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.
These functions scan text to determine where screen lines break, and thus take time proportional to the distance scanned.
function vertical-motion count \&optional window cur-col
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.
count argument can be a cons cell,
(cols . lines), instead of an integer. Then the function moves by
lines screen lines, and puts point
cols columns from the visual start of that screen line. Note that
cols are counted from the visual start of the line; if the window is scrolled horizontally (see Horizontal Scrolling), the column on which point will end is in addition to the number of columns by which the text is scrolled.
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.
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.
function count-screen-lines \&optional beg end count-final-newline window
This function returns the number of screen lines in the text from
end. The number of screen lines may be different from the number of actual lines, due to line continuation, the display table, etc. If
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-
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.
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.
command move-to-window-line count
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.
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.
function move-to-window-group-line count
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.
function compute-motion from frompos to topos width offsets window
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
topos are cons cells of the form
(hpos . vpos).
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
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
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.
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
The return value is a list of five elements:
(pos hpos vpos prevhpos contin)
pos is the buffer position where the scan stopped,
vpos is the vertical screen position, and
hpos is the horizontal screen position.
prevhpos is the horizontal position one character back from
pos. The result
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
to, to limit the scan to the end of the accessible portion of the buffer, and pass
topos. Here’s a function that does this:
(defun coordinates-of-position (col line)
(car (compute-motion (window-start)
'(0 . 0)
(cons col line)
(cons (window-hscroll) 0)
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.