chapter26.tex

% -*- coding: utf-8 -*-
% Translated by inkydragon@github
% Date of translated: 2018-05-10
\documentclass{book}

\input{preamble}
\setcounter{chapter}{25}

\begin{document}

%\chapter{Page Shape}\label{page:shape}
\chapter{页面形状}\label{page:shape}

%This chapter treats some of the parameters that 
%determine the size of the page and how it appears on paper.
本章讨论一些决定页面大小及其在纸面上显示的参数。

%\label{cschap:topskip}\label{cschap:hoffset}\label{cschap:voffset}\label{cschap:vsize2}\label{cschap:maxdepth}\label{cschap:splitmaxdepth2}
%\begin{inventory}
%\item [\cs{topskip}] 
%      Minimum distance between the top of the page box
%      and the baseline of the first box on the page. 
%      Plain \TeX\ default:~\n{10pt}
\label{cschap:topskip}\label{cschap:hoffset}\label{cschap:voffset}\label{cschap:vsize2}\label{cschap:maxdepth}\label{cschap:splitmaxdepth2}
\begin{inventory}
\item [\cs{topskip}] 
      页面盒子的顶部到本页第一个盒子基线的最小距离。
      Plain \TeX\ 默认值:~\n{10pt}

%\item [\cs{hoffset \cs{voffset}}]
%\mdqon
%      Distance by which the page is shifted right/""down 
%\mdqoff
%      with respect to the reference point.
\item [\cs{hoffset \cs{voffset}}]
\mdqon
      页面相对参考点向左/""向下移动的距离。
\mdqoff

%\item [\cs{vsize}] 
%      Height of the page box.
%      Plain \TeX\ default:~\n{8.9in}
\item [\cs{vsize}] 
      页面盒子的高度。
      Plain \TeX\ 默认值:~\n{8.9in}

%\item [\cs{maxdepth}] 
%      Maximum depth of the page box.
%      Plain \TeX\ default:~\n{4pt}
\item [\cs{maxdepth}] 
      页面盒子的最大深度。
      Plain \TeX\ 默认值:~\n{4pt}

%\item [\cs{splitmaxdepth}] 
%      Maximum depth of a box split off by a \cs{vsplit} operation. 
%      Plain \TeX\ default: \cs{maxdimen}
\item [\cs{splitmaxdepth}] 
      一个盒子能被一个 \cs{vsplit} 操作分割的最大深度。
      Plain \TeX\ 默认值: \cs{maxdimen}

%\end{inventory}
\end{inventory}

%%\point The reference point for global positioning
%\section{The reference point for global positioning}
%\point The reference point for global positioning
\section{全局定位基准点}

%The \indexterm{page positioning} on the paper is governed by
%a \TeX\ convention, to which output device drivers
%must adhere, that the top left point of the page is
%one inch from the page edges. Unfortunately this
%may lead to lots of trouble, for instance if a printer
%(or the page description language it uses)
%takes, say, the {\em lower\/} left corner as the
%reference point, and is factory set to US paper sizes,
%but is used with European standard A4 paper.
纸张上的页面定位(\indexterm{page positioning})由 \TeX\ 的惯例决定：即页面的左上角距离页边1英寸，这也是输出设备必须如实反映的。
不幸的是，这可能会导致许多麻烦，考虑以下情形：一台打印机，或打印机使用的页面描述语言把\emph{左下角}当作基准点，并且打印机出厂默认设为美国纸张大小，但打印机用了欧洲的标准A4纸。

%The page is shifted on the paper if one assigns non-zero
%values to \csidx{hoffset} or \csidx{voffset}: positive values
%shift to the right and down respectively.
如果指定 \csidx{hoffset} 或 \csidx{voffset} 为一个非零的值，页面会在纸面上移动：正值代表向右下移动，水平和竖直方向的移动是独立的。

%%\point \cs{topskip}
%\section{\protect\cs{topskip}}
%\point \cs{topskip}
\section{\protect\cs{topskip}}

%The \csidx{topskip} ensures to a certain point
%that the first baseline of a page
%will be at the same location from page to page,
%even if font sizes
%are switched between pages or if the first line has
%no ascenders.
\csidx{topskip} 确保了一点：每一页的第一条基线都处于同一位置，即使页面之间的字号不相同或者第一行不含有升部\footnote{译注(woclass)：升部指的是某字母比字母x还要高的部分}的字母。

%Before the first box on each page some glue is inserted.
%This glue has the same stretch and shrink as \cs{topskip}, but
%the natural size is the natural size of \cs{topskip}
%minus the height of the first box, or zero if this
%would be negative.
每页第一个盒子之前都嵌入了铅空。
铅空可以和 \cs{topskip} 一起伸缩，通常它的大小是 \cs{topskip} 减去第一个盒子的高度，当结果为负数时，其值为零。

%Plain \TeX\ sets \cs{topskip} to {\tt 10pt}.
%Thus the top lines of pages will have their baselines
%at the same place if
%the top portion of the characters is ten point or less.
%For the Computer Modern fonts this condition is satisfied
%if the font size is less than (about) 13~points; 
%for larger fonts
%the baseline of the top line will drop.
Plain \TeX\ 将 \cs{topskip} 设为 {\tt 10pt}。因此，如果最上面一行的字母的顶部小于等于10点，每一页顶行的基线就处于相同的位置。
对于现代计算机字体来说，字号小于大约13点就能满足这个条件；对于大号字体来说，顶行的基线会下沉。

