29.22 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 ordinarly 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 section 29.9 Input Focus).

See section 29.19 Color Names, 29.20 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-only 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-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.

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-pixel-width &optional display

This function returns the width of the screen in pixels.

Function: display-mm-width &optional display

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

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 colours 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, gray-scale, static-color, pseudo-color, true-color, and direct-color.

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.

Function: x-server-vendor &optional display

This function returns the vendor that provided the X server software.