您的代码可以正常工作。发现一个错误，即对于平坦度，我们需要检查绝对值。还有一个打字错误，即p[1].y被使用了两次而不是一次。因此，它将如下所示：

static int32_t quadratic_flatness(const Vector2 p[3]) {
    return abs((int32_t) p[1].x - (((int32_t) p[0].x + (int32_t) p[2].x) / 2))
         + abs((int32_t) p[1].y - (((int32_t) p[0].y + (int32_t) p[2].y) / 2));
}

JS中的实时示例（单击“运行代码段”按钮，然后开始在画布上单击鼠标左键）：

var canvas = document.createElement('canvas');
document.body.appendChild(canvas);
document.body.style.margin = 0;
canvas.style.position = 'fixed';

// get canvas 2D context and set him correct size
var ctx = canvas.getContext('2d');
resize();

let points = [];

window.addEventListener('resize', resize);
document.addEventListener('mousedown', setPosition);

function quadratic_split_curve(p) { // out[2][3]
    const p12 = midpoint(p[0], p[1]);
    const p23 = midpoint(p[1], p[2]);
    const p123 = midpoint(p12, p23);
  return [
        [p[0], p12, p123],
        [p123, p23, p[2]]
  ];
}

function quadratic_flatness(p) {
    const result = Math.abs(p[1].x - (Math.floor( p[0].x + p[2].x) / 2)) +  Math.abs(p[1].y - (Math.floor( p[0].y +  p[2].y) / 2));
  return result;
}

function quadratic_bezier_curve(p, color) {
    if (quadratic_flatness(p) < 2) {
        line(p[0], p[2], color);
        return;
    } else {
        const split = quadratic_split_curve(p);
        quadratic_bezier_curve(split[0], color);
        quadratic_bezier_curve(split[1], color);
    }
}

function midpoint(a, b) {
    return {
    x: (a.x + b.x)/2,
    y: (a.y + b.y)/2
  }
}

// new position from mouse event
function setPosition(e) {
 if(points.length >= 3) {
        points = [];
  }
  points.push({ x: e.clientX, y: e.clientY });
  draw();
}

function resize() {
  ctx.canvas.width = window.innerWidth;
  ctx.canvas.height = window.innerHeight;
}

function clear() {
    ctx.clearRect(0, 0, window.innerWidth, window.innerHeight);
}
function dot(point) {
    ctx.fillRect(point.x-1,point.y-1,3,3);
}
function line(a, b, color) {
    ctx.strokeStyle = color;
    ctx.beginPath(); 
    ctx.moveTo(a.x, a.y);
    ctx.lineTo(b.x, b.y); 
    ctx.stroke();
}
function draw() {
    clear();
  for(const point of points) {
    dot(point);
  }
  
  if(points.length == 3) {
    ctx.lineWidth = 1;
    quadratic_bezier_curve(points, '#c0392b');
  }
  
  if(points.length > 1) {
  
    ctx.lineWidth = 2;
    ctx.strokeStyle = '#3039cb77';
    ctx.beginPath(); 
    ctx.moveTo(points[0].x, points[0].y);
    
    for(i=1; i<points.length; i++) {

      ctx.lineTo(points[i].x, points[i].y); 
    }
    ctx.stroke();
  }
  
  return;
}