Motion by Screen Lines

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 section 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 section Usual Display Conventions.

These functions scan text to determine where screen lines break, and thus take time proportional to the distance scanned. If you intend to use them heavily, Emacs provides caches which may improve the performance of your code. See section Truncation.

Function: vertical-motion count &optional window

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.

vertical-motion returns the number of screen lines over which it moved point. 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.

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. 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).

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 window line number point has moved to, with the top line in the window numbered 0.

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 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. Use the value returned by window-width for the window of your choice; normally, use (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 vpos hpos 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 section Minibuffer Miscellany.

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