示意如下：
实际工作中，会发现事情没这么简单。除了图示之外：
- Max Advance
advance 是字符串基线上最左边的点到最右边的点之间的距离。string 的 advance 不一定是它所有字符的 advance 之和。这个不是字符属性吗？
- Max Ascent
* Determines the maximum ascent of the Font described by * this FontMetrics object. No character extends further * above the font's baseline than this height.
- Max Descent
* Determines the maximum descent of the Font described by * this FontMetrics object. No character extends further * below the font's baseline than this height.
