Skip to main content

39.10 Size of Displayed Text

Since not all characters have the same width, these functions let you check the width of a character. See Primitive Indent, and Screen Lines, for related functions.

function char-width char

This function returns the width in columns of the character char, if it were displayed in the current buffer (i.e., taking into account the buffer’s display table, if any; see Display Tables). The width of a tab character is usually tab-width (see Usual Display).

function string-width string

This function returns the width in columns of the string string, if it were displayed in the current buffer and the selected window.

function truncate-string-to-width string width \&optional start-column padding ellipsis

This function returns the part of string that fits within width columns, as a new string.

If string does not reach width, then the result ends where string ends. If one multi-column character in string extends across the column width, that character is not included in the result. Thus, the result can fall short of width but cannot go beyond it.

The optional argument start-column specifies the starting column. If this is non-nil, then the first start-column columns of the string are omitted from the value. If one multi-column character in string extends across the column start-column, that character is not included.

The optional argument padding, if non-nil, is a padding character added at the beginning and end of the result string, to extend it to exactly width columns. The padding character is used at the end of the result if it falls short of width. It is also used at the beginning of the result if one multi-column character in string extends across the column start-column.

If ellipsis is non-nil, it should be a string which will replace the end of string (including any padding) if it extends beyond width, unless the display width of string is equal to or less than the display width of ellipsis. If ellipsis is non-nil and not a string, it stands for the value of the variable truncate-string-ellipsis.

(truncate-string-to-width "\tab\t" 12 4)
⇒ "ab"
(truncate-string-to-width "\tab\t" 12 4 ?\s)
⇒ " ab "

The following function returns the size in pixels of text as if it were displayed in a given window. This function is used by fit-window-to-buffer and fit-frame-to-buffer (see Resizing Windows) to make a window exactly as large as the text it contains.

function window-text-pixel-size \&optional window from to x-limit y-limit mode-and-header-line

This function returns the size of the text of window’s buffer in pixels. window must be a live window and defaults to the selected one. The return value is a cons of the maximum pixel-width of any text line and the maximum pixel-height of all text lines. This function exists to allow Lisp programs to adjust the dimensions of window to the buffer text it needs to display.

The optional argument from, if non-nil, specifies the first text position to consider, and defaults to the minimum accessible position of the buffer. If from is t, it stands for the minimum accessible position that is not a newline character. The optional argument to, if non-nil, specifies the last text position to consider, and defaults to the maximum accessible position of the buffer. If to is t, it stands for the maximum accessible position that is not a newline character.

The optional argument x-limit, if non-nil, specifies the maximum X coordinate beyond which text should be ignored; it is therefore also the largest value of pixel-width that the function can return. If x-limit nil or omitted, it means to use the pixel-width of window’s body (see Window Sizes); this default means that text of truncated lines wider than the window will be ignored. This default is useful when the caller does not intend to change the width of window. Otherwise, the caller should specify here the maximum width window’s body may assume; in particular, if truncated lines are expected and their text needs to be accounted for, x-limit should be set to a large value. Since calculating the width of long lines can take some time, it’s always a good idea to make this argument as small as needed; in particular, if the buffer might contain long lines that will be truncated anyway.

The optional argument y-limit, if non-nil, specifies the maximum Y coordinate beyond which text is to be ignored; it is therefore also the maximum pixel-height that the function can return. If y-limit is nil or omitted, it means to considers all the lines of text till the buffer position specified by to. Since calculating the pixel-height of a large buffer can take some time, it makes sense to specify this argument; in particular, if the caller does not know the size of the buffer.

The optional argument mode-and-header-line nil or omitted means to not include the height of the mode- or header-line of window in the return value. If it is either the symbol mode-line or header-line, include only the height of that line, if present, in the return value. If it is t, include the height of both, if present, in the return value.

window-text-pixel-size treats the text displayed in a window as a whole and does not care about the size of individual lines. The following function does.

function window-lines-pixel-dimensions \&optional window first last body inverse left

This function calculates the pixel dimensions of each line displayed in the specified window. It does so by walking window’s current glyph matrix—a matrix storing the glyph (see Glyphs) of each buffer character currently displayed in window. If successful, it returns a list of cons pairs representing the x- and y-coordinates of the lower right corner of the last character of each line. Coordinates are measured in pixels from an origin (0, 0) at the top-left corner of window. window must be a live window and defaults to the selected one.

If the optional argument first is an integer, it denotes the index (starting with 0) of the first line of window’s glyph matrix to be returned. Note that if window has a header line, the line with index 0 is that header line. If first is nil, the first line to be considered is determined by the value of the optional argument body: If body is non-nil, this means to start with the first line of window’s body, skipping any header line, if present. Otherwise, this function will start with the first line of window’s glyph matrix, possibly the header line.

If the optional argument last is an integer, it denotes the index of the last line of window’s glyph matrix that shall be returned. If last is nil, the last line to be considered is determined by the value of body: If body is non-nil, this means to use the last line of window’s body, omitting window’s mode line, if present. Otherwise, this means to use the last line of window which may be the mode line.

The optional argument inverse, if nil, means that the y-pixel value returned for any line specifies the distance in pixels from the left edge (body edge if body is non-nil) of window to the right edge of the last glyph of that line. inverse non-nil means that the y-pixel value returned for any line specifies the distance in pixels from the right edge of the last glyph of that line to the right edge (body edge if body is non-nil) of window. This is useful for determining the amount of slack space at the end of each line.

The optional argument left, if non-nil means to return the x- and y-coordinates of the lower left corner of the leftmost character on each line. This is the value that should be used for windows that mostly display text from right to left.

If left is non-nil and inverse is nil, this means that the y-pixel value returned for any line specifies the distance in pixels from the left edge of the last (leftmost) glyph of that line to the right edge (body edge if body is non-nil) of window. If left and inverse are both non-nil, the y-pixel value returned for any line specifies the distance in pixels from the left edge (body edge if body is non-nil) of window to the left edge of the last (leftmost) glyph of that line.

This function returns nil if the current glyph matrix of window is not up-to-date which usually happens when Emacs is busy, for example, when processing a command. The value should be retrievable though when this function is run from an idle timer with a delay of zero seconds.

function line-pixel-height

This function returns the height in pixels of the line at point in the selected window. The value includes the line spacing of the line (see Line Height).

When a buffer is displayed with line numbers (see Display Custom in The GNU Emacs Manual), it is sometimes useful to know the width taken for displaying the line numbers. The following function is for Lisp programs which need this information for layout calculations.

function line-number-display-width \&optional pixelwise

This function returns the width used for displaying the line numbers in the selected window. If the optional argument pixelwise is the symbol columns, the return value is a float number of the frame’s canonical columns; if pixelwise is t or any other non-nil value, the value is an integer and is measured in pixels. If pixelwise is omitted or nil, the value is the integer number of columns of the font defined for the line-number face, and doesn’t include the 2 columns used to pad the numbers on display. If line numbers are not displayed in the selected window, the value is zero regardless of the value of pixelwise. Use with-selected-window (see Selecting Windows) if you need this information about another window.