https://github.com/cran/fields
Tip revision: 04279c16ce718b82025aae97f6fcd30ba3a8b1c5 authored by Doug Nychka on 05 September 2011, 20:18:33 UTC
version 6.6
version 6.6
Tip revision: 04279c1
image.plot.Rd
% fields, Tools for spatial data
% Copyright 2004-2011, Institute for Mathematics Applied Geosciences
% University Corporation for Atmospheric Research
% Licensed under the GPL -- www.gpl.org/licenses/gpl.html
\name{image.plot}
\alias{image.plot}
\title{
Draws image plot with a legend strip for the color scale based on either
a regular grid or a grid of quadrilaterals.
}
\description{
This function combines the R image function with some automatic
placement of a legend. This is done by splitting the plotting region
into two parts. Putting the image in one and the legend in the other.
It also allows for plotting quadrilateral cells in the image format that
often arise from regular grids transformed with a map projection.
}
\usage{
image.plot(...,
add = FALSE, nlevel = 64, horizontal = FALSE,
legend.shrink = 0.9, legend.width = 1.2, legend.mar =
ifelse(horizontal, 3.1, 5.1), legend.lab = NULL,
graphics.reset = FALSE, bigplot = NULL, smallplot =
NULL, legend.only = FALSE, col = tim.colors(nlevel),
lab.breaks = NULL, axis.args = NULL, legend.args =
NULL, midpoint=FALSE, border=NA, lwd=1.0)}
\arguments{ \item{\dots}{
The usual arguments to the \code{image} function as x,y,or z or as a list with x,y,z as components. One can also include a
\code{breaks} argument for an unequal color scale with color scale
boundaries at the breaks (see example below). If a quadrilateral grid,
arguments must be explicitly x,y and z with x, and y being matrices of
dimension equal or one more than z giving the grid locations. The basic
concept is that the coordinates of x and y still define a grid but the image
cells are general quadrilaterals rather than being restricted to rectangles.
See details below as to how one handles whether the quads are specified by their
vertices or by their midpoints.
}
\item{add}{
If true add image and a legend strip to the existing plot.
}
\item{nlevel}{
Number of color levels used in legend strip
}
\item{legend.shrink}{
Amount to shrink the size of legend relative to the full height or width
of the plot.
}
\item{legend.width}{
Width in characters of the legend strip. Default is 1.2, a little bigger
that the width of a character. }
\item{legend.mar}{
Width in characters of legend margin that has the axis. Default is 5.1
for a vertical legend and 3.1 for a horizontal legend.}
\item{legend.lab}{ Label for the axis of the color legend. Default is no
label as this is usual evident from the plot title.}
\item{graphics.reset}{
If FALSE (default) the plotting region ( plt in par) will not be reset
and one can add more information onto the image plot. (e.g. using functions
such as points or lines.) If TRUE will reset plot parameters to the
values before entering the function.
}
\item{horizontal}{
If false (default) legend will be a vertical strip on the right side. If
true the legend strip will be along the bottom.
}
\item{bigplot}{
Plot coordinates for image plot. If not passed
these will
be determined within the function.
}
\item{smallplot }{
Plot coordinates for legend. If not passed these will be determined within
the function.
}
\item{legend.only}{
If TRUE just add the
legend to a the plot in the plot region defined by the coordinates in
smallplot. In the absence of other information the range for the legend
is determined from the \code{zlim} argument.
}
\item{col}{
Color table to use for image ( see help file on image for details).
Default is a pleasing range of 64 divisions suggested by Tim Hoar and is similar to
the MATLAB (TM) jet color scheme.
}
\item{lab.breaks}{ If breaks are supplied these are text string labels
to put at each break value. This is intended to label axis on a
transformed scale such as logs.}
\item{axis.args}{Additional arguments for the axis function used to
create the legend axis. (See example below adding a log scaling.)}
\item{legend.args}{Arguments for a complete specification of the
legend label. This is in the form of list and is just passed to the
mtext function. Usually this will not be needed.
(See example below.)}
\item{midpoint}{
This option for the case of unequally spaced grids with x and y being
matrices of grid point locations.
If FALSE (default) the quadralaterals will be extended
to surround the z locations as midpoints. If TRUE z values will be averaged
to yield a midpoint value and the original grid points be used to define the
quadralaterals. (See help on poly.image for details). In most cases
midpoint should be FALSE to preserve exact values for z and
let the grid polygons be modified.
}
\item{border}{This only works if x and y are matrices -- if NA the quadralaterals will have no borders. Otherwise this specifies the color to use.}
\item{lwd}{Line width of borders around pixels if border is not NA.}
}
\section{Side Effects}{
After exiting, the
plotting region may be changed to make it possible to add more features to
the plot. To be explicit, \code{par()\$plt} may be changed to reflect a
smaller plotting region that has accommodated room for the legend subplot.
