emacs/man/display.texi

@c This is part of the Emacs manual.
@c Copyright (C) 1985, 86, 87, 93, 94, 95, 1997 Free Software Foundation, Inc.
@c See file emacs.texi for copying conditions.
@node Display, Search, Registers, Top
@chapter Controlling the Display

  Since only part of a large buffer fits in the window, Emacs tries to
show a part that is likely to be interesting.  Display-control commands
allow you to specify which part of the text you want to see, and how to
display it.

@menu
* Scrolling::	           Moving text up and down in a window.
* Horizontal Scrolling::   Moving text left and right in a window.
* Follow Mode::            Follow mode lets two windows scroll as one.
* Selective Display::      Hiding lines with lots of indentation.
* Optional Mode Line::     Optional mode line display features.
* Text Display::           How text characters are normally displayed.
* Display Vars::           Information on variables for customizing display.
@end menu

@node Scrolling
@section Scrolling

  If a buffer contains text that is too large to fit entirely within a
window that is displaying the buffer, Emacs shows a contiguous portion of
the text.  The portion shown always contains point.

@cindex scrolling
  @dfn{Scrolling} means moving text up or down in the window so that
different parts of the text are visible.  Scrolling forward means that text
moves up, and new text appears at the bottom.  Scrolling backward moves
text down and new text appears at the top.

  Scrolling happens automatically if you move point past the bottom or top
of the window.  You can also explicitly request scrolling with the commands
in this section.

@table @kbd
@item C-l
Clear screen and redisplay, scrolling the selected window to center
point vertically within it (@code{recenter}).
@item C-v
Scroll forward (a windowful or a specified number of lines) (@code{scroll-up}).
@item @key{NEXT}
Likewise, scroll forward.
@item M-v
Scroll backward (@code{scroll-down}).
@item @key{PRIOR}
Likewise, scroll backward.
@item @var{arg} C-l
Scroll so point is on line @var{arg} (@code{recenter}).
@item C-M-l
Scroll heuristically to bring useful information onto the screen
(@code{reposition-window}).
@end table

@kindex C-l
@findex recenter
  The most basic scrolling command is @kbd{C-l} (@code{recenter}) with
no argument.  It clears the entire screen and redisplays all windows.
In addition, it scrolls the selected window so that point is halfway
down from the top of the window.

@kindex C-v
@kindex M-v
@kindex NEXT
@kindex PRIOR
@findex scroll-up
@findex scroll-down
  The scrolling commands @kbd{C-v} and @kbd{M-v} let you move all the text
in the window up or down a few lines.  @kbd{C-v} (@code{scroll-up}) with an
argument shows you that many more lines at the bottom of the window, moving
the text and point up together as @kbd{C-l} might.  @kbd{C-v} with a
negative argument shows you more lines at the top of the window.
@kbd{M-v} (@code{scroll-down}) is like @kbd{C-v}, but moves in the
opposite direction.  The function keys @key{NEXT} and @key{PRIOR} are
equivalent to @kbd{C-v} and @kbd{M-v}.

  The names of scroll commands are based on the direction that the text
moves in the window.  Thus, the command to scroll forward is called
@code{scroll-up} because it moves the text upward on the screen.

@vindex next-screen-context-lines
  To read the buffer a windowful at a time, use @kbd{C-v} with no argument.
It takes the last two lines at the bottom of the window and puts them at
the top, followed by nearly a whole windowful of lines not previously
visible.  If point was in the text scrolled off the top, it moves to the
new top of the window.  @kbd{M-v} with no argument moves backward with
overlap similarly.  The number of lines of overlap across a @kbd{C-v} or
@kbd{M-v} is controlled by the variable @code{next-screen-context-lines}; by
default, it is 2.

@vindex scroll-preserve-screen-position
  Some users like the full-screen scroll commands to keep point at the
same screen line.  To enable this behavior, set the variable
@code{scroll-preserve-screen-position} to a non-@code{nil} value.  This
mode is convenient for browsing through a file by scrolling by
screenfuls; if you come back to the screen where you started, point goes
back to the line where it started.  However, this mode is inconvenient
when you move to the next screen in order to move point to the text
there.

