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29.23 Display Feature Testing

The functions in this section describe the basic capabilities of a particular display. Lisp programs can use them to adapt their behavior to what the display can do. For example, a program that ordinarily uses a popup menu could use the minibuffer if popup menus are not supported.

The optional argument display in these functions specifies which display to ask the question about. It can be a display name, a frame (which designates the display that frame is on), or nil (which refers to the selected frame's display, see Input Focus).

See Color Names, Text Terminal Colors, for other functions to obtain information about displays.

— Function: display-popup-menus-p &optional display

This function returns t if popup menus are supported on display, nil if not. Support for popup menus requires that the mouse be available, since the user cannot choose menu items without a mouse.

— Function: display-graphic-p &optional display

This function returns t if display is a graphic display capable of displaying several frames and several different fonts at once. This is true for displays that use a window system such as X, and false for text terminals.

— Function: display-mouse-p &optional display

This function returns t if display has a mouse available, nil if not.

— Function: display-color-p &optional display

This function returns t if the screen is a color screen. It used to be called x-display-color-p, and that name is still supported as an alias.

— Function: display-grayscale-p &optional display

This function returns t if the screen can display shades of gray. (All color displays can do this.)

— Function: display-supports-face-attributes-p attributes &optional display

This function returns non-nil if all the face attributes in attributes are supported (see Face Attributes).

The definition of `supported' is somewhat heuristic, but basically means that a face containing all the attributes in attributes, when merged with the default face for display, can be represented in a way that's

  1. different in appearance than the default face, and
  2. `close in spirit' to what the attributes specify, if not exact.

Point (2) implies that a :weight black attribute will be satisfied by any display that can display bold, as will :foreground "yellow" as long as some yellowish color can be displayed, but :slant italic will not be satisfied by the tty display code's automatic substitution of a `dim' face for italic.

— Function: display-selections-p &optional display

This function returns t if display supports selections. Windowed displays normally support selections, but they may also be supported in some other cases.

— Function: display-images-p &optional display

This function returns t if display can display images. Windowed displays ought in principle to handle images, but some systems lack the support for that. On a display that does not support images, Emacs cannot display a tool bar.

— Function: display-screens &optional display

This function returns the number of screens associated with the display.

— Function: display-pixel-height &optional display

This function returns the height of the screen in pixels. On a character terminal, it gives the height in characters.

For graphical terminals, note that on “multi-monitor” setups this refers to the pixel width for all physical monitors associated with display. See Multiple Terminals.

— Function: display-pixel-width &optional display

This function returns the width of the screen in pixels. On a character terminal, it gives the width in characters.

For graphical terminals, note that on “multi-monitor” setups this refers to the pixel width for all physical monitors associated with display. See Multiple Terminals.

— Function: display-mm-height &optional display

This function returns the height of the screen in millimeters, or nil if Emacs cannot get that information.

— Function: display-mm-width &optional display

This function returns the width of the screen in millimeters, or nil if Emacs cannot get that information.

— User Option: display-mm-dimensions-alist

This variable allows the user to specify the dimensions of graphical displays returned by display-mm-height and display-mm-width in case the system provides incorrect values.

— Function: display-backing-store &optional display

This function returns the backing store capability of the display. Backing store means recording the pixels of windows (and parts of windows) that are not exposed, so that when exposed they can be displayed very quickly.

Values can be the symbols always, when-mapped, or not-useful. The function can also return nil when the question is inapplicable to a certain kind of display.

— Function: display-save-under &optional display

This function returns non-nil if the display supports the SaveUnder feature. That feature is used by pop-up windows to save the pixels they obscure, so that they can pop down quickly.

— Function: display-planes &optional display

This function returns the number of planes the display supports. This is typically the number of bits per pixel. For a tty display, it is log to base two of the number of colors supported.

— Function: display-visual-class &optional display

This function returns the visual class for the screen. The value is one of the symbols static-gray (a limited, unchangeable number of grays), gray-scale (a full range of grays), static-color (a limited, unchangeable number of colors), pseudo-color (a limited number of colors), true-color (a full range of colors), and direct-color (a full range of colors).

— Function: display-color-cells &optional display

This function returns the number of color cells the screen supports.

These functions obtain additional information specifically about X displays.

— Function: x-server-version &optional display

This function returns the list of version numbers of the X server running the display. The value is a list of three integers: the major and minor version numbers of the X protocol, and the distributor-specific release number of the X server software itself.

— Function: x-server-vendor &optional display

This function returns the “vendor” that provided the X server software (as a string). Really this means whoever distributes the X server.

When the developers of X labeled software distributors as “vendors”, they showed their false assumption that no system could ever be developed and distributed noncommercially.