var linew = 20; var lineoffset = 30; var space = linew + lineoffset;

function setup() { var canvas = createCanvas(400, 400); strokeCap(PROJECT); }

function draw() { background(255);

// lines
strokeWeight(linew);
for (let i = 0; i < 1.5 * width / space; i++) {
    if (i % 2 == 0) {
        stroke(255, 0, 0);
    } else {
        stroke(0, 0, 255);
    }

    let xoff = i * space + frameCount % (2 * space);
    line(-width / 2 + xoff,
        height / 4,
        xoff,
        3 * height / 4);
}

// hiding blocks
if (!mouseIsPressed) {
    noStroke();
    fill(255);
    rect(0, 0, width, height / 4);
    rect(0, 3 * height / 4, width, height / 4);
    rect(0, 0, 2 * width / 5, height);
    rect(3 * width / 5, 0, 2 * width / 5, height);
}
strokeWeight(1);
stroke(0);
noFill();
rect(2 * width / 5, height / 4, width / 5, height / 2);

}