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% Copyright 2019 by Till Tantau
%
% This file may be distributed and/or modified
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% 1. under the LaTeX Project Public License and/or
% 2. under the GNU Free Documentation License.
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% 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 ``set up
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 coordinate 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}
\begin{command}{\pgfsys@pictureboxsynced\marg{box number}}
Basically, this should do the same as doing a (scoped) low level sync
followed by inserting the box \meta{box number} directly into the output
stream. However, the default implementation uses |\pgfsys@hboxsynced| in
conjunction with |\pgfsys@beginpicture| to ensure that, if possible,
hyperlinks survive in \textsc{pdf}s. Drivers that are sensitive to
picture-in-picture scopes should replace this implementation by
%
\begin{codeexample}[code only]
\pgfsys@beginscope\pgflowlevelsynccm\box#1\pgfsys@endscope
\end{codeexample}
%
\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 protocolled, see Section~\ref{section-protocols}.
\end{command}
\begin{command}{\pgfsys@endscope}
Restores the last saved graphic state.
This command is protocolled, see Section~\ref{section-protocols}.
\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 protocolled, 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 protocolled, 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 protocolled, 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 protocolled, 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 protocolled, 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 protocolled, 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 protocolled, 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 protocolled, see Section~\ref{section-protocols}.
\end{command}
\begin{command}{\pgfsys@viewboxmeet\marg{$x_1$}\marg{$y_1$}\marg{$x_2$}\marg{$y_2$}%
\marg{$x'_1$}\marg{$y'_1$}\marg{$x'_2$}\marg{$y'_2$}%
}
Starts a ``view box'' scope, which must be ended using |\pgfsys@endviewbox|
later on (with matching scopes).
The effect of this command is as follows: Consider the rectangles $R$ with
lower left corner $(x_1,y_1)$ and upper right corner $(x_2,y_2)$ and $R'$
with corners $(x'_1,y'_1)$ and $(x'_2,y'_2)$. The command will install a
canvas translation and uniform scaling such that $R'$ then has the same
center as~$R$ and additionally, has maximum size such that it still fits
inside~$R$. (Think of this as ``viewing'' $R'$ through $R$ such that the
aspect ratio is kept.)
This command has a default implementation. Its main purpose is to allow
animations of the view box; for static drawings it is better to compute the
necessary transformations directly.
\end{command}
\begin{command}{\pgfsys@viewboxslice\marg{$x_1$}\marg{$y_1$}\marg{$x_2$}\marg{$y_2$}%
\marg{$x'_1$}\marg{$y'_1$}\marg{$x'_2$}\marg{$y'_2$}%
}
Works like the previous command, but now $R'$ has minimal size such that it
encompasses all of $R$.
\end{command}
\begin{command}{\pgfsys@endviewbox}
Ends a viewbox previously started using |\pgfsys@viewboxmeet| or the
|...slice| variant.
\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 protocolled, 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 protocolled, 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 protocolled, 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 protocolled, 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 protocolled, 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 protocolled, see Section~\ref{section-protocols}.
\end{command}
\begin{command}{\pgfsys@buttcap}
Sets the cap to a butt cap. See |\pgfsys@stroke|.
This command is protocolled, see Section~\ref{section-protocols}.
\end{command}
\begin{command}{\pgfsys@roundcap}
Sets the cap to a round cap. See |\pgfsys@stroke|.
This command is protocolled, see Section~\ref{section-protocols}.
\end{command}
\begin{command}{\pgfsys@rectcap}
Sets the cap to a rectangular cap. See |\pgfsys@stroke|.
This command is protocolled, see Section~\ref{section-protocols}.
\end{command}
\begin{command}{\pgfsys@miterjoin}
Sets the join to a miter join. See |\pgfsys@stroke|.
This command is protocolled, 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 protocolled, see Section~\ref{section-protocols}.
\end{command}
\begin{command}{\pgfsys@roundjoin}
Sets the join to a round join. See |\pgfsys@stroke|.
This command is protocolled, see Section~\ref{section-protocols}.
\end{command}
\begin{command}{\pgfsys@beveljoin}
Sets the join to a bevel join. See |\pgfsys@stroke|.
This command is protocolled, 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 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'' and ``off'' phases of the dashing. 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 protocolled, 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}
}
\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 separately. 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 protocolled, 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@...| command. Thus, each |\color|
command will reset both the stroking and filling colors by calling
|\pgfsys@color@reset|.
