% Copyright 2006 by Till Tantau
%
% This file may be distributed and/or modified
%
% 1. under the LaTeX Project Public License and/or
% 2. under the GNU Free Documentation License.
%
% See the file doc/generic/pgf/licenses/LICENSE for more details.


\section{Commands of the System Layer}

\makeatletter

\subsection{Beginning and Ending a Stream of System Commands}

A ``user'' of the \pgfname\ system layer (like the basic layer or a
frontend) will interface with the system layer by calling a stream of
commands starting with |\pgfsys@|. From the system layer's point of
view, these commands form a long stream. Between calls to the system
layer, control goes back to the user.

The driver files implement system layer commands by inserting
|\special| commands that implement the desired operation. For example,
|\pgfsys@stroke| will be mapped to |\special{pdf: S}| by the driver
file for |pdftex|.

For many drivers, when such a stream of specials starts, it is
necessary to install an appropriate transformation and perhaps perform
some more bureaucratic tasks. For this reason, every stream will start
with a |\pgfsys@beginpicture| and will end with a corresponding ending
command.

\begin{command}{\pgfsys@beginpicture}
  Called at the beginning of a |{pgfpicture}|. This command should
  ``setup things.''

  Most drivers will need to implement this command.
\end{command}

\begin{command}{\pgfsys@endpicture}
  Called at the end of a pgfpicture. 

  Most drivers will need to implement this command.
\end{command}

\begin{command}{\pgfsys@typesetpicturebox\marg{box}}
  Called \emph{after} a |{pgfpicture}| has been typeset. The picture
  will have been put in box \meta{box}. This command should insert the
  box into the normal text. The box \meta{box} will still be a ``raw''
  box that contains only the |\special|'s that make up the description
  of the picture. The  job of this command is to resize and shift
  \meta{box} according to the  baseline shift and the size of the
  box. 

  This command has a default implementation and need not be
  implemented by a driver file.
\end{command}

\begin{command}{\pgfsys@beginpurepicture}
  This version of the |\pgfsys@beginpicture| picture command can be
  used for pictures that are guaranteed not to contain any escaped
  boxes (see below). In this case, a driver might provide a more
  compact version of the command. 
  
  This command has a default implementation and need not be
  implemented by a driver file.
\end{command}

\begin{command}{\pgfsys@endpurepicture}
  Called at the end of a ``pure'' |{pgfpicture}|.
  
  This command has a default implementation and need not be
  implemented by a driver file.
\end{command}

Inside a stream it is sometimes necessary to ``escape'' back into
normal typesetting mode; for example to insert some normal text, but
with all of the current transformations and clippings being in
force. For this escaping, the following command is used:

\begin{command}{\pgfsys@hbox\marg{box number}}
  Called to insert a (horizontal) TeX box inside a
  |{pgfpicture}|.
  
  Most drivers will need to (re-)implement this command.
\end{command}

\begin{command}{\pgfsys@hboxsynced\marg{box number}}
  Called to insert a (horizontal) TeX box inside a
  |{pgfpicture}|, but with the current coordiante transformation
  matrix synced with the canvas transformation matrix. 

  This command should do the same as if you used
  |\pgflowlevelsynccm| followed by |\pgfsys@hbox|. However, the default
  implementation of this command will use a ``TeX-translation'' for
  the translation part of the transformation matrix. This will ensure
  that hyperlinks ``survive'' at least translations. On the other
  hand, a driver may choose to revert to a simpler
  implementation. This is done, for example, for the \textsc{svg}
  implementation, where a \TeX-translation makes no sense.
\end{command}



\subsection{Path Construction System Commands}

\begin{command}{\pgfsys@moveto\marg{x}\marg{y}}
  This command is used to start a path at a specific point
  $(x,y)$ or to move the current point of the current path to  $(x,y)$
  without drawing anything upon stroking (the current path is
  ``interrupted'').

  Both \meta{x} and \meta{y} are given as \TeX\ dimensions. It is the
  driver's job to transform these to the coordinate system of the
  backend. Typically, this means converting the \TeX\ dimension into a
  dimensionless multiple of $\frac{1}{72}\mathrm{in}$. The function
  |\pgf@sys@bp| helps with this conversion.

