elisp: Display Feature Testing
28.24 Display Feature Testing
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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, 
Input Focus).
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 menu is popped up by clicking the mouse on
some portion of the Emacs display.
-- 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 (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 height for all physical monitors associated
with DISPLAY. 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. 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.
For graphical terminals, note that on multi-monitor setups this
refers to the height for all physical monitors associated with
DISPLAY. Multiple Terminals.
-- Function: display-mm-width &optional display
This function returns the width of the screen in millimeters, or
‘nil’ if Emacs cannot get that information.
For graphical terminals, note that on multi-monitor setups this
refers to the width for all physical monitors associated with
DISPLAY. Multiple Terminals.
-- 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 about the window system
in use where Emacs shows the specified DISPLAY. (Their names begin with
‘x-’ for historical reasons.)
-- Function: x-server-version &optional display
This function returns the list of version numbers of the GUI window
system running on DISPLAY, such as the X server on GNU and Unix
systems. The value is a list of three integers: the major and
minor version numbers of the protocol, and the distributor-specific
release number of the window system software itself. On GNU and
Unix systems, these are normally the version of the X protocol and
the distributor-specific release number of the X server software.
On MS-Windows, this is the version of the Windows OS.
-- Function: x-server-vendor &optional display
This function returns the vendor that provided the window system
software (as a string). On GNU and Unix systems this really means
whoever distributes the X server. On MS-Windows this is the vendor
ID string of the Windows OS (Microsoft).
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.
Info Catalog
elisp: Resources
elisp: Frames