[Dev-luatex] Modifying the LuaTeX Engine

Arthur Reutenauer arthur.reutenauer at normalesup.org
Tue Dec 30 17:08:08 CET 2008


> Can you show me an example of vertical typesetting in LuaTeX?

  Try the attached file.  It demonstrates vertical typesetting with LuaTeX in
three steps; unfortunately it only works in DVI mode for the moment
because the PDF output is completely buggy, I discussed with Taco about
that.

  1. At first, you simply need to switch to vertical directions, using the
primitives inherited from Omega.  Here I used LTL as the general direction: the
first letter indicates the first edge of the page is at the left (lines of text
move globally from left to right); 'T' means the second edge of the page is at
the top (inside a single line, text moves from the top downwards).  The last
letter defines the notion of "top" for individual letters, I chose left, more
or less randomly.  It has some influence, as we will see later, but not as much
as the first two letters.

  Then you can set English text vertically, with the columns running from left
to right.  It's akward to use OpenType fonts in DVI mode, so I simply used
Computer Modern Typewriter, stacking the letters on top of each other.  Of
course it's weird to set English text vertically without rotating the letters,
but it is possible.  There is a problem, though, because the text looks
completely clumsy: the reason is the font's metrics are not designed for
vertical typesetting, so you need to change them.  The way metrics work in
Omega's directional model is that width runs along the direction of the line,
here top-to-bottom; hence a glyph's width, in that example, is actually the sum
of its height and its depth in the original font designed for horizontal
typesetting. Its height and depth, in turn, need to be chosen so that their sum
is equal to the horizontal width, and I think it's a good choice to make them
equal to each other, so that each glyph is centered on the vertical base line.

  2. On the second page I demonstrate a first way to do that, with Lua code:
you can use the define_font callback to redefine width, height and depth for
each glyph in the font, and LuaTeX will use these new metrics.

  Note that the vertical width should, actually, be a little bit more than
horizontal height + horizontal depth, because these dimensions are not meant to
blend well with stacks of letters, unlike the horizontal width: the former are
really more like the dimensions of a bounding box, whereas the latter is the
distance by which you should advance the line of text in order for the text not
to look to crowded.  Thus, I cheated a bit, by stretching the vertical width by
30% (chosen, again, at random by trial and error).

  But that's still not enough: as you can see, letters with descender, like 'g'
in the first line, sticks into the next letter.  That's because it has a
horizontal depth, and its origin is thus too low with respect to its enclosing
box: we need to raise those letters by their horizontal depth.

  3.  We could do this by editing the font and modify every glyph so that it
sits on the baseline, but we can do better by using virtual fonts.  In LuaTeX,
there are two ways to use virtual fonts: you can use a .vf file, or you can
define it with Lua commands in the font data.  Unfortunately, the latter
doesn't work in DVI mode because your Lua code is lost once the DVI file is
produced, and your DVI reader / driver doesn't know how to create the font, so
we have to stick to the traditional method and produce a .vf file.  I wrote a
small script to that effect which you will find in the attached zip, but the
.vf and .tfm files are also attached anyway.  Now the text looks quite nice, in
my opinion.  It's still hard to read, but that's of course because English is
not supposed to be typeset that way.

  The (figurative) bottom line of all this is that you need to typeset vertical
text with fonts that have vertical metrics.  Once the problems with PDF mode
are fixed, I can try and do that with Chinese or Japanese fonts; I expect it
would be quite straightforward.

	Arthur
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