  \example Draw a line from $(10\mathrm{pt},10\mathrm{pt})$ to the
  origin of the picture. 
\begin{codeexample}[code only]
\pgfsys@moveto{10pt}{10pt}
\pgfsys@lineto{0pt}{0pt}
\pgfsys@stroke
\end{codeexample}

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}


\begin{command}{\pgfsys@lineto\marg{x}\marg{y}}
  Continue the current path to $(x,y)$ with
  a straight line.

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}


\begin{command}{\pgfsys@curveto\marg{$x_1$}\marg{$y_1$}\marg{$x_2$}\marg{$y_2$}\marg{$x_3$}\marg{$y_3$}}
  Continue the current path to $(x_3,y_3)$
  with a Bézier curve that has the two control points  $(x_1,y_1)$ and  $(x_2,y_2)$.

  \example Draw a good approximation of a quarter circle:
\begin{codeexample}[code only]
\pgfsys@moveto{10pt}{0pt}
\pgfsys@curveto{10pt}{5.55pt}{5.55pt}{10pt}{0pt}{10pt}
\pgfsys@stroke
\end{codeexample}

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}


\begin{command}{\pgfsys@rect\marg{x}\marg{y}\marg{width}\marg{height}}
  Append a rectangle to the current path whose lower left corner is
  at $(x,y)$ and whose width and height in
  big points are  given by \meta{width} and \meta{height}.

  This command can be ``mapped back'' to |\pgfsys@moveto| and
  |\pgfsys@lineto| commands, but it is included since \pdf\ has a
  special, quick version of this command. 

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}


\begin{command}{\pgfsys@closepath}
  Close the current path. This results in joining the current point of
  the path with the point specified by the last |\pgfsys@moveto|
  operation. Typically, this is preferable over using |\pgfsys@lineto|
  to the last point specified by a |\pgfsys@moveto|, since the line
  starting at this point and the line ending at this point will be
  smoothly joined by |\pgfsys@closepath|.

  \example Consider
\begin{codeexample}[code only]
\pgfsys@moveto{0pt}{0pt}
\pgfsys@lineto{10bp}{10bp}
\pgfsys@lineto{0bp}{10bp}
\pgfsys@closepath
\pgfsys@stroke
\end{codeexample}
  and
\begin{codeexample}[code only]
\pgfsys@moveto{0bp}{0bp}
\pgfsys@lineto{10bp}{10bp}
\pgfsys@lineto{0bp}{10bp}
\pgfsys@lineto{0bp}{0bp}
\pgfsys@stroke
\end{codeexample}
  
  The difference between the above will be that in the second triangle
  the corner at the origin will be wrong; it will just be the overlay
  of two lines going in different directions, not a sharp pointed
  corner.

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}




\subsection{Canvas Transformation System Commands}

\begin{command}{\pgfsys@transformcm\marg{a}\marg{b}\marg{c}\marg{d}\marg{e}\marg{f}}
  Perform a concatenation of the canvas transformation matrix with the
  matrix given by the values \meta{a} to \meta{f}, see the \pdf\ or
  PostScript manual for details. The values \meta{a} to \meta{d} are
  dimensionless factors, \meta{e} and \meta{f} are \TeX\ dimensions 

  \example |\pgfsys@transformcm{1}{0}{0}{1}{1cm}{1cm}|.

  This command is protocoled, see Section~\ref{section-protocols}.  
\end{command}


\begin{command}{\pgfsys@transformshift\marg{x displacement}\marg{y displacement}}
  This command will change the origin of the canvas to $(x,y)$.

  This command has a default implementation and need not be
  implemented by a driver file.

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}

\begin{command}{\pgfsys@transformxyscale\marg{x scale}\marg{y scale}}
  This command will scale the canvas (and  everything that is drawn)
  by a factor of \meta{x scale} in the $x$-direction and \meta{y
    scale} in the  $y$-direction. Note that this applies to
  everything, including  lines. So a scaled line will have a different
  width and may even have a different width when going along the
  $x$-axis and when going along the $y$-axis, if the scaling is
  different in these directions. Usually, you do not want this.

  This command has a default implementation and need not be
  implemented by a driver file.