  Another way to do scrolling is with @kbd{C-l} with a numeric argument.
@kbd{C-l} does not clear the screen when given an argument; it only scrolls
the selected window.  With a positive argument @var{n}, it repositions text
to put point @var{n} lines down from the top.  An argument of zero puts
point on the very top line.  Point does not move with respect to the text;
rather, the text and point move rigidly on the screen.  @kbd{C-l} with a
negative argument puts point that many lines from the bottom of the window.
For example, @kbd{C-u - 1 C-l} puts point on the bottom line, and @kbd{C-u
- 5 C-l} puts it five lines from the bottom.  Just @kbd{C-u} as argument,
as in @kbd{C-u C-l}, scrolls point to the center of the selected window.

@kindex C-M-l
@findex reposition-window
  The @kbd{C-M-l} command (@code{reposition-window}) scrolls the current
window heuristically in a way designed to get useful information onto
the screen.  For example, in a Lisp file, this command tries to get the
entire current defun onto the screen if possible.

@vindex scroll-conservatively
  Scrolling happens automatically if point has moved out of the visible
portion of the text when it is time to display.  Normally, automatic
scrolling centers point vertically within the window.  However, if you
set @code{scroll-conservatively} to a small number @var{n}, then if you
move point just a little off the screen---less than @var{n} lines---then
Emacs scrolls the text just far enough to bring point back on screen.
By default, @code{scroll-conservatively} is 0.

@cindex aggressive scrolling
@vindex scroll-up-aggressively
@vindex scroll-down-aggressively 
  If you prefer a more aggresive scrolling, customize the values of the
variables @code{scroll-up-aggressively} and
@code{scroll-down-aggressively}.  The value of
@code{scroll-up-aggressively} should be either nil or a fraction @var{f}
between 0 and 1.  If it is a fraction, that specifies where on the
screen to put point when scrolling upward.  More precisely, when a
window scrolls up because point is above the window start, the new start
position is chosen to put point @var{f} part of the window height from
the top.  The larger @var{f}, the more aggressive the scrolling.

A value of @code{nil} is equivalent to .5, since its effect is to center
point.

Likewise, @code{scroll-down-aggressively} is used for scrolling down.
The value, @var{f}, specifies how far point should be placed from the
bottom of the window; thus, as with @code{scroll-up-aggressively}, a
larger value scrolls more aggressively.

@vindex scroll-margin
  The variable @code{scroll-margin} restricts how close point can come
to the top or bottom of a window.  Its value is a number of screen
lines; if point comes within that many lines of the top or bottom of the
window, Emacs recenters the window.  By default, @code{scroll-margin} is
0.

@node Horizontal Scrolling
@section Horizontal Scrolling
@cindex horizontal scrolling

  @dfn{Horizontal scrolling} means shifting all the lines sideways
within a window---so that some of the text near the left margin
is not displayed at all.

@table @kbd
@item C-x <
Scroll text in current window to the left (@code{scroll-left}).
@item C-x >
Scroll to the right (@code{scroll-right}).
@end table

  When a window has been scrolled horizontally, text lines are truncated
rather than continued (@pxref{Continuation Lines}), with a @samp{$}
appearing in the first column when there is text truncated to the left,
and in the last column when there is text truncated to the right.

@kindex C-x <
@kindex C-x >
@findex scroll-left
@findex scroll-right
  The command @kbd{C-x <} (@code{scroll-left}) scrolls the selected
window to the left by @var{n} columns with argument @var{n}.  This moves
part of the beginning of each line off the left edge of the window.
With no argument, it scrolls by almost the full width of the window (two
columns less, to be precise).

  @kbd{C-x >} (@code{scroll-right}) scrolls similarly to the right.  The
window cannot be scrolled any farther to the right once it is displayed
normally (with each line starting at the window's left margin);
attempting to do so has no effect.  This means that you don't have to
calculate the argument precisely for @w{@kbd{C-x >}}; any sufficiently large
argument will restore the normal display.

@cindex horizontal scrolling
@vindex automatic-hscrolling
  Emacs automatically scrolls a window horizontally whenever that is
necessary to keep point visible and not too far from the left or right
edge.  If you don't want this, customize the variable
@code{automatic-hscrolling} and set it to nil.

@node Follow Mode
@section Follow Mode
@cindex Follow mode
@cindex mode, Follow

  @dfn{Follow mode} is a minor mode that makes two windows showing the
same buffer scroll as one tall ``virtual window.''  To use Follow mode,
go to a frame with just one window, split it into two side-by-side
windows using @kbd{C-x 3}, and then type @kbd{M-x follow-mode}.  From
then on, you can edit the buffer in either of the two windows, or scroll
either one; the other window follows it.

