Because our eyes always try to see shapes, we can see a shape without it being fully closed. In this example a corner is missing. Can you still see the triangle?
function setup() {
var canvas = createCanvas(500, 500);
canvas.parent('sketch');
colorMode(HSB, 360, 100, 100, 100);
background(150, 75, 90, 100);
fill(20);
rect(0, 0, width / 2, height);
noStroke();
fill(150, 75, 90, 100);
triangle(150, 100, 100, 300, 380, 250);
}
function draw() {
}