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}


\subsection{Stroking, Filling, and Clipping System Commands}

\begin{command}{\pgfsys@stroke}
  Stroke the current path (as if it were drawn with a pen). A number
  of graphic state parameters influence this, which can be
  set using appropriate system commands described later.

  \begin{description}
  \item[Line width]
    The ``thickness'' of the line. A width of 0 is the thinnest width
    renderable on the device. On a high-resolution printer this may
    become invisible and should be avoided. A good choice is 0.4pt,
    which is the default.

  \item[Stroke color]
    This special color is used for stroking. If it is not set, the
    current color is used.
 
  \item[Cap]
    The cap describes how the endings of lines are drawn. A round cap
    adds a little half circle to these endings. A butt cap ends the
    lines exactly at the end (or start) point without anything
    added. A rectangular cap ends the lines like the butt cap, but the
    lines protrude over the endpoint by the line thickness. (See also
    the \pdf\ manual.) If the path has been closed, no cap
    is drawn.
 
  \item[Join]
    This describes how a bend (a join) in a path is rendered. A round
    join draws bends using small arcs. A bevel join just draws the two
    lines and then fills the join minimally so that it becomes
    convex. A miter join extends the lines so that they form a single
    sharp corner, but only up to a certain miter limit. (See the \pdf\
    manual once more.)
 
  \item[Dash]
    The line may be dashed according to a dashing pattern.
 
  \item[Clipping area]
    If a clipping area is established, only those parts of the path
    that are inside the clipping area will be drawn.
  \end{description}
  
  In addition to stroking a path, the path may also be used for
  clipping after it has been stroked. This will happen if the
  |\pgfsys@clipnext| is used prior to this command, see there for
  details.

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}


\begin{command}{\pgfsys@closestroke}
  This command should have the same effect as first closing the path
  and then stroking it.

  This command has a default implementation and need not be
  implemented by a driver file.

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}


\begin{command}{\pgfsys@fill}
  This command fills the area surrounded by the current path. If the
  path has not yet been closed, it is closed prior to filling. The
  path itself is not stroked. For self-intersecting paths or paths
  consisting of multiple parts, the nonzero winding number rule is
  used to determine whether a point is inside or outside the
  path, except if |\ifpgfsys@eorule| holds -- in which case the
  even-odd rule should be used. (See the \pdf\ or PostScript manual
  for details.)  
 
  The following graphic state parameters influence the filling:
 
  \begin{description}
  \item[Interior rule]
    If |\ifpgfsys@eorule| is set, the even-odd rule is used, otherwise
    the non-zero winding number rule.
 
  \item[Fill color]
    If the fill color is not especially set, the current color is
    used. 
 
  \item[Clipping area]
    If a clipping area is established, only those parts of the filling
    area that are inside the clipping area will be drawn.
  \end{description}

  In addition to filling the path, the path will also be used for
  clipping if |\pgfsys@clipnext| is used prior to this command.

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}

\begin{command}{\pgfsys@fillstroke}
  First, the path is filled, then the path is stroked. If the fill and
  stroke colors are the same (or if they are not specified and the
  current color is used), this yields almost the same as a
  |\pgfsys@fill|. However, due to the line thickness of the stroked
  path, the fill-stroked area will be slightly larger.

  In addition to stroking and filling the path, the path will also be
  used for clipping if |\pgfsys@clipnext| is used prior to this command.

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}


\begin{command}{\pgfsys@discardpath}
 Normally, this command should ``throw away'' the current path.
 However, after |\pgfsys@clipnext| has been called, the current path
 should subsequently be used for clipping. See |\pgfsys@clipnext| for 
 details. 

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}


\begin{command}{\pgfsys@clipnext}
  This command should be issued after a path has been constructed, but
  before it has been stroked and/or filled or discarded. When the
  command is used, the next stroking/filling/discarding command will
  first be executed normally. Then, afterwards, the just-used path
  will be used for subsequent clipping. If there has already been a
  clipping region, this region is intersected with the new clipping
  path (the clipping cannot get bigger). The nonzero winding number
  rule is used to determine whether a point is inside or outside the
  clipping area or the even-odd rule, depending on whether
  |\ifpgfsys@eorule| holds.
\end{command}




\subsection{Graphic State Option System Commands}

\begin{command}{\pgfsys@setlinewidth\marg{width}}
  Sets the width of lines, when stroked, to \meta{width}, which must
  be a \TeX\ dimension.

