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If you want to create a picture made out of text characters (for example,
a picture of the division of a register into fields, as a comment in a
program), use the command edit-picture to enter Picture mode.
In Picture mode, editing is based on the quarter-plane model of text, according to which the text characters lie studded on an area that stretches infinitely far to the right and downward. The concept of the end of a line does not exist in this model; the most you can say is where the last nonblank character on the line is found.
Of course, Emacs really always considers text as a sequence of characters, and lines really do have ends. But in Picture mode most frequently-used keys are rebound to commands that simulate the quarter-plane model of text. They do this by inserting spaces or by converting tabs to spaces.
Most of the basic editing commands of Emacs are redefined by Picture mode to do essentially the same thing but in a quarter-plane way. In addition, Picture mode defines various keys starting with the C-c prefix to run special picture editing commands.
One of these keys, C-c C-c, is pretty important. Often a picture
is part of a larger file that is usually edited in some other major mode.
M-x edit-picture records the name of the previous major mode, and
then you can use the C-c C-c command (picture-mode-exit) to
restore that mode. C-c C-c also deletes spaces from the ends of
lines, unless given a numeric argument.
The commands used in Picture mode all work in other modes (provided the `picture' library is loaded), but are not bound to keys except in Picture mode. Note that the descriptions below talk of moving "one column" and so on, but all the picture mode commands handle numeric arguments as their normal equivalents do.
Turning on Picture mode calls the value of the variable picture-mode-hook
as a function, with no arguments, if that value exists and is non-nil.
Most keys do the same thing in Picture mode that they usually do, but do
it in a quarter-plane style. For example, C-f is rebound to run
picture-forward-column, which is defined to move point one column to
the right, by inserting a space if necessary, so that the actual end of the
line makes no difference. C-b is rebound to run
picture-backward-column, which always moves point left one column,
converting a tab to multiple spaces if necessary. C-n and C-p
are rebound to run picture-move-down and picture-move-up,
which can either insert spaces or convert tabs as necessary to make sure
that point stays in exactly the same column. C-e runs
picture-end-of-line, which moves to after the last nonblank
character on the line. There is no need to change C-a, as the choice
of screen model does not affect beginnings of lines.
Insertion of text is adapted to the quarter-plane screen model through
the use of Overwrite mode (see section Minor Modes). Self-inserting characters
replace existing text, column by column, rather than pushing existing text
to the right. RET runs picture-newline, which just moves to
the beginning of the following line so that new text will replace that
line.
Deletion and killing of text are replaced with erasure. DEL
(picture-backward-clear-column) replaces the preceding character
with a space rather than removing it. C-d
(picture-clear-column) does the same thing in a forward direction.
C-k (picture-clear-line) really kills the contents of lines,
but does not ever remove the newlines from the buffer.
To do actual insertion, you must use special commands. C-o
(picture-open-line) still creates a blank line, but does so after
the current line; it never splits a line. C-M-o, split-line,
makes sense in Picture mode, so it is not changed. LFD
(picture-duplicate-line) inserts below the current line another line
with the same contents.
Real deletion can be done with C-w, or with C-c C-d (which is
defined as delete-char, as C-d is in other modes), or with one
of the picture rectangle commands (see section Picture Mode Rectangle Commands).
Since "self-inserting" characters in Picture mode just overwrite and move point, there is no essential restriction on how point should be moved. Normally point moves right, but you can specify any of the eight orthogonal or diagonal directions for motion after a "self-inserting" character. This is useful for drawing lines in the buffer.
picture-movement-left).
picture-movement-right).
picture-movement-up).
picture-movement-down).
picture-movement-nw).
picture-movement-ne).
picture-movement-sw).
picture-movement-se).
Two motion commands move based on the current Picture insertion
direction. The command C-c C-f (picture-motion) moves in the
same direction as motion after "insertion" currently does, while C-c
C-b (picture-motion-reverse) moves in the opposite direction.
Two kinds of tab-like action are provided in Picture mode.
Context-based tabbing is done with M-TAB
(picture-tab-search). With no argument, it moves to a point
underneath the next "interesting" character that follows whitespace in
the previous nonblank line. "Next" here means "appearing at a
horizontal position greater than the one point starts out at". With an
argument, as in C-u M-TAB, this command moves to the next such
interesting character in the current line. M-TAB does not
change the text; it only moves point. "Interesting" characters are
defined by the variable picture-tab-chars, which contains a string
whose characters are all considered interesting. Its default value is
"!-~".
TAB itself runs picture-tab, which operates based on the
current tab stop settings; it is the Picture mode equivalent of
tab-to-tab-stop. Normally it just moves point, but with a numeric
argument it clears the text that it moves over.
The context-based and tab-stop-based forms of tabbing are brought
together by the command C-c TAB, picture-set-tab-stops.
This command sets the tab stops to the positions which M-TAB
would consider significant in the current line. The use of this command,
together with TAB, can get the effect of context-based tabbing. But
M-TAB is more convenient in the cases where it is sufficient.
Picture mode defines commands for working on rectangular pieces of the text in ways that fit with the quarter-plane model. The standard rectangle commands may also be useful (see section Rectangles).
picture-clear-rectangle). With
argument, kill it.
picture-clear-rectangle-to-register).
picture-yank-rectangle). With argument,
insert instead.
picture-yank-rectangle-from-register).
The picture rectangle commands C-c C-k
(picture-clear-rectangle) and C-c C-w
(picture-clear-rectangle-to-register) differ from the standard
rectangle commands in that they normally clear the rectangle instead of
deleting it; this is analogous with the way C-d is changed in Picture
mode.
However, deletion of rectangles can be useful in Picture mode, so these commands delete the rectangle if given a numeric argument.
The Picture mode commands for yanking rectangles differ from the standard
ones in overwriting instead of inserting. This is the same way that
Picture mode insertion of other text is different from other modes.
C-c C-y (picture-yank-rectangle) inserts (by overwriting) the
rectangle that was most recently killed, while C-c C-x
(picture-yank-rectangle-from-register) does likewise for the
rectangle found in a specified register.
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