Re: gamegrid.el and some games

[Francesco Potorti` <pot@gnu.org>]

What follows is a request for explanation, and it may highlight a
shortcoming of the emacs lisp manual.  Sorry if sometimes my wording is
not correct, I am new to this area of Emacs lisp.

I am trying to understand whether Emacs can use an arbitrary image in a
display table on a window system.  That is, if it is possible to write a
graphical image (a glyph) in a given position of a display table.

I found this in the manual:

| Glyphs
| ------
| 
|    A "glyph" is a generalization of a character; it stands for an image
| that takes up a single character position on the screen.  Glyphs are
| represented in Lisp as integers, just as characters are.
| 
|    The meaning of each integer, as a glyph, is defined by the glyph
| table, which is the value of the variable `glyph-table'.
| 
|  - Variable: glyph-table
|      The value of this variable is the current glyph table.  It should
|      be a vector; the Gth element defines glyph code G.  If the value
|      is `nil' instead of a vector, then all glyphs are simple (see
|      below).  The glyph table is not used on windowed displays.

What is a `windowed display'?  I know about `character terminal' and
`window system', and in fact this wording is used below, but I do not
know what a windowed display is.  

I suspect it is a window system, and if this is true, it means that the
answer to my initial question is NO.

| 
|    Here are the possible types of elements in the glyph table:
| 
| STRING
|      Send the characters in STRING to the terminal to output this
|      glyph.  This alternative is available on character terminals, but
|      not under a window system.

Okay.  So this is for character terminals.  So I suppose that the two
following alternatives do work on window systems, because nothing is
specified there.

| 
| INTEGER
|      Define this glyph code as an alias for glyph code INTEGER.  You
|      can use an alias to specify a face code for the glyph; see below.
| 
| `nil'
|      This glyph is simple.  The glyph code mod 524288 is the character
|      to output, and the glyph code divided by 524288 specifies the face
|      number (*note Face Functions::) to use while outputting it.
|      (524288 is 2**19.)  *Note Faces::.
| 
|    If a glyph code is greater than or equal to the length of the glyph
| table, that code is automatically simple.
| 
|  - Function: create-glyph string
|      This function returns a newly-allocated glyph code which is set up
|      to display by sending STRING to the terminal.

So here I am not yet sure whether it is possible to specify a graphical
glyph inside a display table, and if yes, how to do it.