drawArc奇怪的行为

Question

代码：

int width = canvas.getWidth();
int height = canvas.getHeight();

int shift = 0;
RectF rect = new RectF(0 + shift, 0 + shift, width - 1 - shift, height - 1 - shift);

Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);
paint.setStyle(Paint.Style.STROKE);
paint.setStrokeWidth(30);
paint.setColor(0xff009900);

float angle_step = 180 / 5;

for (int i=0; i<5; i++) {
  canvas.drawArc(rect, 180 + angle_step * i, angle_step, true, paint);
}

结果：

问题： 如何使内部笔划而不是外部？

由于