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}

\begin{command}{\pgfsys@buttcap}
  Sets the cap to a butt cap. See |\pgfsys@stroke|.

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}

\begin{command}{\pgfsys@roundcap}
  Sets the cap to a round cap. See |\pgfsys@stroke|.

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}

\begin{command}{\pgfsys@rectcap}
  Sets the cap to a rectangular cap. See |\pgfsys@stroke|.

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}

\begin{command}{\pgfsys@miterjoin}
  Sets the join to a miter join. See |\pgfsys@stroke|.

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}

\begin{command}{\pgfsys@setmiterlimit\marg{factor}}
  Sets the miter limit of lines to \meta{factor}. See
  the \pdf\ or PostScript for details on what the miter limit is.

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}

\begin{command}{\pgfsys@roundjoin}
  Sets the join to a round join. See |\pgfsys@stroke|.

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}

\begin{command}{\pgfsys@beveljoin}
  Sets the join to a bevel join. See |\pgfsys@stroke|.

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}

\begin{command}{\pgfsys@setdash\marg{pattern}\marg{phase}}
  Sets the dashing patter. \meta{pattern} should be a list of \TeX\
  dimensions lengths separated by commas. \meta{phase} should be a
  single dimension.

  \example |\pgfsys@setdash{3pt,3pt}{0pt}|
 
  The list of values in \meta{pattern} is used to determine the
  lengths of the ``on'' phases of the dashing and of the ``off''
  phases. For example, if \meta{pattern} is |3bp,4bp|, then the dashing
  pattern is ``3bp on followed by 4bp off, followed by 3bp on,
  followed by 4bp off, and so on.'' A pattern of |.5pt,4pt,3pt,1.5pt| means
  ``.5pt on, 4pt off, 3pt on, 1.5pt off, .5pt on, \dots'' If the
  number of entries is odd, the last one is used twice, so |3pt| means
  ``3pt on, 3pt off, 3pt on, 3pt off, \dots'' An empty list 
  means  ``always on.''
 
  The second argument determines the ``phase'' of the pattern. For
  example, for a pattern of |3bp,4bp| and a phase of |1bp|, the pattern
  would start: ``2bp on, 4bp off, 3bp on, 4bp off, 3bp on, 4bp off,
  \dots''

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}

{\let\ifpgfsys@eorule=\relax
\begin{command}{\ifpgfsys@eorule}
  Determines whether the even odd rule is used for filling and
  clipping or not.
\end{command}
}

\begin{command}{\pgfsys@stroke@opacity\marg{value}}
  Sets the opacity of stroking operations.
\end{command}

\begin{command}{\pgfsys@fill@opacity\marg{value}}
  Sets the opacity of filling operations.
\end{command}


\subsection{Color System Commands}

The \pgfname\ system layer provides a number of system commands for
setting colors. These command coexist with commands from the |color|
and |xcolor| package, which perform similar functions. However, the
|color| package does not support having two different colors for
stroking and filling, which is a useful feature that is supported by
\pgfname. For this reason, the \pgfname\ system layer offers commands for
setting these colors separatedly. Also, plain \TeX\ profits from the
fact that \pgfname\ can set colors.

For \pdf, implementing these color commands is easy since \pdf\ 
supports different stroking and filling colors directly. For
PostScript, a more complicated approach is needed in which the colors
need to be stored in special PostScript variables that are set
whenever a stroking or a filling operation is done.

\begin{command}{\pgfsys@color@rgb\marg{red}\marg{green}\marg{blue}}
  Sets the color used for stroking and filling operations to the given 
  red/green/blue tuple (numbers between 0 and 1).

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}

\begin{command}{\pgfsys@color@rgb@stroke\marg{red}\marg{green}\marg{blue}}
  Sets the color used for stroking operations to the given
  red/green/blue tuple (numbers between 0 and 1). 