%The height of the page box for a page containing only
%text (and assuming a zero \cs{parskip})
%will be the \cs{topskip} plus a number of times
%the \cs{baselineskip}. Thus one can define a macro
%to compute the \cs{vsize} from the number of lines
%on a page:
%\howto Specify page height in lines\par
%\begin{verbatim}
%\def\HeightInLines#1{\count@=#1\relax
%    \advance\count@ by -1\relax
%    \vsize=\baselineskip
%    \multiply\vsize by \count@
%    \advance\vsize by \topskip}
%\end{verbatim}
%Calculating the \cs{vsize} this way will prevent
%underfull boxes for text-only pages.
假设 \cs{parskip} 为零，对于一个只含有文字的页面来说，页面盒子的高度等于 \cs{topskip} 加上 \cs{baselineskip} 的倍数。
因此我们可以定义一个宏，通过页面的行数来计算 \cs{vsize}：
\howto Specify page height in lines\par
\begin{verbatim}
\def\HeightInLines#1{\count@=#1\relax
    \advance\count@ by -1\relax
    \vsize=\baselineskip
    \multiply\vsize by \count@
    \advance\vsize by \topskip}
\end{verbatim}
对于纯文本页面，像这样计算 \cs{vsize} 可以避免未充满的盒子。

%In cases where the page does not start with a line of text
%(for instance a rule), the topskip may give unwanted effects.
%To prevent these, start the page with
%\begin{verbatim}
%\hbox{}\kern-\topskip
%\end{verbatim}
%followed by what you wanted on top. 
当页面的首行不是文字时(例如有奇怪的规定要求这样做)，topskip可能造成非预期的影响。为了避免这个问题，我们可以以
\begin{verbatim}
\hbox{}\kern-\topskip
\end{verbatim}
作为一页的开头，后面再写你想在页面顶部出现的东西。

%Analogous to the \cs{topskip}, there is a \cs{splittopskip}
%for pages generated by a \cs{vsplit} operation; see
%the next chapter.
类似 \cs{topskip}，还有一个 \cs{splittopskip}，它适用于由 \cs{vsplit} 操作产生的页面，详见下一章。

%%\point Page height and depth
%\section{Page height and depth}
%\point Page height and depth
\section{页面高度与深度}

%\index{page!height|(}
%\index{page!depth|(}
\index{page!height|(}
\index{page!depth|(}

%\TeX\ tries to build pages as a \cs{vbox} of height \csidx{vsize};
%\alt
%see also \cs{pagegoal} in the next chapter.
\TeX\ 会尝试将页面构建为一个高 \csidx{vsize}的 \cs{vbox} ；参见下下一章的 \cs{pagegoal}。

%If the last item on a page has an excessive depth,
%that page would be noticeably longer than other pages.
%To prevent this phenomenon \TeX\ uses \csidx{maxdepth} as
%the maximum depth of the page box. If adding an item to the
%page would make the depth exceed this quantity, then the
%reference point of the page is moved down to make the depth
%exactly \cs{maxdepth}.
如果某页的最后一项深度过长，这一页会明显比其他页更长。
为了避免这种现象 \TeX\ 使用 \csidx{maxdepth} 限制页面盒子的最大深度。
如果继续添加一项会导致页面深度超限，这时参考点会下移，直至页面深度恰好等于 \cs{maxdepth}。

%The `raggedbottom' effect is obtained in plain \TeX\
%\cstoidx raggedbottom\par
%by giving the \cs{topskip} some finite stretchability:
%\hbox{\n{10pt plus 60pt}}.
%Thus the natural height of box~255 can vary when it reaches
%the output routine.
%Pages are then shipped out (more or less) as
%\begin{verbatim}
%\dimen0=\dp255 \unvbox255
%\ifraggedbottom \kern-\dimen0 \vfil \fi 
%\end{verbatim}
%The \cs{vfil} causes the topskip to be set at natural
%width, so the effect is one of a fixed top line  and a
%variable bottom line of the page.
通过赋给 \cs{topskip} 无限的弹性：\hbox{\n{10pt plus 60pt}}，我们可以在plain \TeX\ 中观察到“辣鸡页底”的效应。
因此当 box~255 到达输出程序时，它的自然高度通常有变化。
随后，页面以差不多如下形式送出：
\begin{verbatim}
\dimen0=\dp255 \unvbox255
\ifraggedbottom \kern-\dimen0 \vfil \fi 
\end{verbatim}
\cs{vfil} 使 \cs{topskip} 设置为自然宽度，这样做会产生以下两种效果之一：页面有固定顶部线或可变底部线。

%Before \cs{box255} is unboxed in the plain \TeX\ output routine,
%\cs{boxmaxdepth} is set to \cs{maxdepth}
%so that this box will made under the same assumptions
%that the page builder used when putting together \cs{box255}.
在 \cs{box255} 于 plain \TeX\ 的输出程序中被解包之前，\cs{boxmaxdepth} 被设成 \cs{maxdepth}，这样创建这个盒子时，页面生成器使用的假设会和把 \cs{box255} 放在一起时相同。

%The depth of box split off by a \cs{vsplit} operation
%is controlled by the \cs{splitmaxdepth} parameter.
\cs{vsplit} 操作分割盒子的深度由 \cs{splitmaxdepth} 参数控制。

%\index{page!height|)}
%\index{page!depth|)}
\index{page!height|)}
\index{page!depth|)}

%\endofchapter
\endofchapter

\end{document}