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diff --git a/doc/context/sources/general/manuals/mk/mk-itworks.tex b/doc/context/sources/general/manuals/mk/mk-itworks.tex new file mode 100644 index 000000000..0a9d5fce8 --- /dev/null +++ b/doc/context/sources/general/manuals/mk/mk-itworks.tex @@ -0,0 +1,142 @@ +% language=uk + +\environment mk-environment + +\startcomponent mk-itworks + +\chapter{It works!} + +One of the more powerful commands in \CONTEXT\ is \type {\framed}. +You can pass quite some parameters that control the spacing, +alignment, backgrounds and more. This command is used all over the +place (although often hidden for the user) which means that it also has +to be quite stable. However, there is one nasty bit of code that +is hard to get right. Calculating the height of a box is not that +complex: the height that \TEX\ reports is indeed the height. +However, the width of box is determined by the value of \type +{\hsize} at the time of typesetting. The actual content can be +smaller. In the \type {\framed} macro by default the width is +calculated automatically. + +\startbuffer +\framed + [align=middle,width=fit] + {Out beyond the ethernet the spectrum spreads \unknown} +\stopbuffer + +\typebuffer + +this shows up as:\footnote{Taken from \quote {Casino Nation} by Jackson Browne.} + +\startlinecorrection +\getbuffer +\stoplinecorrection + +Or take this quote:\footnote{Taken from \quote {A World Without Us} by Alan Weisman.} + +\startbuffer +\hsize=.6\hsize \framed [align=middle,width=fit] {\input weisman } +\stopbuffer + +\typebuffer + +This gives a multi|-|line paragraph: + +\startlinecorrection +\getbuffer +\stoplinecorrection + +Here the outer \type {\hsize} was made a bit smaller. As you can +see the frame is determined by the widest line. Because it was one +of the first features we needed, the code in \CONTEXT\ that is +involved in determining the maximum natural width is pretty old. +It boils down to unboxing a \type {\vbox} and stepwise grabbing +the last box, penalty, kern and skip. You unwind the box +backwards. However, you cannot grab everything or in \TEX\ speak: +there is only a limited number of \type {\lastsomething} commands. +Special nodes, like whatsits cannot be grabbed and they make the +analyzer abort its analysis. There is no way that we can solve +this in traditional \TEX\ and in \CONTEXT\ \MKII. + +So how about \LUATEX\ and \CONTEXT\ \MKIV ? The macro used in the +\type {\framed} commands is: + +\starttyping +\doreshapeframedbox{do something with \box\framebox} +\stoptyping + +In \LUATEX\ we can manipulate box content at the \LUA\ level. Instead +of providing a truckload of extra primitives (which would also introduce +new data types at the \TEX\ end) we just delegate the job to \LUA. + +\starttyping +\def\doreshapeframedbox + {\ctxlua{commands.doreshapeframedbox(\number\framebox)}} +\stoptyping + +Here \type {\ctxlua} is our reserved instance and \type {commands} +provides the namespace for commands that we delegate to \LUA\ (so, +there are more of them). The amount of \LUA\ code is way less than +the \TEX\ code which we will not show here; it's in \type +{supp-box.tex} if you really want to see it. + +\starttyping +function commands.doreshapeframedbox(n) + local box_n = tex.box[n] + if box_n.width ~= 0 then + local hpack = node.hpack + local free = node.free + local copy = node.copy_list + local noflines, lastlinelength, width = 0, 0, 0 + local list = box_n.list + local done = false + for h in node.traverse_id('hlist',list) do + done = true + local p = hpack(copy(h.list)) + lastlinelength = p.width + if lastlinelength > width then + width = lastlinelength + end + free(p) + end + if done then + if width ~= 0 then + for h in node.traverse_id('hlist',list) do + if h.width ~= width then + h.list = hpack(h.list,width,'exactly') + h.width = width + end + end + end + box_n.width = width + end + -- we can also do something with lastlinelength + end +end +\stoptyping + +In the first loop we inspect all lines (nodes with type \type +{hlist}) and repack them to their natural width with \type +{node.hpack}. In the process we keep track of the maximum natural +width. In the second loop we repack the content again, but this +time permanently. Now we use the maximum encountered width which +is forced by the keyword \type {exactly}. Because all glue is +still present we automatically get the desired alignment. We +create local shortcuts to some node functions which makes it run +faster; keep in mind that this is a core function called many +times in a regular \CONTEXT\ job. + +In \CONTEXT\ \MKIV\ you will find quite some \LUA\ code and often +it looks rather complex, especially if you have no clue why it's +needed. Think of \OPENTYPE\ font handling which involves locating +fonts, loading and caching them, storing features and later on +applying them to node lists, etc. However, once we are beyond the +stage of developing all the code that is needed to support the +basics, we will start doing the things that more relate to the +typesetting process itself, like the previous code. One of the +candidates for a similar \LUA\ based solution is for instance +column balancing. From the previous example code you can deduce +that manipulating the node lists from \LUA\ can make that easier. +Of course we're a few more years down the road then. + +\stopcomponent |