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Re: A definition for "character cell" is not provide in the spec

From: Jonathan Watt <jwatt@jwatt.org>
Date: Thu, 08 Feb 2007 04:20:10 +0100
Message-ID: <45CA96EA.5020705@jwatt.org>
To: Chris Lilley <chris@w3.org>
Cc: www-svg@w3.org

Hi Chris,

Chris Lilley wrote:
> JW> I really can't see how "character cell"/"glyph cell" can be
> JW> equated with "bounding box" based on any version of the spec.
> JW> (BTW, can the spec choose one of the former two terms and stick
> JW> with it please - assuming "character cell" and "glyph cell" are
> JW> actually interchangeable of course!)
> 
> They aren't interchangable.
> 
> The (misnamed) character cell, also termed the em square, is the
> design space of all the glyphs in a font. So its constant in size for
> a given font; its then scasled uniformly to the size at which the text
> is set.

Ah, great. "em square" is a concept I'm comfortable with as a term with a clear 
meaning outside of SVG. Can the SVG spec be changed to use the term instead of 
"character cell" along with a definition of "em square" please? The definition 
should at least mention that it's a commonly used typographic concept, or better 
still actually provide details. (And as a term it should be directly linkable, 
not embedded in some paragraph.)

> A glyph cell is the bounding box of an individual glyph. Even for
> Latin text these vary, depending on
> whether the glyph has descenders (like j, g, y) ascenders (d,t)  or
> neither (a, e, o). Once on adds an accent or tone mark, or indeed
> multiple of them as with Vietnamese or (IIRC) Thai, its clear that
> they vary a lot. And of course they cary in with too, compare i and w
> in a proportional font.

So "glyph cell" is the box that perfectly fits tightly around a specific glyph? 
Excluding stroke I presume? Again, as a term, can this be given a directly 
linkable definition please? ("bounding box" seems like a poor term for any such 
definition IMHO given other parts of the spec which use it as a concept for 
certain elements only.)

> JW> Defining the term "glyph cell" inside the definition for "bounding box" is the
> JW> wrong place to do it. In my opinion this sentence:
> 
> JW>    For example, for horizontal text, the calculations must assume
> JW>    that each glyph extends vertically to the full ascent and descent
> JW>    values for the font.
> 
> Which is therefore talking of the em square, not the glyph cell.

Here I'm confused. You just said "glyph cell" is not the same as em square, but 
here the text seems to be defining glyph cell.

Regards,
Jonathan
Received on Thursday, 8 February 2007 03:20:20 GMT

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