}
\details{
Relationship of x, y and z:
If the z component is a matrix then the user should be aware that
this function locates the matrix element z[i,j] at the grid locations
(x[i], y[j]) this is very different than simply listing out the
matrix in the usual row column tabular form. See the example below
for more details of this difference in formatting. What does one do
if you don't really have the "z" values on a regular grid? See the
functions \code{quilt.plot.Rd} and \code{as.image} to discretise
irregular observations to a grid.
If x and y are matrices then
z[i,j] is rendered at a quadrilateral that is centered at x[i,j] and
y[i,j] (\code{midpoint} TRUE). The details of how this cell is found
are buried in \code{poly.image}. If \code{midpoint} is FALSE then x
and y are interpreted as the corners of the quadrilateral cells. But
what about z? The four values of z are now averaged to represent a
value at the midpoint of the cell and this is what is used for
plotting. Quadrilateral grids was added to help with plotting
the gridded output of geophysical models where the regular grid is
defined according to one map projection by the plotting is required
in another projection. Typically the regular grid becomes distorted in
a smooth way when this happens. See the regional climate example for
a illustration of this application. One can add border colors in this case
easily because these choices are jsut passed onto the polygon function.
For adding the grid of pixel borders to a rectangular image try this example
\code{
dx<- x[2] - x[1]
dy <- y[2]-y[1]
xtemp<- seq( min( x)- dx/2, max(x)+ dx/2,, length(x) +1)
ytemp<- seq( min( y)- dy/2, max(y)+ dy/2,, length(y) +1)
xline( xtemp, col="grey50", lwd=2); yline( ytemp, col="grey50", lwd=2)
}
where \code{x} and \code{y} here are the x and y grid values from the image list.
Fine tuning color scales: This function gives some flexibility in
tuning the color scale to fit the rendering of z values. This can
either be specially designed color scale with specific colors ( see
help on \code{designer.colors}), positioning the colors at specific
points on the [0,1] scale, or mapping distinct colors to intervals of
z. The examples below show how to do each of these. In addition, by
supplying \code{lab.break} strings or axis parameters one can
annotate the legend axis in an informative matter.
Dividing up the plotting real estate:
It is surprising how hard it is to automatically add the
legend! All "plotting coordinates" mentioned here are in device
coordinates. The plot region is assumed to be [0,1]X[0,1] and plotting
regions are defined as rectangles within this square. We found these
easier to work with than user coordinates.
\code{legend.width} and \code{legend.mar} are in units of character
spaces. These units are helpful in thinking about axis labels that
will be put into these areas. To add more or less space between the
legend and the image plot alter the mar parameters. The default mar
settings (5.1,5.1,5.1,2.1) leaves 2.1 spaces for vertical legends and
5.1 spaces for horizontal legends. Changing the plot margins directly
replaces the \code{offset} argument in the older version of this function.
There are always problems with
default solutions to placing information on graphs but the choices made
here may be useful for most cases. The most annoying thing is that after
using plot.image and adding information the next plot that is made may
have the slightly smaller plotting region set by the image plotting.
The user should set \code{reset.graphics=TRUE} to avoid the plotting size
from changing. The disadvantage, however, of resetting the graphics
is that one can no longer add additional graphics elements to the image
plot. Note that filled.contour always resets the graphics but provides
another mechanism to pass through plotting commands. Apparently
filled.contour, while very pretty, does not work for multiple plots.
\code{levelplot} that is part of the lattice package has a very
similar function to image.plot and a formula syntax in the call.
How this function works:
The strategy for \code{image.plot} is simple, divide the plotting region
into two smaller regions \code{bigplot} and \code{smallplot}. The image
goes in one and the legend in the other. This way there is always room for
the legend. Some adjustments are made to this rule by not shrinking the
\code{bigplot} if there is already room for the legend strip and also
sticking the legend strip close to the image plot. One can specify the
plot regions explicitly by \code{bigplot} and \ \code{smallplot} if the
default choices do not work. There may be problems with small plotting
regions in fitting both of these elements in the plot region and one may
have to change the default character sizes or margins to make things fit.
By keeping the zlim argument the same across images one can generate the
same color scale. (See the image help file.) One useful technique for a
panel of images is to just draw the images with \code{image}
and then use image.plot to add a legend to the last plot. (See example
below for messing with the outer margins to make this work.)
Usually a square plot (\code{pty="s"}) done in a rectangular plot region will
have room for the legend stuck to the right side without any other
adjustments. See the examples below and the code for \code{plot.Wimage}
for more complicated arrangements of multiple image plots and summary
legends.
Adding just the legend strip:
Note that to add just the legend strip all the numerical information one
needs is the \code{zlim} argument! We like \code{tim.colors} as a default
color scale. The the topographic color scale (\code{topo.colors}) is
also a close second showing our geophysical basis. See also
\code{terrain.colors} for a subset and \code{designer.colors} to "roll
your own". One nice option of this last one is to fix colors at
particular quantiles of the data rather than at equally spaced
intervals. For color choices see how the \code{nlevels} argument figures
into the legend and main plot number of colors.