  To turn off Follow mode, type @kbd{M-x follow-mode} a second time.

@node Selective Display
@section Selective Display
@findex set-selective-display
@kindex C-x $

  Emacs has the ability to hide lines indented more than a certain number
of columns (you specify how many columns).  You can use this to get an
overview of a part of a program.

  To hide lines, type @kbd{C-x $} (@code{set-selective-display}) with a
numeric argument @var{n}.  Then lines with at least @var{n} columns of
indentation disappear from the screen.  The only indication of their
presence is that three dots (@samp{@dots{}}) appear at the end of each
visible line that is followed by one or more hidden ones.

  The commands @kbd{C-n} and @kbd{C-p} move across the hidden lines as
if they were not there.

  The hidden lines are still present in the buffer, and most editing
commands see them as usual, so you may find point in the middle of the
hidden text.  When this happens, the cursor appears at the end of the
previous line, after the three dots.  If point is at the end of the
visible line, before the newline that ends it, the cursor appears before
the three dots.

  To make all lines visible again, type @kbd{C-x $} with no argument.

@vindex selective-display-ellipses
  If you set the variable @code{selective-display-ellipses} to
@code{nil}, the three dots do not appear at the end of a line that
precedes hidden lines.  Then there is no visible indication of the
hidden lines.  This variable becomes local automatically when set.

@node Optional Mode Line
@section Optional Mode Line Features

@cindex Line Number mode
@cindex mode, Line Number
@findex line-number-mode
  The current line number of point appears in the mode line when Line
Number mode is enabled.  Use the command @kbd{M-x line-number-mode} to
turn this mode on and off; normally it is on.  The line number appears
before the buffer percentage @var{pos}, with the letter @samp{L} to
indicate what it is.  @xref{Minor Modes}, for more information about
minor modes and about how to use this command.

@vindex line-number-display-limit
@cindex line number display, removing the limit
  If the buffer is very large (larger than the value of
@code{line-number-display-limit}), then the line number doesn't appear.
Emacs doesn't compute the line number when the buffer is large, because
that would be too slow.  Set it to @code{nil} to remove the limit.  If
you have narrowed the buffer (@pxref{Narrowing}), the displayed line
number is relative to the accessible portion of the buffer.

@cindex Column Number mode
@cindex mode, Column Number
@findex column-number-mode
  You can also display the current column number by turning on Column
Number mode.  It displays the current column number preceded by the
letter @samp{C}.  Type @kbd{M-x column-number-mode} to toggle this mode.

@findex display-time
@cindex time (on mode line)
  Emacs can optionally display the time and system load in all mode
lines.  To enable this feature, type @kbd{M-x display-time} or customize
the option @code{display-time-mode}.  The information added to the mode
line usually appears after the buffer name, before the mode names and
their parentheses.  It looks like this:

@example
@var{hh}:@var{mm}pm @var{l.ll}
@end example

@noindent
@vindex display-time-24hr-format
Here @var{hh} and @var{mm} are the hour and minute, followed always by
@samp{am} or @samp{pm}.  @var{l.ll} is the average number of running
processes in the whole system recently.  (Some fields may be missing if
your operating system cannot support them.)  If you prefer time display
in 24-hour format, set the variable @code{display-time-24hr-format}
to @code{t}.

@cindex mail (on mode line)
@vindex display-time-use-mail-icon
@vindex display-time-mail-face
  The word @samp{Mail} appears after the load level if there is mail
for you that you have not read yet.  On a graphical display you can use
an icon instead of @samp{Mail} by customizing
@code{display-time-use-mail-icon}; this may save some space on the mode
line.  You can customize @code{display-time-mail-face} to make the mail
indicator prominent.

@node Text Display
@section How Text Is Displayed
@cindex characters (in text)

  ASCII printing characters (octal codes 040 through 0176) in Emacs
buffers are displayed with their graphics.  So are non-ASCII multibyte
printing characters (octal codes above 0400).

  Some ASCII control characters are displayed in special ways.  The
newline character (octal code 012) is displayed by starting a new line.
The tab character (octal code 011) is displayed by moving to the next
tab stop column (normally every 8 columns).

  Other ASCII control characters are normally displayed as a caret
(@samp{^}) followed by the non-control version of the character; thus,
control-A is displayed as @samp{^A}.

