@Section
   @Title { "@Graphic" }
   @Tag { graphic }
@Begin
@PP
graphic.sym @Index { @@Graphic symbol }
diagrams @Index { Diagrams }
Lout does not provide the vast repertoire of graphical objects (lines,
circles, boxes, etc.) required by diagrams.  Instead, it provides an
escape route to some other language that does have these features, via
its @@Graphic symbol:
postscript.graphic @SubIndex { used by @@Graphic }
@ID @OneRow @OneRow @Code {
"{ 0 0 moveto"
"  0 ysize lineto"
"  xsize ysize lineto"
"  xsize 0 lineto"
"  closepath"
"  stroke"
"}"
"@Graphic"
"{ //0.2c"
"  ||0.2c hello, world ||0.2c"
"  //0.2c"
"}"
}
The result of the above invocation of the symbol @@Graphic is
@ID @OneRow @OneRow {
  @BackEnd @Case {
    PostScript @Yield {
      { 0     0     moveto
        0     ysize lineto
        xsize ysize lineto
        xsize 0     lineto
        closepath
        stroke
      }
      @Graphic
      { //0.2c
        ||0.2c hello, world ||0.2c
        //0.2c
      }
    }
    PDF @Yield {
      { 0       0       m
        0       __ysize l
        __xsize __ysize l
        __xsize 0       l
        s
      }
      @Graphic
      { //0.2c
        ||0.2c hello, world ||0.2c
        //0.2c
      }
    }
  }
}
@PP
The right parameter always appears as part of the result, and indeed the
result is always an object whose size is identical to the size of the
right parameter with @@OneCol and @@OneRow applied to
it.  From now on we refer to this part of the result as the {@I base}.
@PP
The left parameter is implementation-dependent:  that is, its
meaning is not defined by Lout, and different implementations could
require different values for it.  The following description applies to
Basser Lout, which uses the PostScript page description language
@Cite { $adobe1990ps }.  Similar but more restricted possibilities exist
with the PDF back end (see a separate document distributed with Lout);
to include both, use the @@BackEnd symbol like this:
@ID @OneRow @Code {
"{ @BackEnd @Case {"
"      PostScript @Yield"
"      {"
"          ..."
"      }"
"      PDF @Yield"
"      {"
"          ..."
"      }"
"  }"
"  @Graphic"
"  {"
"      ..."
"  }"
"}"
}
Returning to PostScript, the left parameter refers to a coordinate system
whose origin is the bottom left-hand corner of the base.  It may use the symbols
@Code xsize and @Code ysize to denote the horizontal and vertical size
of the base; similarly, @Code xmark and @Code ymark denote the positions
of the base's column and row marks:
@ID @OneRow 9p @Font @Fig {
   { &1rt @I ysize /0ik &1rt @I ymark /0ik &1rt 0 } |0.4c
   {  /
      |0ik @ShowMarks { 1c @High 1.5c @Wide ^| 3c @Wide ^/ 2c @High }
      |0ik /
   }
   /0.2c
   | 0 | @I xmark | @I xsize
}
In addition to these four symbols and 0, lengths may be denoted in
centimetres, inches, points, ems, f's, v's and s's using the notation
@ID @OneRow @Tab
    vmargin { 0.5vx }
    hmargin { 1m }
    @Fmta { @Col {@I l  @Code A} ! @Col {instead of Lout's} ! @Col {{@I l}B} }
{
@Rowa A { cm } B { c }
@Rowa A { in } B { i }
@Rowa A { pt } B { p }
@Rowa A { em } B { m }
@Rowa A { ft } B { f }
@Rowa A { vs } B { v }
@Rowa A { sp } B { s }
}
Note that there must be a space between the number and its unit,
unlike Lout proper.
@PP
A point within the base (and, with care, a point outside it) may
be denoted by a pair of lengths.  For example,
@ID @OneRow @Code {
"xmark  ymark"
}
is the point where the marks cross, and
@ID @OneRow @Code {
"0   2 cm"
}
is a point on the left edge, two centimetres above the bottom left-hand
corner.  These two numbers are called the @I {x coordinate} and the
@I {y coordinate} of the point.
@PP
The first step in specifying a graphic object is to define a
{@I path}.  A path can be thought of as the track of a pen moving over
the page.  The pen may be up (not drawing) or down (drawing a line or
curve) as it moves.  The entire path is a sequence of the following
items:
@LP
2i @Wide { |1rt @I {x y} @Code moveto }
|2m Lift the pen and move it to the indicated point.
@JP
2i @Wide { |1rt @I {x y} @Code lineto }
|2m Put the pen down and draw a straight line to the indicated point.
@JP
2i @Wide { |1rt @I {x y r angle1 angle2} @Code arc }
|2m Put the pen down and draw a circular arc whose centre has
coordinates @I x and @I y and whose radius is {@I r}.  The arc begins
at the angle @I angle1 measuring counterclockwise from the point
directly to the right of the centre, and proceeds counterclockwise to
{@I angle2}.  If the arc is not the first thing on the path, a straight
line will be drawn connecting the current point to the start of the arc.