  \example Make stroked text dark red: |\pgfsys@color@rgb@stroke{0.5}{0}{0}|

  The special stroking color is only used if the stroking color has
  been set since the last |\color| or |\pgfsys@color@xxx|
  command. Thus, each |\color| command will reset both the stroking
  and filling colors by calling |\pgfsys@color@reset|. 

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}

\begin{command}{\pgfsys@color@rgb@fill\marg{red}\marg{green}\marg{blue}}
  Sets the color used for filling operations to the given
  red/green/blue tuple (numbers between 0 and 1). This color may be
  different from the stroking color.

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}

\begin{command}{\pgfsys@color@cmyk\marg{cyan}\marg{magenta}\marg{yellow}\marg{black}}
  Sets the color used for stroking and filling operations to the given
  cymk tuple (numbers between 0 and 1). 

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}

\begin{command}{\pgfsys@color@cmyk@stroke\marg{cyan}\marg{magenta}\marg{yellow}\marg{black}}
  Sets the color used for stroking operations to the given cymk tuple
  (numbers between 0 and 1). 

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}

\begin{command}{\pgfsys@color@cmyk@fill\marg{cyan}\marg{magenta}\marg{yellow}\marg{black}}
  Sets the color used for filling operations to the given cymk tuple
  (numbers between 0 and 1). 

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}

\begin{command}{\pgfsys@color@cmy\marg{cyan}\marg{magenta}\marg{yellow}}
  Sets the color used for stroking and filling operations to the given
  cym tuple (numbers between 0 and 1). 

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}

\begin{command}{\pgfsys@color@cmy@stroke\marg{cyan}\marg{magenta}\marg{yellow}}
  Sets the color used for stroking operations to the given cym tuple
  (numbers between 0 and 1). 

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}

\begin{command}{\pgfsys@color@cmy@fill\marg{cyan}\marg{magenta}\marg{yellow}}
  Sets the color used for filling operations to the given cym tuple
  (numbers between 0 and 1). 

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}

\begin{command}{\pgfsys@color@gray\marg{black}}
  Sets the color used for stroking and filling operations to the given
  black value, where 0 means black and 1 means white.

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}

\begin{command}{\pgfsys@color@gray@stroke\marg{black}}
  Sets the color used for stroking operations to the given black value,
  where 0 means black and 1 means white.

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}

\begin{command}{\pgfsys@color@gray@fill\marg{black}}
  Sets the color used for filling operations to the given black value,
  where 0 means black and 1 means white.

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}

\begin{command}{\pgfsys@color@reset}
  This command will be called when the |\color| command is used. It
  should purge any internal settings of stroking and filling
  color. After this call, till the next use of a command like
  |\pgfsys@color@rgb@fill|, the current color installed by the
  |\color| command should be used.

  If the \TeX-if |\pgfsys@color@reset@inorder| is set to true, this
  command may ``assume'' that any call to a color command that sets
  the fill or stroke color came ``before'' the call to this command
  and may try to optimize the output accordingly.

  An example of an incorrect ``out of order'' call would be using
  |\pgfsys@color@reset| at the beginning of a box that is constructed
  using |\setbox|. Then, when the box is constructed, no special fill
  or stroke color might be in force. However, when the box is later on
  inserted at some point, a special fill color might already have been
  set. In this case, this command is not guaranteed to reset the color
  correctly. 
\end{command}

\begin{command}{\pgfsys@color@reset@inordertrue}
  Sets the optimized ``in order'' version of the color resetting. This
  is the default.
\end{command}

\begin{command}{\pgfsys@color@reset@inorderfalse}
  Switches off the optimized color resetting. 
\end{command}

\begin{command}{\pgfsys@color@unstacked\marg{\LaTeX\ color}}
  This slightly obscure command causes the color stack to be
  tricked. When called, this command should set the current color to
  \meta{\LaTeX\ color} without causing any change in the color stack.

  \example |\pgfsys@color@unstacked{red}|
\end{command}




\subsection{Pattern System Commands}


\begin{command}{\pgfsys@declarepattern
    \marg{name}\marg{$x_1$}\marg{$y_1$}\marg{$x_2$}\marg{$y_2$}
    \marg{$x$ step}\marg{$y$ step}\marg{code}\marg{flag}}
  This command declares a new colored or uncolored pattern, depending
  on whether \meta{flag} is |0|, which means uncolored, or |1|, which
  means colored. Uncolored patterns have no inherent color, the color
  is provided when they are set. Colored patters have an inherent
  color.

