计算字体truetype hhea值（上升，下降） [英] Computing font truetype hhea values (ascender, descender)

问题描述

我试图解析truetype字体为建立和存储上升，为每个字体大小的下行，我使用 http://nodebox.github.io/opentype.js/ 做了一个了不起的工作，但我不明白如何计算返回的值。

示例字体Blackoak（adobe）
我有[Ascender，Descender] = 1900和-500

我认为这些数字表示与em空间的基线的距离，但是我需要其他元信息来计算这些值吗？

提前Thx

解决方案

如果要计算ascender /以字体为单位的下拉字体，然后你需要为 sTypoAscender 和<$ c $查找 OS / 2 c> sTypoDescender 值（按 W3C ）。 hhea 表中的上升和下降 字体设计师的意图 必然对应于实际的计算值。关于价值类型，你是正确的，上行和下行是在UPM单位，负面意味着低于基准。



为了将这些数字转换为特定字体大小的像素，还需要提取字体中每个em 的单位数。您可以通过从头表中读取 unitsPerEm 值来实现。通常，对于OTF，其1000个UPM，对于TTF-1024或2048个UPM。 sTypoAscender 和 sTypoDescender 的绝对值应该合计为 unitsPerEm 值。然后它只是一个比例的问题。例如，让我们来看看opentype.js nofollow>字体检查器并假设16px的字体大小。值 unitsPerEm 是2048 UPM， sTypoAscender 和 sTypoDescender 是1536 （1536 + 512 = 2048）：



  Ascender = 16 * 1536/2048 = 12px（高于基线）
 Descender = 16 * -512/2048 = -4px（低于基线）
  




I'm trying to parse truetype font for building and store the ascender,descender for each fontsize, i'm using http://nodebox.github.io/opentype.js/ that did an amazing work, but i don't understand how to compute the returned values.

Exemple font Blackoak (adobe) i've got [Ascender, Descender] = 1900 and -5OO

I presume that these numbers indicates the distance from the basline in em space, but did i need other meta information for computing these values?

Thx in advance

解决方案

If you want to compute ascender/descender in pixels for specific font size, then you need to lookup OS/2 table for sTypoAscender and sTypoDescender values (as instructed by W3C). The ascender and descender values of hhea table are marking only font designer's intention which not necessarily corresponds to real computed values. Concerning value type, you're right, ascender and descender are in UPM units and negative means below the baseline.

In order to convert these numbers to pixels for specific font size you also need to extract how many units per em are in your font. You can do that by reading unitsPerEm value from the head table. Usually, for OTF its 1000 UPM, for TTF - 1024 or 2048 UPM. Absolute values of sTypoAscender and sTypoDescender should add up to unitsPerEm value. And then its just a matter of ratio.

For example, lets take a look at the opentype.js Font Inspector and assume 16px font size. Value unitsPerEm is 2048 UPM, sTypoAscender and sTypoDescender are 1536 and -512 correspondingly (1536 + 512 = 2048):

 Ascender = 16 * 1536/2048 = 12px  (above baseline)
Descender = 16 * -512/2048 = -4px  (below baseline)

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