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Archive: 15 May 2012

Defining ‘ch’

I’m working my way through a rewrite of Two Salmon (more on that anon), and I just recently came to the ch unit.  Its definition in the latest CSS Values and Units module is as follows:

ch unit

Equal to the advance measure of the “0” (ZERO, U+0030) glyph found in the font used to render it.

…and that’s it.  I had never heard the term “advance measure” before, and a bit of Googling for font "advance measure" only led me to copies of the CSS Values and Units module and some configuration files for the Panda 3D game engine.  So I asked the editor and it turns out that “advance measure” is a CSS-ism that corresponds to the term “advance width”, which I had also never heard before but which yielded way more Google results.  Wikipedia’s entry for “Font” has this definition:

Glyph-level metrics include … the advance width (the proper distance between the glyph’s initial pen position and the next glyph’s initial pen position)…

My question for the font geeks in the room is this:  is that the generally accepted definition for “advance width”?  If not, is there a better definition out there; and if so, where?  I’d like to be able to recommend the best known definition for inclusion in the specification; or, if there’s no agreement as to the best, then at least a good one.  The Wikipedia definition certainly sounds good, assuming it’s accurate.  Is it?

In CSS terms, if I’ve understood things correctly, 1ch is equal to the width of the character box for the glyph for “0”.  In other words, if I were to create a floated element with just a “0” and no whitespace between it and the element’s open and close tags, then the float’s width would be precisely 1ch.  But that’s if I’ve understood things correctly.  If I haven’t, I hope I’ll be corrected soon!

May 2012
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