Ellipse. Using processing.js library.
float x = 512;
float x1 = 10;
float y = 288;
float y1 = 5;
void setup() {
size(1024, 576); // 16:9
frameRate(60);
background(0);
}
void draw() {
fill (0, 0, 0, 175);
stroke (255, 0, 0, 175);
strokeWeight(random(15));
ellipse(random(x), random(y), 20, 20);
x = x + x1;
y = y + y1;
if ((x > width -1) || (x < 1)) {
x1 = -x1;
background(random(255), random(255), random(255));
}
if ((y > height - 1) || (y < 1)) {
y1 = -y1;
background(0);
}
}