function draw_stroke(stroke) {
    const points = stroke.points;

    if (points.length === 0) {
        return;
    }

    // console.debug(points)

    storage.ctx0.beginPath();
    storage.ctx0.moveTo(points[0].x, points[0].y);

    for (let i = 1; i < points.length; ++i) {
        const p = points[i];
        storage.ctx0.lineTo(p.x, p.y);
    }

    storage.ctx0.stroke();
}

function redraw_predraw() {
    storage.ctx1.clearRect(0, 0, storage.ctx1.canvas.width, storage.ctx1.canvas.height);
}

function predraw_user(user_id, event) {
    if (!(user_id in storage.predraw)) {
        storage.predraw[user_id] = [];
    }

    storage.ctx1.beginPath();
    if (storage.predraw[user_id].length > 0) {
        const last = storage.predraw[user_id][storage.predraw[user_id].length - 1];
        storage.ctx1.moveTo(last.x, last.y);
        storage.ctx1.lineTo(event.x, event.y);
    } else {
        storage.ctx1.moveTo(event.x, event.y);
    }
    storage.ctx1.stroke();       

    storage.predraw[user_id].push({ 'x': event.x, 'y': event.y });
}