how to use graphic-display-p and window-system

["Drew Adams" <drew.adams@oracle.com>]

> From: Eli Zaretskii Sent: Tuesday, April 14, 2009 4:33 AM
> To: 2993@emacsbugs.donarmstrong.com Subject: bug#2993
> > posn-col-row uses the `line-spacing' value in terminals.  
> > According to the doc `line-spacing' only has an effect
> > in window systems.
> 
> You mean, on graphic displays. Please use `display-graphic-p' 
> rather than assuming that if `window-system' is non-nil, the 
> display is necessarily a graphic one. At least one Emacs 
> configuration violates this assumption.

I'm not familiar with `display-graphic-p'. I've been using `window-system' to
distinguish Emacs running in a console/terminal from Emacs running with a window
mgr. In particular, I want to check whether Emacs supports keys such as S-TAB.

[Excuse me if console/terminal is not the right terminology, but I'm talking
about Emacs with no fancy keys such as S-TAB and less graphic support (colors,
fonts etc.) versus Emacs with no such limitations.]

`display-graphic-p' has apparently been with us since Emacs 22, but there is no
mention of it in the Elisp manual, even in Emacs 23. There is, however, plenty
of mention of `window-system', including node Window Systems.

Could someone please summarize the differences between the two, and when to use
one or the other? Please include how each might relate to use with a console,
-daemon, etc.

And could we then document this stuff in the manual? If it is in fact already
documented somewhere, then we can try to make it more visible or easier to find.

(FYI, I filed a bug about a general lack of clarity in the doc about terms
"graphical", "display", "terminal", and so on. But the bug site seems to be
down, so I don't know the bug #.)