This command is protocolled, 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 protocolled, 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 cmyk
tuple (numbers between 0 and 1).
This command is protocolled, 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 cmyk tuple
(numbers between 0 and 1).
This command is protocolled, 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 cmyk tuple (numbers
between 0 and 1).
This command is protocolled, 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 cmy
tuple (numbers between 0 and 1).
This command is protocolled, 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 cmy tuple (numbers
between 0 and 1).
This command is protocolled, 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 cmy tuple (numbers
between 0 and 1).
This command is protocolled, 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 protocolled, 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 protocolled, 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 protocolled, 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{$a$}\marg{$b$}\marg{$c$}\marg{$d$}\allowbreak\marg{$e$}\marg{$f$}%
\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}.
The parameters \meta{$a$} to \meta{$f$} are entries of the transformation
matrix that is applied to the pattern, see |\pgfsys@patternmatrix| for more
information.
\example
%
\begin{codeexample}[code only]
\pgfsys@declarepattern
{hori}{-.5pt}{0pt}{.5pt}{3pt}{3pt}{3pt}%
{1.0}{0.0}{0.0}{1.0}{0.0pt}{0.0pt}%
{\pgfsys@moveto{0pt}{0pt}\pgfsys@lineto{0pt}{3pt}\pgfsys@stroke}
{0}
\end{codeexample}
%
\end{command}
\begin{command}{\pgfsys@patternmatrix}
For convenience \pgfname\ defines the transformation matrix that is applied
to all patterns defined with |\pgfdeclarepatternformonly| and
|\pgfdeclarepatterninherentlycolored| in a macro. This can be used as an
extension point for ad-hoc transformation of existing patterns. The
default definition is the
identity matrix:
%
\begin{codeexample}[code only]
\def\pgfsys@patternmatrix{{1.0}{0.0}{0.0}{1.0}{0.0pt}{0.0pt}}
\end{codeexample}
%
The entries of the enclosed array
|{|\meta{$a$}|}{|\meta{$b$}|}{|\meta{$c$}|}{|\meta{$d$}|}{|\meta{$e$}|}{|\meta{$f$}|}|
are entries in the transformation matrix, identified as in the following
transformation prescription:
\begin{equation*}
\begin{pmatrix}
x' \\
y' \\
1 \\
\end{pmatrix}
=
\begin{pmatrix}
a & c & e \\
b & d & f \\
0 & 0 & 1 \\
\end{pmatrix}
\begin{pmatrix}
x \\
y \\
1 \\
\end{pmatrix} .
\end{equation*}
Carrying out the matrix multiplication results in the following system of
equations
\begin{align*}
x' &= a x + c y + e , \\
y' &= b x + d y + f .
\end{align*}
Evidently, the parameters \marg{$a$} to \marg{$d$} have to be dimensionless
because they are scaling factors, but the parameters \marg{$e$} and
\marg{$f$} are offsets, therefore they have to carry a unit.
\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{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 only one 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 set up 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}
\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
set up 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}
\begin{command}{\pgfsys@functionalshading\marg{name}\marg{lower left corner}\meta{upper right corner}\marg{type 4 function}}
Declares a shading using a PostScript-like function that provides a color
for each point. Like the previous macros, this command should set up the
macro |\@pgfshading|\meta{name}|!| so that it will produce a box containing
the desired shading.
Parameter \meta{name} is the name of the shading. Parameter \meta{type 4
function} is a Postscript-like function (type 4 function of the PDF
specification) as described in Section~3.9.4 of the PDF specification
version 1.7. Parameters \meta{lower left corner} and \meta{upper right
corner} are \pgfname\ points that specifies the lower left and upper right
corners of the shading, respectively.
When \meta{type 4 function} is evaluated, the coordinate of the current
point will be on the (virtual) PostScript stack in bp units. After the
function has been evaluated, the stack should consist of three numbers (not
integers! -- the Apple PDF renderer is broken in this regard, so add cvrs
at the end if needed) that represent the red, green, and blue components of
the color.
A buggy function will result is \emph{totally unpredictable chaos} during
rendering.
\end{command}
\subsection{Transparency System Commands}
\begin{command}{\pgfsys@opacity\marg{value}}
Sets the opacity of all operations, treating stroking and filling as a
transparency group. Some drivers support this operations, others do not and
set the fill and stroke individually. This difference can only be seen when
a path is stroked and filled at the same time: When the drawing and fill
opacities are set individually, the effect of filling and drawing a path at
the same time is the same as first filling the path and then drawing it. On
the other, if the opacity is set using this command, the effect should
rather be that same as first filling and then drawing the path without any
opacity in an off-screen area and then copying the result to the target
area with a homogeneous opacity of \meta{value}.