  Non-ASCII characters 0200 through 0377 are displayed with octal escape
sequences; thus, character code 0243 (octal) is displayed as
@samp{\243}.  However, if you enable European display, most of these
characters become non-ASCII printing characters, and are displayed using
their graphics (assuming your terminal supports them).
@xref{Single-Byte Character Support}.

@node Display Vars
@section Variables Controlling Display

  This section contains information for customization only.  Beginning
users should skip it.

@vindex mode-line-inverse-video
  The variable @code{mode-line-inverse-video} controls whether the mode
line is displayed in inverse video (assuming the terminal supports it);
@code{nil} means don't do so.  @xref{Mode Line}.  If you specify the
foreground color for the @code{modeline} face, and
@code{mode-line-inverse-video} is non-@code{nil}, then the default
background color for that face is the usual foreground color.
@xref{Faces}.

@vindex inverse-video
  If the variable @code{inverse-video} is non-@code{nil}, Emacs attempts
to invert all the lines of the display from what they normally are.

@vindex visible-bell
  If the variable @code{visible-bell} is non-@code{nil}, Emacs attempts
to make the whole screen blink when it would normally make an audible bell
sound.  This variable has no effect if your terminal does not have a way
to make the screen blink.@refill

@vindex no-redraw-on-reenter
  When you reenter Emacs after suspending, Emacs normally clears the
screen and redraws the entire display.  On some terminals with more than
one page of memory, it is possible to arrange the termcap entry so that
the @samp{ti} and @samp{te} strings (output to the terminal when Emacs
is entered and exited, respectively) switch between pages of memory so
as to use one page for Emacs and another page for other output.  Then
you might want to set the variable @code{no-redraw-on-reenter}
non-@code{nil}; this tells Emacs to assume, when resumed, that the
screen page it is using still contains what Emacs last wrote there.

@vindex echo-keystrokes
  The variable @code{echo-keystrokes} controls the echoing of multi-character
keys; its value is the number of seconds of pause required to cause echoing
to start, or zero meaning don't echo at all.  @xref{Echo Area}.

@vindex ctl-arrow
  If the variable @code{ctl-arrow} is @code{nil}, control characters in
the buffer are displayed with octal escape sequences, except for newline
and tab.  Altering the value of @code{ctl-arrow} makes it local to the
current buffer; until that time, the default value is in effect.  The
default is initially @code{t}.  @xref{Display Tables,, Display Tables,
elisp, The Emacs Lisp Reference Manual}.

@vindex tab-width
  Normally, a tab character in the buffer is displayed as whitespace which
extends to the next display tab stop position, and display tab stops come
at intervals equal to eight spaces.  The number of spaces per tab is
controlled by the variable @code{tab-width}, which is made local by
changing it, just like @code{ctl-arrow}.  Note that how the tab character
in the buffer is displayed has nothing to do with the definition of
@key{TAB} as a command.  The variable @code{tab-width} must have an
integer value between 1 and 1000, inclusive.

@c @vindex truncate-lines  @c No index entry here, because we have one
@c in the continuation section.
  If the variable @code{truncate-lines} is non-@code{nil}, then each
line of text gets just one screen line for display; if the text line is
too long, display shows only the part that fits.  If
@code{truncate-lines} is @code{nil}, then long text lines display as
more than one screen line, enough to show the whole text of the line.
@xref{Continuation Lines}.  Altering the value of @code{truncate-lines}
makes it local to the current buffer; until that time, the default value
is in effect.  The default is initially @code{nil}.

@c @vindex truncate-partial-width-windows  @c Idx entry is in Split Windows.
  If the variable @code{truncate-partial-width-windows} is
non-@code{nil}, it forces truncation rather than continuation in any
window less than the full width of the screen or frame, regardless of
the value of @code{truncate-lines}.  For information about side-by-side
windows, see @ref{Split Window}.  See also @ref{Display,, Display,
elisp, The Emacs Lisp Reference Manual}.

@vindex baud-rate
  The variable @code{baud-rate} holds the output speed of the
terminal, as far as Emacs knows.  Setting this variable does not change
the speed of actual data transmission, but the value is used for
calculations such as padding.  It also affects decisions about whether
to scroll part of the screen or redraw it instead---even when using a
window system.  (We designed it this way, despite the fact that a window
system has no true ``output speed,'' to give you a way to tune these
decisions.)

  You can customize the way any particular character code is displayed
by means of a display table.  @xref{Display Tables,, Display Tables,
elisp, The Emacs Lisp Reference Manual}.