  The \meta{name} is a name for later use when the pattern is to be
  shown. The pairs $(x_1,y_1)$ and $(x_2,y_2)$ must describe a
  bounding box of the pattern \meta{code}.

  The tiling step of the pattern is given by \meta{$x$ step} and
  \meta{$y$ step}.

  \example
\begin{codeexample}[code only]
\pgfsys@declarepattern{hori}{-.5pt}{0pt}{.5pt}{3pt}{3pt}{3pt}
{\pgfsys@moveto{0pt}{0pt}\pgfsys@lineto{0pt}{3pt}\pgfsys@stroke}
{0}
\end{codeexample}
\end{command}

\begin{command}{\pgfsys@setpatternuncolored\marg{name}\marg{red}\marg{green}\marg{blue}}
  Sets the fill color to the pattern named \meta{name}. This pattern
  must previously have been declared with \meta{flag} set to |0|. The
  color of the pattern is given in the parameters \meta{red},
  \meta{green}, and \meta{blue} in the usual way.

  The fill color ``pattern''  will persist till the next color command
  that modifies the fill color.
\end{command}

\begin{command}{\pgfsys@setpatterncolored\marg{name}}
  Sets the fill color to the pattern named \meta{name}. This pattern 
  must have been declared with the |1| flag.
\end{command}



\subsection{Scoping System Commands}

The scoping commands are used to keep changes of the graphics state
local.

\begin{command}{\pgfsys@beginscope}
  Saves the current graphic state on a graphic state stack. All
  changes to the graphic state parameters mentioned for |\pgfsys@stroke|
  and |\pgfsys@fill| will be local to the current graphic state and 
  the old values will be restored after |\pgfsys@endscope| is used.
 
  \emph{Warning:} \pdf\ and PostScript differ with respect to the
  question of whether the current path is part of the graphic state or
  not. For this reason, you should never use this command unless the
  path is currently empty. For example, it might be a good idea to use 
  |\pgfsys@discardpath| prior to calling this command. 

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}

\begin{command}{\pgfsys@endscope}
  Restores the last saved graphic state.

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}







\subsection{Image System Commands}

The system layer provides some commands for image inclusion.

\begin{command}{\pgfsys@imagesuffixlist}
  This macro should expand to a list of suffixes, separated by `:',
  that will be tried when searching for an image.

  \example |\def\pgfsys@imagesuffixlist{eps:epsi:ps}|
\end{command}


\begin{command}{\pgfsys@defineimage}
  Called, when an image should be defined. 
 
  This command does not take any parameters. Instead, certain macros
  will be preinstalled with appropriate values when this command is
  invoked. These are:
 
  \begin{itemize}
  \item\declare{|\pgf@filename|}
    File name of the image to be defined.

  \item\declare{|\pgf@imagewidth|}
    Will be set to the desired (scaled) width of the image.

  \item\declare{|\pgf@imageheight|}
    Will be set to the desired (scaled) height of the image.
 
    If this macro and also the height macro are empty, the image
    should have its ``natural'' size.
 
    If exactly only of them is specified, the undefined value the
    image is scaled so that the aspect ratio is kept.
 
    If both are set, the image is scaled in both directions
    independently, possibly changing the aspect ratio.
  \end{itemize}
 
  The following macros presumable mostly make sense for drivers that
  can handle \pdf: 

  \begin{itemize}
  \item \declare{|\pgf@imagepage|}
    The desired page number to be extracted from a multi-page
    ``image.''

  \item\declare{|\pgf@imagemask|}
    If set, it will be set to |/SMask x 0 R| where |x| is the \pdf\ 
    object number of a soft mask to be applied to the image.

  \item\declare{|\pgf@imageinterpolate|}
    If set, it will be set to |/Interpolate true| or
    |/Interpolate false|, indicating whether the image should be
    interpolated in \pdf. 
  \end{itemize}
 
  The command should now setup the macro |\pgf@image| such that calling
  this macro will result in typesetting the image. Thus, |\pgf@image| is
  the ``return value'' of the command.