Since \textsc{pdf} does not support this form of opacity, this command is
only present on the system layer and not supported in the basic layer.
\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}
\begin{command}{\pgfsys@blend@mode\marg{value}}
Sets the blend mode, see Section~7.2.4 of the \textsc{pdf} Specification,
Version~1.7.
\end{command}
\begin{command}{\pgfsys@transparencygroupfrombox\marg{box}}
This takes a \TeX\ box and converts it into a transparency group. This
means that any transparency settings apply to the box as a whole. For
instance, if a box contains two overlapping black circles and you draw the
box and, thus, the two circles normally with 50\% transparency, then the
overlap will be darker than the rest. By comparison, if the circles are
part of a transparency group, the overlap will get the same color as the
rest.
\end{command}
A transparency group can be \emph{isolated} and/or a \emph{knockout} group (see
Sections~7.3.4 and 7.3.5 of the \textsc{pdf} Specification Version~1.7). Which
of these is the case is dictated by the current settings of the following two
ifs, which must be set before the above command is called:
{\let\ifpgfsys@transparency@group@isolated=\relax
\begin{command}{\ifpgfsys@transparency@group@isolated}
Determines whether a transparency group should be isolated.
\end{command}
}
{\let\ifpgfsys@transparency@group@knockout=\relax
\begin{command}{\ifpgfsys@transparency@group@knockout}
Determines whether a transparency group is a knockout group or not.
\end{command}
}
\begin{command}{\pgfsys@fadingfrombox\marg{name}\marg{box}}
Declares the fading \meta{name}. The \meta{box} is a \TeX-box. Its
content's luminosity determines the opacity of the resulting fading. This
means that the lighter a pixel inside the box, the more opaque the fading
will be at this position.
\end{command}
\begin{command}{\pgfsys@usefading\meta{name}\marg{a}\marg{b}\marg{c}\marg{d}\marg{e}\marg{f}}
Installs a previously declared fading \meta{name} in the current graphics
state. Afterwards, all drawings will be masked by the fading. The fading
should be centered on the origin and have its original size, except that
the parameters \meta{a} to \meta{f} specify a transformation matrix that
should be applied additionally to the fading before it is installed. The
transformation should not apply to the following graphics, however.
\end{command}
\begin{command}{\pgfsys@clipfading}
This command has a default implementation and need not be implemented by
driver files other than |pgfsys-dvips.def|. The macro is called
in |\pgfsetfadingforcurrentpath| and |\pgfsetfadingforcurrentpathstroked|
of the basic layer, where it invokes the current path for clipping the
shading just before installing it as an opacity mask for fading. The
default implementation is actually a non-operation, but with |dvips| it
is used to clip the fading as described.
\end{command}
\begin{command}{\pgfsys@definemask}
This command declares a fading (known as a soft mask in this context) based
on an image and 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 transparency 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{Animation Commands}
The animation system layer command (|\pgfsys@anim...|) are described in a
separate section, Section~\ref{section-pgfsys-anim}.
\subsection{Object Identification System Commands}
\label{section-sys-id}
The system layer provides commands for adding identification labels (ids) to
different objects in a graphic. These can be used for hyperlinking, which is
needed for instance in conjunction with animations.
The following ``objects'' can get an id assigned to them:
%
\begin{enumerate}
\item Graphic scopes (namely when |\pgfsys@begin@idscope| is called),
\item view boxes (namely when |\pgfsys@viewboxmeet| or
|\pgfsys@viewboxslice| are called),
\item paths (namely when |\pgfsys@fill|, |\pgfsys@stroke|, and so on are
called),
\item text boxes (namely when |\pgfsys@hbox| or |\pgfsys@hboxsynced| is
called), and
\item animations (namely when |\pgfsys@animate| is called).
\end{enumerate}
Creating and using ids is a two-step process. First, you create the id using
|\pgfsys@new@id|, which stores a fresh id in a macro. You can now pass this id
around and clone it. Then, at some point, you wish one of the above objects to
actually get this id. For this, you use |\pgfsys@use@id| just \emph{before} the
object since this command always influences the \emph{next} object.