@JP
2i @Wide { |1rt @I {x y r angle1 angle2} @Code arcn }
|2m As for arc, but the arc goes clockwise from @I angle1 to
{@I angle2 }.
@JP
2i @Wide @Code { |1rt closepath }
|2m Draw a straight line back to the point most recently moved to.
@LP
The first item should always be a {@Code moveto}, {@Code arc}, or
{@Code arcn}.  It should be clear from this that the path given earlier:
@ID @OneRow @Code {
"0 0 moveto"
"0 ysize lineto"
"xsize ysize lineto"
"xsize 0 lineto"
"closepath"
}
traces around the boundary of the base with the pen down.
@PP
Once a path is set up, we are ready to @I paint it onto the page.  There 
are two choices: we can either @I stroke it, which means to display it
as described; or we can @I fill it, which means to paint everything
inside it grey or black.  For stroking the two main options are
@IL
@LI {
2i @Wide { |1rt @I length @Code setlinewidth }
|2m The pen will draw lines of the given width.
}
@LI {
2i @Wide { |1rt @Code "[" @I length @Code {"]" 0 setdash} }
|2m The pen will draw dashed lines when it is down, with the dashes each
of the given length.
}
@EL
These options are followed by the word {@Code "stroke"}.  So, for example,
@ID @OneRow @Code {
"{ 0 0 moveto xsize 0 lineto"
"  2 pt setlinewidth [ 5 pt ] 0 setdash stroke"
"}"
"@Graphic { 3i @Wide }"
}
has result
@ID @OneRow {
  @BackEnd @Case {
    PostScript @Yield {
      { 0 0 moveto xsize 0 lineto
        2 pt setlinewidth [ 5 pt ] 0 setdash stroke
      }
      @Graphic { 3i @Wide }
    }
    PDF @Yield {
      { [ __mul(5, __pt) ] 0 d __mul(2, __pt) w 0 0 m __xsize 0 l S
      }
      @Graphic { 3i @Wide }
    }
  }
}
@PP
When filling in the region enclosed by a path, the main option is
{@Code setgray}, which determines the shade of grey to use, on a scale
from 0 (black) to 1 (white).  So, for example,
@ID @OneRow @Code {
"{ 0 0 moveto xsize 0 lineto 0 ysize lineto closepath"
"  0.8 setgray fill"
"}"
"@Graphic"
"{ 2c @Wide  2c @High }"
}
has result
@ID @OneRow {
  @BackEnd @Case {
    PostScript @Yield {
      { 0 0 moveto xsize 0 lineto 0 ysize lineto closepath
        0.8 setgray fill
      }
      @Graphic
      { 2c @Wide  2c @High }
    }
    PDF @Yield {
      { 0 0 m __xsize 0 l 0 __ysize l h
        0.8 g f
      }
      @Graphic
      { 2c @Wide  2c @High }
    }
  }
}
@PP
There are many other options.  The value of the left parameter of
@@Graphic may be any fragment of the PostScript page description language
@Cite { $adobe1990ps }.  Here are two other examples:
@ID @OneRow @Code {
xsize 2 div
}
denoting a length equal to half the horizontal size of the base, and
@ID @OneRow @Code {
gsave fill grestore stroke
}
which both fills and strokes the path.  Since Basser Lout does not check
that the left parameter is valid PostScript, it is possible to cause
mysterious errors in the printing device, resulting in no output, if an
incorrect value is given.  It is a good idea to encapsulate graphics
objects in carefully tested definitions, like those of the Diag figure
drawing package @Cite { $kingston1995lout.user, Chapter 9 },
diag @Index { Diag diagram-drawing package }
to be sure of avoiding these errors.
@PP
PostScript experts may find the following information helpful when
designing advanced graphics features.  The left parameter of @@Graphic
may have two parts, separated by {@Code "//"}:
@ID @OneRow {
@Code "{" @I {first part} @Code "//" @I {second part} @Code "} @Graphic"
@I object
}
If there is no {@Code "//"}, the second part is taken to be empty.  The
PostScript output has the form
@ID @OneRow lines @Break {
@Code gsave
@I x @I y @Code translate
@I {Code which defines {@Code xsize}, {@Code ysize}, {@Code xmark}, {@Code ymark}, {@Code ft}, {@Code vs}, and {@Code sp} }
@Code gsave
@I {first part}
@Code grestore
@I {Code which renders the right parameter in translated coordinates}
@I {second part}
@Code grestore
}
where @Eq {x, y} is the position of the lower left corner of the
base.  Having two parts permits bracketing operations, like @Code save
and @Code restore or @Code begin and {@Code end}, to enclose an
object.  See the source file of the Diag package for examples.
@End @Section