  This command has a default implementation and need not be
  implemented by a driver file.
\end{command}


\begin{command}{\pgfsys@definemask}
  This command declares a mask for usage with images. It works similar
  to |\pgfsys@defineimage|: Certain macros are set when the command is
  called. The result should be to set the macro |\pgf@mask| to a pdf
  object count that can subsequently be used as a soft mask. The
  following macros will be set when this command is invoked:
 
  \begin{itemize}
  \item \declare{|\pgf@filename|}
    File name of the mask to be defined.

  \item \declare{|\pgf@maskmatte|}
    The so-called matte of the mask (see the \pdf\ documentation for
    details). The matte is a color specification consisting of 1, 3 or
    4 numbers between 0 and 1. The number of numbers depends on the
    number of color channels in the image (not in the mask!). It will
    be assumed that the image has been preblended with this color.
  \end{itemize}
\end{command}


\subsection{Shading System Commands}


\begin{command}{\pgfsys@horishading\marg{name}\marg{height}\marg{specification}}
  Declares a horizontal shading for later use. The effect of this
  command should be the definition of a macro called |\@pgfshading|\meta{name}|!|
  (or |\csname @pdfshading|\meta{name}|!\endcsname|, to be
  precise). When invoked, this new macro should insert a shading at
  the current position. 
 
  \meta{name} is the name of the shading, which is also used in the
  output macro name. \meta{height} is the height of the shading and
  must be given as a TeX dimension like |2cm| or
  |10pt|. \meta{specification} is a shading color 
  specification as specified in Section~\ref{section-shadings}. The
  shading specification implicitly fixes the width of the shading. 
 
  When |\@pgfshading|\meta{name}|!| is invoked, it should insert a box
  of height \meta{height} and the width implicit in the shading
  declaration. 
\end{command}


\begin{command}{\pgfsys@vertshading\marg{name}\marg{width}\marg{specification}}
  Like the horizontal version, only for vertical shadings. This time,
  the height of the shading is implicit in \meta{specification} and
  the width is given as \meta{width}.
\end{command}

\begin{command}{\pgfsys@radialshading\marg{name}\marg{starting point}\marg{specification}}
  Declares a radial shading. Like the previous macros, this command
  should setup the macro |\@pgfshading|\meta{name}|!|, which upon
  invocation should insert a radial shading whose size is implicit in
  \meta{specification}.

  The parameter \meta{starting point} is a \pgfname\ point
  specifying the inner starting point of the shading.
\end{command}


\subsection{Reusable Objects System Commands}

\begin{command}{\pgfsys@invoke\marg{literals}}
  This command gets protocoled literals and should insert them into
  the |.pdf| or |.dvi| file using an appropriate |\special|.
\end{command}

\begin{command}{\pgfsys@defobject\marg{name}\marg{lower
      left}\marg{upper right}\marg{code}}
  Declares an object for later use. The idea is that the object can be
  precached in some way and then be rendered more quickly when used
  several times. For example, an arrow head might be defined and
  prerendered in this way.
 
  The parameter \meta{name} is the name for later use. \meta{lower
  left} and \meta{upper right} are \pgfname\ points specifying a bounding
  box for the object. \meta{code} is the code for the object. The code
  should not be too fancy.

  This command has a default implementation and need not be
  implemented by a driver file.
\end{command}

\begin{command}{\pgfsys@useobject\marg{name}\marg{extra code}}
  Renders a previously declared object. The first parameter is the
  name of the the object. The second parameter is extra code that
  should be executed right \emph{before} the object is
  rendered. Typically, this will be some transformation code.

  This command has a default implementation and need not be
  implemented by a driver file.
\end{command}


\subsection{Invisibility System Commands}

All drawing or stroking or text rendering between calls of the
following commands should be suppressed. A similar effect can be
achieved by clipping against an empty region, but the following
commands do not open a graphics scope and can be opened and closed
``orthogonally'' to other scopes.

\begin{command}{\pgfsys@begininvisible}
  Between this command and the closing |\pgfsys@endinvisible| all
  output should be suppressed. Nothing should be drawn at all, which
  includes all paths, images and shadings. However, no groups (neither
  \TeX\ groups nor graphic state groups) should be opened by this
  command. 