The basic id management gets more powerful when you use \emph{id types}. The
idea is as follows: In reality, the objects from above do not get assigned only
an id, but rather a combination of an id and a type -- and you can set the type
independently of the id. This is used, for instance, to allow easy access to
the different parts of a node in animations: Each node has a single id, but
consists of several graphic objects (normally, at least a background path and a
text). Each of these uses the same underlying id of the node, but the path has
the type |path| (actually |background.path|) while the text has the type
|text|. The advantage is that for each node only one id must be stored instead
of a great number of the many different possible parts of a node.
\begin{command}{\pgfsys@new@id\marg{macro}}
Creates a new id for later use and stores it in \meta{macro}. It is an
internal text created by the driver and may not be changed or modified.
\end{command}
\begin{command}{\pgfsys@use@id\marg{id}}
``Uses'' an id previously created using |\pgfsys@new@id|. This causes the
\emph{next} graphic object to get the \meta{id} (not the current one). Once
used, the id-type-pair becomes \emph{invalid} and will not be attached to
any other graphics objects. It is, however, not an error to try this. If
\meta{id} is empty, no id-type-pair is attached to the next object.
\end{command}
\begin{command}{\pgfsys@use@type\marg{type}}
Changes the type used with the next graphic object. As mentioned earlier,
the id assigned to the next object is actually a pair consisting of the
currently used id and the currently used type.
\end{command}
\begin{command}{\pgfsys@append@type\marg{text}}
Appends the \meta{text} to the current type.
\end{command}
\begin{command}{\pgfsys@push@type}
Pushes the current type on a global ``stack of types'' without opening a
\TeX\ scope. The is useful when you temporarily wish to change the type
(for instance, by appending something to it), but you cannot create a new
scope.
\end{command}
\begin{command}{\pgfsys@pop@type}
Restores the most recently pushed type.
\end{command}
\begin{command}{\pgfsys@begin@idscope}
Starts a (graphics) scope whose sole purpose is to assign it an
id-type-pair so that it can be referenced later. Note that this command
does not always produce a graphics scope: If not id is currently in use or
if the id-type-pair has already been used, a graphic scope may or may not
be created as defined by the driver (but always a \TeX\ scope). This allows
drivers to minimize the number of graphic scopes created.
When an id scope is created, any code that has been ``attached'' to it
using |\pgfsys@attach@to@id| gets executed, see that command.
Note that |\pgfsys@beginscope| does not use the current id-type-pair. You
need to call this command to attach an id to a group.
\end{command}
\begin{command}{\pgfsys@end@idscope}
Ends the graphics id scope started by |\pgfsys@end@idscope|. It must nest
correctly with other graphic scopes and \TeX\ scopes.
\end{command}
\begin{command}{\pgfsys@attach@to@id\marg{id}\marg{type}\marg{begin code}\marg{end code}\marg{setup code}}
Attaches codes to the \meta{id}-\meta{type}-pair, where \meta{id} must have
been created using |\pgfsys@new@id|. The effect is that just before the id
scope for this pair is created, the \meta{setup code} is executed, then the
scope is started, then the \meta{begin code} is executed at the beginning,
and, finally, \meta{end code} gets executed just before the scope ends.
Multiple calls of this macro accumulated.
\end{command}
\subsection{Resource Description Framework Annotations (RDFa)}
\label{section-sys-rdf}
With certain output formats (in particular, with \textsc{svg}) you can insert
annotations into the output file following the standard set by the
\emph{resource description framework} (\textsc{rdf}), please consult the
literature on \textsc{rdf} for an introduction to resource descriptions and
ontologies.
The support for \textsc{rdf} annotations works as follows in \pgfname: You use
the following commands before you create an id scope (using
|\pgfsys@begin@idscope|). Then the attributes set by the commands will be added
as an annotation to that object. Here is an example:
%
\begin{codeexample}[code only]
\pgfsys@rdf@resource{/fruits/apple}
\pgfsys@begin@idscope
...
\pgfsys@end@idscope
\end{codeexample}
If \textsc{svg} output is produced, this results in the following code in the
\textsc{svg} file:
%
\begin{codeexample}[code only]
<g resource="/fruits/apple">
...
