The line functions in the previous section count text lines, delimited only by newline characters. By contrast, these functions count screen lines, which are defined by the way the text appears on the screen. A text line is a single screen line if it is short enough to fit the width of the selected window, but otherwise it may occupy several screen lines.
In some cases, text lines are truncated on the screen rather than
continued onto additional screen lines. In these cases,
vertical-motion moves point much like forward-line.
See Truncation.
Because the width of a given string depends on the flags that control
the appearance of certain characters, vertical-motion behaves
differently, for a given piece of text, depending on the buffer it is
in, and even on the selected window (because the width, the truncation
flag, and display table may vary between windows). See Usual Display Conventions.
These functions scan text to determine where screen lines break, and thus take time proportional to the distance scanned.
This function moves point to the start of the screen line count screen lines down from the screen line containing point. If count is negative, it moves up instead. If count is zero, point moves to the visual start of the current screen line.
The count argument can be a cons cell, (cols . lines), instead of an integer. Then the function moves by
lines screen lines, as described for count above, and puts
point cols columns from the visual start of that screen line.
The value of cols can be a float, and is interpreted in units of
the frame’s canonical character width (see Frame Font); this
allows specifying accurate horizontal position of point when the
target screen line uses variable fonts. Note that cols are
counted from the visual start of the line; if the window is
scrolled horizontally (see Horizontal Scrolling), the column where
point will end is in addition to the number of columns by which the
text is scrolled, and if the target line is a continuation line, its
leftmost column is considered column zero (unlike column-oriented
functions, see Counting Columns).
The return value is the number of screen lines over which point was moved. The value may be less in absolute value than count if the beginning or end of the buffer was reached.
The window window is used for obtaining parameters such as the
width, the horizontal scrolling, and the display table. But
vertical-motion always operates on the current buffer, even if
window currently displays some other buffer.
The optional argument cur-col specifies the current column when the function is called. This is the window-relative horizontal coordinate of point, measured in units of font width of the frame’s default face. Providing it speeds up the function, especially in very long lines, because the function doesn’t have to go back in the buffer in order to determine the current column. Note that cur-col is also counted from the visual start of the line.
This function returns the number of screen lines in the text from
beg to end. The number of screen lines may be different
from the number of actual lines, due to line continuation, the display
table, etc. If beg and end are nil or omitted,
they default to the beginning and end of the accessible portion of the
buffer.
If the region ends with a newline, that is ignored unless the optional
third argument count-final-newline is non-nil.
The optional fourth argument window specifies the window for obtaining parameters such as width, horizontal scrolling, and so on. The default is to use the selected window’s parameters.
Like vertical-motion, count-screen-lines always uses the
current buffer, regardless of which buffer is displayed in
window. This makes possible to use count-screen-lines in
any buffer, whether or not it is currently displayed in some window.
This function moves point with respect to the text currently displayed in the selected window. It moves point to the beginning of the screen line count screen lines from the top of the window; zero means the topmost line. If count is negative, that specifies a position −count lines from the bottom (or the last line of the buffer, if the buffer ends above the specified screen position); thus, count of −1 specifies the last fully visible screen line of the window.
If count is nil, then point moves to the beginning of the
line in the middle of the window. If the absolute value of count
is greater than the size of the window, then point moves to the place
that would appear on that screen line if the window were tall enough.
This will probably cause the next redisplay to scroll to bring that
location onto the screen.
In an interactive call, count is the numeric prefix argument.
The value returned is the screen line number point has moved to, relative to the top line of the window.
This function is like move-to-window-line, except that when the
selected window is a part of a group of windows (see Window Group), move-to-window-group-line will move to a position with
respect to the entire group, not just the single window. This
condition holds when the buffer local variable
move-to-window-group-line-function is set to a function. In
this case, move-to-window-group-line calls the function with
the argument count, then returns its result.
This function scans the current buffer, calculating screen positions. It scans the buffer forward from position from, assuming that is at screen coordinates frompos, to position to or coordinates topos, whichever comes first. It returns the ending buffer position and screen coordinates.
The coordinate arguments frompos and topos are cons cells of
the form (hpos . vpos).
The argument width is the number of columns available to display
text; this affects handling of continuation lines. nil means
the actual number of usable text columns in the window, which is
equivalent to the value returned by (window-width window).
The argument offsets is either nil or a cons cell of the
form (hscroll . tab-offset). Here hscroll is
the number of columns not being displayed at the left margin; most
callers get this by calling window-hscroll. Meanwhile,
tab-offset is the offset between column numbers on the screen and
column numbers in the buffer. This can be nonzero in a continuation
line, when the previous screen lines’ widths do not add up to a multiple
of tab-width. It is always zero in a non-continuation line.
The window window serves only to specify which display table to
use. compute-motion always operates on the current buffer,
regardless of what buffer is displayed in window.
The return value is a list of five elements:
(pos hpos vpos prevhpos contin)
Here pos is the buffer position where the scan stopped, vpos is the vertical screen position, and hpos is the horizontal screen position.
The result prevhpos is the horizontal position one character back
from pos. The result contin is t if the last line
was continued after (or within) the previous character.
For example, to find the buffer position of column col of screen line
line of a certain window, pass the window’s display start location
as from and the window’s upper-left coordinates as frompos.
Pass the buffer’s (point-max) as to, to limit the scan to
the end of the accessible portion of the buffer, and pass line and
col as topos. Here’s a function that does this:
(defun coordinates-of-position (col line)
(car (compute-motion (window-start)
'(0 . 0)
(point-max)
(cons col line)
(window-width)
(cons (window-hscroll) 0)
(selected-window))))
When you use compute-motion for the minibuffer, you need to use
minibuffer-prompt-width to get the horizontal position of the
beginning of the first screen line. See Minibuffer Contents.