, ypos2;
int xpos, xpos2;

PGraphics masker;
PImage img;
String basetext;
String masktext;
void setup() {
  size(400, 410);
  img = createImage();
  f = createFont("sans-serif", 500);
  textFont(f);
  masker = createGraphics(width, height);
  basetext = letters[int(random(letters.length))];
  masktext = letters[int(random(letters.length))];

  ypos = height*.95;
  ypos2 = height + 200;
  xpos = width*.5;
  xpos2 = width*.5;
  frameRate(30);
  smooth();
}
void draw () {
  background(255);

  masker.beginDraw();
  masker.textAlign(CENTER);
  masker.textFont(f);
  masker.background(255);
  masker.fill(0);
  masker.text(masktext, xpos, ypos);
  masker.endDraw();

  textAlign(CENTER);
  textFont(f);
  fill(0);
  text(basetext, xpos2, ypos2);
  img = masker.get();
  blend(img, 0, 0, masker.width, masker.height, 0, 0, width, height, ADD);
  textSize(16);
  text(str(basetext) + " as seen through " + str(masktext), width*.5, height-textDescent()-2);

  ypos2 -= 5;

  if (ypos2 < 10) {
    ypos2 = height + 400;
    basetext = letters[int(random(letters.length))];
    masktext = letters[int(random(letters.length))];
  }
}