</g>
\end{codeexample}
Note that a call to |\pgfsys@begin@idscope| adds all the set attributes, but
then clears the settings (globally). Thus, you should set all attributes more
or less right before the id scope is created. For most of these command, if you
call them multiple times before starting the id scope, the ``last call wins'',
that is, later values overwrite earlier ones. In contrast, for the commands
|\pgfsys@rdf@property|, |\pgfsys@rdf@rel|, |\pgfsys@rdf@rev|, as well as
|\pgfsys@rdf@typeof|, the calls accumulate, that is, the texts passed in each
call will all be added to the output, properly separated to form a list of
values. Consider for instance:
%
\begin{codeexample}[code only]
\pgfsys@rdf@resource{/fruits/apple}
\pgfsys@rdf@resource{/fruits/watermelon}
\pgfsys@rdf@property{http://foo.com/props/juicy}
\pgfsys@rdf@property{http://foo.com/props/green}
\pgfsys@begin@idscope
...
\pgfsys@end@idscope
\end{codeexample}
In the resulting id scope, we will have:
%
\begin{codeexample}[code only]
<g resource="/fruits/watermelon"
property="http://foo.com/props/juicy http://foo.com/props/green">
...
</g>
\end{codeexample}
\begin{command}{\pgfsys@rdf@about\marg{text}}
Adds the \textsc{rdf} attribute |about="|\meta{text}|"| to the next id
scope (please see the \textsc{rdf}a specification for details on the
semantics of |about| in the context of the resource description framework).
\end{command}
The following commands work the same way as the above command, except that the
set attribute is different. Please see the \textsc{rdf}a specification for
details on these attributes. Note that the |\pgfsys@rdf@inlist| command is the
only one that takes no argument.
\begin{command}{\pgfsys@rdf@content\marg{text}}
\end{command}
\begin{command}{\pgfsys@rdf@datatype\marg{text}}
\end{command}
\begin{command}{\pgfsys@rdf@href\marg{text}}
\end{command}
\begin{command}{\pgfsys@rdf@inlist}
\end{command}
\begin{command}{\pgfsys@rdf@prefix\marg{text}}
\end{command}
\begin{command}{\pgfsys@rdf@property\marg{text}}
\end{command}
\begin{command}{\pgfsys@rdf@rel\marg{text}}
\end{command}
\begin{command}{\pgfsys@rdf@resource\marg{text}}
\end{command}
\begin{command}{\pgfsys@rdf@rev\marg{text}}
\end{command}
\begin{command}{\pgfsys@rdf@src\marg{text}}
\end{command}
\begin{command}{\pgfsys@rdf@typeof\marg{text}}
\end{command}
\begin{command}{\pgfsys@rdf@vocab\marg{text}}
\end{command}
\subsection{Reusable Objects System Commands}
\begin{command}{\pgfsys@invoke\marg{literals}}
This command gets protocolled 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 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}
\begin{command}{\pgfsys@marker@declare\marg{macro}\marg{code}}
Declares a \emph{marker} symbol for later use. The command is very similar
to |\pgfsys@defobject|, but the use case is slightly different: The graphic
object defined using the \meta{code} is stored in such a way that it can be
used as an \emph{arrow tip marker symbol} in animations. The \meta{macro}
is set to an identifier by which the marker can be referenced later on.
This command has a default implementation and need not be implemented by a
driver file.
\end{command}
\begin{command}{\pgfsys@marker@use\marg{macro}}
Adds the marker object referenced by the \meta{macro} to the current
output.
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 protocolled, 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 protocolled, see Section~\ref{section-protocols}.
\end{command}
\subsection{Page Size Commands}
The following commands can be used to set the page size of a document in a
``portable'' way. Note, however, that many packages also (try to) set the page
size.
These commands are typically not given inside a |{pgfpicture}|, but on the
outer level of compilation.
\begin{command}{\pgfsys@papersize\marg{width}\marg{height}}
Inserts the necessary |\special|s for the current driver into the output
stream to ``locally'' change the page size. Whether such a ``local'' change
is possible depends strongly on the driver. For instance, |dvips| will
honor the first call to this command that is part of the shipped-out
document and will ignore all other uses. In contrast, |pdftex| will use the
current value of the paper size for each page and, additionally, setting
the papersize is local to the current \TeX\ group.
\end{command}
\begin{command}{\pgfsys@global@papersize\marg{width}\marg{height}}
Like the previous command, only for drivers where setting the paper size
parameters is a \TeX-group-local operation, |\global| is prefixed to the
setting of the page sizes.
\end{command}
\begin{command}{\pgfsys@thepageheight}
This macro expands to the current page's height, provided \LaTeX\ is used,
otherwise a best guess is returned (currently just |\the\vsize|).
\end{command}
\begin{command}{\pgfsys@thepagewidth}
As above.
\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 an |.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
|\mymacro|. It will be 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 of 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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