[XeTeX] lineskip is inconsistent
jonathan_kew at sil.org
Sun Feb 19 19:53:36 CET 2006
On 19 Feb 2006, at 5:54 pm, Steffen Wolfrum wrote:
> coming from ConTeXt I am still experimenting with different
> features of Xe(Con)TeX(t).
> One thing I have found today makes me wonder:
> Even though the lineskip is defined as 14 pt it differs totally
> from font to font. And only Times and Helvetica are on the grid.
Does ConTeXt redefine \lineskip to mean something other than its
built-in TeX meaning? If not, then I think you're using the wrong
In TeX, the "normal" spacing of text lines is given by the parameter
\baselineskip; successive lines of text will have their baselines
exactly this far apart, *provided* there aren't any particularly tall/
If TeX determines that the bottom of one line and the top of the next
would come closer than \lineskiplimit (default in Plain TeX: 0pt),
then it abandons \baselineskip and instead inserts \lineskip
*between* the adjacent lines (not their baselines, but between the
bottom of one and the top of the next).
An additional complication with XeTeX is that the height and depth of
each line is computed not on the basis of the actual shapes of the
characters, but is based on the "ascent" and "descent" metrics
provided by the font. In some cases, these metrics include some
whitespace above/below the normal extent of the glyphs, and so the
lines will logically "touch" (and \lineskip will kick in) even before
the actual visible glyphs would have touched.
When using such fonts, it can be helpful to set \lineskiplimit to a
*negative* value, to allow a certain amount of "overlap" between the
logical extent of adjacent lines, before the normal \baselineskip
spacing is abandoned and \lineskip used instead.
OK, that's the Plain TeX picture (actually, these are built-in TeX
parameters, not even Plain definitions). I don't know, however, to
what extent ConTeXt overrides this stuff. I'd be a bit surprised,
though, if it actually redefined these command names to mean
something quite different.
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