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 <code>Font</code> described by
* this <code>FontMetrics</code> object. No character extends further
* above the font's baseline than this height.
Max Descent
* Determines the maximum descent of the <code>Font</code> described by
* this <code>FontMetrics</code> object. No character extends further
* below the font's baseline than this height.
