\HeaderA{trellis.par.get}{Graphical Parameters for Trellis Displays}{trellis.par.get}
\aliasA{show.settings}{trellis.par.get}{show.settings}
\aliasA{trellis.par.set}{trellis.par.get}{trellis.par.set}
\keyword{dplot}{trellis.par.get}
\begin{Description}\relax
Functions used to query, display and modify graphical parameters for
fine control of Trellis displays.  Modifications are made to the
settings for the currently active device only.
\end{Description}
\begin{Usage}
\begin{verbatim}
trellis.par.set(name, value, ..., theme, warn = TRUE)
trellis.par.get(name = NULL)
show.settings(x = NULL)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{name}] character giving the name of a component. If unspecified,
\code{names(trellis.par.get())} returns a list containing all the
current settings (this can be used to get the valid values for
\code{name})

\item[\code{value}] a list giving the desired value of the component. Components that
are already defined as part of the current settings but are not
mentioned in \code{value} will remain unchanged.

\item[\code{theme}] a list decribing how to change the settings, similar to what is
returned by \code{trellis.par.get()}.  This is purely for
convenience, allowing multiple calls to \code{trellis.par.set} to be
condensed into one.  The name of each component must be a valid
\code{name} as described above, with the corresponding value a valid
\code{value} as described above.

As in \code{\LinkA{trellis.device}{trellis.device}}, \code{theme} can also be a
function that produces such a list when called.  The function name
can be supplied as a quoted string.

\item[\code{...}] Multiple settings can be specified in \code{name = value} form.
Equivalent to calling with \code{theme = list(...)}

\item[\code{warn}] logical, indicating whether a warning should be issued when
\code{trellis.par.get} is called when no graphics device is open

\item[\code{x}] optional list of components that change the settings (any valid
value of \code{theme}).  These are used to modify the current
settings (obtained by \code{trellis.par.get}) before they are
displayed.

\end{ldescription}
\end{Arguments}
\begin{Details}\relax
The various graphical parameters (color, line type, background etc)
that control the look and feel of Trellis displays are highly
customizable. Also, R can produce graphics on a number of devices, and
it is expected that a different set of parameters would be more suited
to different devices. These parameters are stored internally in a
variable named \code{lattice.theme}, which is a list whose components
define settings for particular devices. The components are idenified
by the name of the device they represent (as obtained by
\code{.Device}), and are created as and when new devices are opened
for the first time using \code{trellis.device} (or Lattice plots are
drawn on a device for the first time in that session).

The initial settings for each device defaults to values appropriate
for that device. In practice, this boils down to three distinct
settings, one for screen devices like \code{x11} and \code{windows},
one for black and white plots (mostly useful for \code{postscript})
and one for color printers (color \code{postcript}, \code{pdf}).

Once a device is open, its settings can be modified. When another
instance of the same device is opened later using
\code{trellis.device}, the settings for that device are reset to its
defaults, unless otherwise specified in the call to
\code{trellis.device}. But settings for different devices are treated
separately, i.e., opening a postscript device will not alter the x11
settings, which will remain in effect whenever an x11 device is
active.

The functions \code{trellis.par.*} are meant to be interfaces to the
global settings. They always apply on the settings for the currently
ACTIVE device.

\code{trellis.par.get}, called without any arguments, returns the full
list of settings for the active device. With the \code{name} argument
present, it returns that component only. \code{trellis.par.get} sets
the value of the \code{name} component of the current active device
settings to \code{value}.

\code{trellis.par.get} is usually used inside trellis functions to get
graphical parameters before plotting. Modifications by users via
\code{trellis.par.set} is traditionally done as follows:

\code{add.line <- trellis.par.get("add.line")}

\code{add.line\$col <- "red"}

\code{trellis.par.set("add.line", add.line)}

More convenient (but not S compatible) ways to do this are

\code{trellis.par.set(list(add.line = list(col = "red")))}

and

\code{trellis.par.set(add.line = list(col = "red"))}


The actual list of the components in \code{trellis.settings} has not
been finalized, so I'm not attempting to list them here. The current
value can be obtained by \code{print(trellis.par.get())}. Most names
should be self-explanatory.

\code{show.settings} provides a graphical display summarizing some of
the values in the current settings.
\end{Details}
\begin{Value}
\code{trellis.par.get} returns a list giving parameters for that
component. If \code{name} is missing, it returns the full list.
\end{Value}
\begin{Note}\relax
In some ways, \code{trellis.par.get} and \code{trellis.par.set}
together are a replacement for the \code{\LinkA{par}{par}} function used in
traditional R graphics.  In particular, changing \code{par} settings
has little (if any) effect on lattice output.  Since lattice plots are
implemented using Grid graphics, its parameter system \emph{does} have
an effect unless overridden by a suitable lattice parameter setting.
Such parameters can be specified as part of a lattice theme by
including them in the \code{grid.pars} component (see
\code{\LinkA{gpar}{gpar}} for a list of valid parameter names).


One of the uses of \code{par} is to set \code{par(ask = TRUE)} making
R wait for user input before starting a new graphics page.  For Grid
graphics, this is done using
\code{\LinkA{grid.prompt}{grid.prompt}}.  Lattice has no separate
interface for this, and the user must call \code{grid.prompt}
directly.  If the grid package is not attached (lattice itself
only loads the grid namespace), this may be done using
\code{grid::grid.prompt(TRUE)}.
\end{Note}
\begin{Author}\relax
Deepayan Sarkar \email{Deepayan.Sarkar@R-project.org}
\end{Author}
\begin{SeeAlso}\relax
\code{\LinkA{trellis.device}{trellis.device}}, \code{\LinkA{Lattice}{Lattice}},
\code{\LinkA{grid.prompt}{grid.prompt}},
\code{\LinkA{gpar}{gpar}}
\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
show.settings()
\end{ExampleCode}
\end{Examples}

