{\bf \noindent Note: For simplicity, this project uses plain \TeX{} compiled using the Lua\TeX{} engine.\vskip10mm}
\let\lua\directlua % Don't change this.
\catcode`\~=12
{\noindent \bf Here is a typeset paragraph\vskip3mm }
\noindent For example, when \TeX{} typesets a paragraph of text and breaks it into a series of lines, it considers the paragraph's text as a sequence of boxes and uses the dimensions of those character boxes to find the best linebreaks. Each line of the paragraph is itself a box (containing other boxes---e.g., characters) and the typeset paragraph lines (boxes) are stacked vertically to produce the paragraph. Eventually, the largest box of all is produced: the typeset page. Clearly, this is a very simplified picture because you also need the ability to arbitrarily position those boxes and \TeX{} does this using so-called glue. Knuth commented (page 70 of The \TeX book) that ``glue'' probably should have been referred to as ``spring'' but the term glue was adopted early on and, pardon the pun, stuck.
\vskip10mm
{\noindent \bf Here is the same paragraph processed by a Lua\TeX{} callback \vskip3mm}
\lua{%
local HLIST = node.id("hlist")
local RULE = node.id("rule")
local GLUE = node.id("glue")
local GLYPH = node.id("glyph")
local GSPEC = node.id("glue_spec")
local KERN = node.id("kern")
local WHAT = node.id("whatsit")
function lineparse(line)
local box_order=line.glue_order
local box_ratio=line.glue_set
local box_sign=line.glue_sign %0 = normal, 1 = stretching, 2 = shrinking
% This loop uses a function to look for all lines of the typeset paragraph
% This function scans all the items in a typeset line of the paragraph
for n in node.traverse(line.head) do
% We have seen some glue: calculate its size, draw a box
if n.id == GLUE then
% glue parameters
local gwidth=n.width %horizontal or vertical displacement
local gstr=n.stretch %extra (+ve) displacement or stretch amount
local gstrord=n.stretch_order %factor applied to stretch amount
local gshr=n.shrink %extra (-ve) displacement or shrink amount
local gshrord=n.shrink_order %factor applied to shrink amount
local width=0
% This section is how TeX calculates glue settings
if box_sign==0 then %(normal)
width = gwidth
elseif box_sign==1 then %(stretch)
if gstrord == box_order then
width = gwidth + (box_ratio * gstr)
else
width = gwidth
end
elseif box_sign == 2 then %(shrink)
if gshrord == box_order then
width = gwidth - (box_ratio * gshr)
else
width = gwidth
end
end %if box_sign
% now calculate the glue's width in bp + draw a box.
if width > 0 then % don't bother with zero width glues
local pdfn=node.new(WHAT, "pdf_literal")
local wp = (width/65536)*(72/72.27)
pdfn.data=" 0.2 w 0 0 "..wp.." 5 re q 0.9 0.9 0.0 rg f Q "
pdfn.mode=0
% insert into the typeset paragraph
local np = n.prev
np.next=pdfn
pdfn.prev=np
pdfn.next=n
n.prev=pdfn
end
end % if n.id == GLUE
% Within the typeset line we've seen another hbox
if n.id == HLIST then
local nw = n.width
local nd= n.depth
local nh = n.height
local ns = n.shift
local hw = ((nw/65536)*(72/72.27))
local hd = ((nd/65536)*(72/72.27))
local hh = ((nh/65536)*(72/72.27))
local hs= ((ns/65536)*(72/72.27))
local pdfn=node.new(WHAT, "pdf_literal")
local tot=hd+hh %total height + depth
local ts = hd + hs %total height + any shift
pdfn.data=" 0.2 w 0 -"..ts.." "..hw.. " "..tot.." re q 0.9 0.0 0.0 rg f Q"
pdfn.mode=0
% insert into the typeset paragraph
local np = n.prev
np.next=pdfn
pdfn.prev=np
pdfn.next=n
n.prev=pdfn
end % if n.id == HLIST then
if n.id == GLYPH then
local nw = n.width
local nd= n.depth
local nh = n.height
local hw = ((nw/65536)*(72/72.27))
local hd = ((nd/65536)*(72/72.27))
local hh = ((nh/65536)*(72/72.27))
local tot=hd+hh %total height + depth
local pdfn=node.new(WHAT, "pdf_literal")
pdfn.data=" 0.2 w 0 -"..hd.." "..hw.. " "..tot.." re q 0.0 0.0 0.0 RG S Q"
pdfn.mode=0
% insert into the typeset paragraph
local np = n.prev
if np ~= nil then
np.next=pdfn
pdfn.prev=np
pdfn.next=n
n.prev=pdfn
elseif np == nil then
%First glyph at start of the list: special steps needed... May not always work...?
n.prev=pdfno
pdfn.next=n
line.head=pdfn
end
end % if n.id == GLYPH then
% Insert the line bounding box
local lw = line.width
local ld = line.depth
local lh = line.height
local lhw = ((lw/65536)*(72/72.27))
local lhd = ((ld/65536)*(72/72.27))
local lhh = ((lh/65536)*(72/72.27))
local ltot=lhd+lhh
local pdfn=node.new(WHAT, "pdf_literal")
pdfn.data=" 0.1 w 0 -"..lhd.." "..lhw.. " "..ltot.." re q 0.9 g f Q"
pdfn.mode=0
%local ln=line.head.next
pdfn.next=line.head
line.head=pdfn
end % for n in node.traverse
end % end of function
parsepara = function (head)
for line in node.traverse_id(HLIST, head) do
lineparse(line)
end
return head
end
}
\lua{callback.register("post_linebreak_filter", parsepara)}
\noindent For example, when \TeX{} typesets a paragraph of text and breaks it into a series of lines, it considers the paragraph's text as a sequence of boxes and uses the dimensions of those character boxes to find the best linebreaks. Each line of the paragraph is itself a box (containing other boxes---e.g., characters) and the typeset paragraph lines (boxes) are stacked vertically to produce the paragraph. Eventually, the largest box of all is produced: the typeset page. Clearly, this is a very simplified picture because you also need the ability to arbitrarily position those boxes and \TeX{} does this using so-called glue. Knuth commented (page 70 of The \TeX book) that ``glue'' probably should have been referred to as ``spring'' but the term glue was adopted early on and, pardon the pun, stuck.
\bye
