codingMath03c

float centerY, centerX, offset, speed, angle;
float baseAlpha;

void setup() {
  size(400, 400);
  noStroke();

  centerY = height / 2;
  centerX = width / 2;
  offset = 127; // amplitude
  speed = 0.1;
  angle = 0;

  baseAlpha = 128;
}

void draw() {
  background(255); // clear canvas

  float alpha = baseAlpha + (sin(angle) * offset);

  fill(0, 0, 0, alpha);

  ellipse(centerX, centerY, 50, 50);

  angle += speed;
}