  This command has a default implementation and need not be
  implemented by a driver file.

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}
  
\begin{command}{\pgfsys@endinvisible}
  Ends the invisibility section, unless invisibility blocks have been
  nested. In this case, only the ``last'' one restores visibility.

  This command has a default implementation and need not be
  implemented by a driver file.

  This command is protocoled, see Section~\ref{section-protocols}.
\end{command}


\subsection{Position Tracking Commands}

The following commands are used to determine the position of text on a
page. This is a rather complicated process in general since at the
moment when the text is read by \TeX\ the final position cannot be
determined, yet. For example, the text might be put in a box which is
later put in the headline or perhaps in the footline or perhaps even
on a different page.

For these reasons, position tracking is typically a two-stage
process. In a first stage you indicate that a certain position is of
interest by \emph{marking} it. This will (depending on the details of
the backend driver) cause page coordinates or this position to be
written to a |.aux| file when the page is shipped. Possibly, the
position might also be determined at an even later stage. Then, on a
second run of \TeX, the position is read from the |.aux| file and can
be used.

\begin{command}{\pgfsys@markposition\marg{name}}
  Marks a position on the page. This command should be given while
  normal typesetting is done such as in
\begin{codeexample}[code only]
The value of $x$ is \pgfsys@markposition{here}important.
\end{codeexample}
  It causes the position |here| to be saved when the page is shipped
  out.
\end{command}

\begin{command}{\pgfsys@getposition\marg{name}\marg{macro}}
  This command retrieves a position that has been marked on an earlier
  run of \TeX\ on the current file. The \meta{macro} must be a macro
  name such as |\mymarco|. It will redefined such that it is
  \begin{itemize}
  \item either just |\relax| or
  \item a |\pgfpoint...| command.
  \end{itemize}
  The first case will happen when the position has not been marked at
  all or when the file is typeset for the first time, when the
  coordinates are not yet available.

  In the second case, executing \meta{macro} yields the position on
  the page that is to be interpreted as follows: A coordinate like
  |\pgfpoint{2cm}{3cm}| means ``2cm to the right and 3cm up from the
  origin of the page.'' The position of the origin of the page is not
  guaranteed to be at the lower left corner, it is only guaranteed
  that all pictures on a page use the same origin.

  To determine the lower left corner of a page, you can call
  |\pgfsys@getposition| with \meta{name} set to the special name
  |pgfpageorigin|. By shifting all positions by the amount returned by
  this call you can position things absolutely on a page.

  \example Referencing a point or the page:
\begin{codeexample}[code only]
The value of $x$ is \pgfsys@markposition{here}important.

Lots of text.

\hbox{\pgfsys@markposition{myorigin}%
\begin{pgfpicture}
  % Switch of size protocol
  \pgfpathmoveto{\pgfpointorigin}
  \pgfusepath{use as bounding box}

  \pgfsys@getposition{here}{\hereposition}
  \pgfsys@getposition{myorigin}{\thispictureposition}

  \pgftransformshift{\pgfpointscale{-1}{\thispictureposition}}
  \pgftransformshift{\hereposition}

  \pgfpathcircle{\pgfpointorigin}{1cm}
  \pgfusepath{draw}
\end{pgfpicture}}
\end{codeexample}
\end{command}


\subsection{Internal Conversion Commands}

The system commands take \TeX\ dimensions as input, but the dimensions
that have to be inserted into \pdf\ and PostScript files need to be
dimensionless values that are interpreted as multiples of
$\frac{1}{72}\mathrm{in}$. For example, the \TeX\ dimension $2bp$
should be inserted as |2| into a \pdf\ file and the \TeX\ dimension
$10\mathrm{pt}$ as |9.9626401|. To make this conversion easier, the following
command may be useful:

\begin{command}{\pgf@sys@bp\marg{dimension}}
  Inserts how many multiples of $\frac{1}{72}\mathrm{in}$ the
  \meta{dimension} is into the current protocol stream (buffered).

  \example |\pgf@sys@bp{\pgf@x}| or |\pgf@sys@bp{1cm}|.
\end{command}

Note that this command is \emph{not} a system command that can/needs
to be overwritten by a driver. 

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