}
\seealso{ image, poly.image,filled.contour, quilt.plot, plot.surface,
add.image, colorbar.plot, tim.colors, designer.colors }
\examples{
x<- 1:10; y<- 1:15; z<- outer( x,y,"+")
image.plot(x,y,z)
# or obj<- list( x=x,y=y,z=z); image.plot(obj)
# now add some points on diagonal with some clipping anticipated
points( 5:12, 5:12, pch="X", cex=3)
image.plot(x,y,z, legend.lab="inches")
# adding breaks and distinct colors for intervals of z
# with and without lab.breaks
brk<- quantile( c(z))
image.plot(x,y,z, breaks=brk, col=rainbow(4))
# annotate legend strip just at break values
image.plot(x,y,z, breaks=brk, col=rainbow(4),
lab.breaks=names(brk))
#
# compare to
quantile(c(z), c( .05, .1,.5, .9,.95))-> zp
image.plot(x,y,z,
axis.args=list( at=zp, labels=names(zp) ) )
# a log scaling for the colors
ticks<- c( 1, 2,4,8,16,32)
image.plot(x,y,log(z), axis.args=list( at=log(ticks), labels=ticks))
# see help file for designer.colors to generate a color scale that adapts to
# quantiles of z.
#
#fat (5 characters wide) and short (50\% of figure) color bar on the bottom
image.plot( x,y,z,legend.width=5, legend.shrink=.5, horizontal=TRUE)
# adding label with all kinds of additional arguments.
# use side=4 for vertical legend and side= 1 for horizontal legend
# to be parallel to axes. See help(mtext).
image.plot(x,y,z,
legend.args=list( text="unknown units",
col="magenta", cex=1.5, side=4, line=2))
#### example using a irregular quadrilateral grid
data( RCMexample)
image.plot( RCMexample$x, RCMexample$y, RCMexample$z[,,1])
ind<- 50:75 # make a smaller image to show bordering lines
image.plot( RCMexample$x[ind,ind], RCMexample$y[ind,ind], RCMexample$z[ind,ind,1],
border="grey50", lwd=2)
#### multiple images with a common legend
set.panel()
# Here is quick but quirky way to add a common legend to several plots.
# The idea is leave some room in the margin and then over plot in this margin
par(oma=c( 0,0,0,4)) # margin of 4 spaces width at right hand side
set.panel( 2,2) # 2X2 matrix of plots
# now draw all your plots using usual image command
for ( k in 1:4){
image( matrix( rnorm(150), 10,15), zlim=c(-4,4), col=tim.colors())
}
par(oma=c( 0,0,0,1))# reset margin to be much smaller.
image.plot( legend.only=TRUE, zlim=c(-4,4))
# image.plot tricked into plotting in margin of old setting
set.panel() # reset plotting device
#
# Here is a more learned strategy to add a common legend to a panel of
# plots consult the split.screen help file for more explanations.
# For this example we draw two
# images top and bottom and add a single legend color bar on the right side
# first divide screen into the figure region (left) and legend region (right)
split.screen( rbind(c(0, .8,0,1), c(.8,1,0,1)))
# now subdivide up the figure region into two parts
split.screen(c(2,1), screen=1)-> ind
zr<- range( 2,35)
# first image
screen( ind[1])
image( x,y,z, col=tim.colors(), zlim=zr)
# second image
screen( ind[2])
image( x,y,z+10, col=tim.colors(), zlim =zr)
# move to skinny region on right and draw the legend strip
screen( 2)
image.plot( zlim=zr,legend.only=TRUE, smallplot=c(.1,.2, .3,.7),
col=tim.colors())
close.screen( all=TRUE)
# you can always add a legend arbitrarily to any plot;
# note that here the plot is too big for the vertical strip but the
# horizontal fits nicely.
plot( 1:10, 1:10)
image.plot( zlim=c(0,25), legend.only=TRUE)
image.plot( zlim=c(0,25), legend.only=TRUE, horizontal =TRUE)
# combining the usual image function and adding a legend
# first change margin for some more room
\dontrun{
par( mar=c(10,5,5,5))
image( x,y,z, col=topo.colors(64))
image.plot( zlim=c(0,25), nlevel=64,legend.only=TRUE, horizontal=TRUE,
col=topo.colors(64))
}
#
#
# sorting out the difference in formatting between matrix storage
# and the image plot depiction
A<- matrix( 1:48, ncol=6)
# Note that matrix(c(A), ncol=6) == A
image.plot(1:8, 1:6, A)
# add labels to each box
text( c( row(A)), c( col(A)), A)
# and the indices ...
text( c( row(A)), c( col(A))-.25,
paste( "(", c(row(A)), ",",c(col(A)),")", sep=""), col="grey")
# "columns" of A are horizontal and rows are ordered from bottom to top!
#
# matrix in its usual tabular form where the rows are y and columns are x
image.plot( t( A[6:1,]), axes=FALSE)
}
\keyword{hplot}
% docclass is function