raster support in graphics devices

Hi


baptiste auguie wrote:

Dear all,

It seems to me that grid.raster is a special case of grid.rect as far
as the intended visual output is concerned. The example below
illustrates how both can be used to produce an image of the volcano
data,

I disagree. ?A "rect" grob is a vector object and a "raster" grob is a
raster object and I think they should be kept distinct. ?You could possibly
create a higher-level "image" object that is agnostic with respect to how it
is implemented and have both "rect"-based and "raster"-based versions of
that, but "rect" and "raster" are graphical primitives and at that level I
think the distinction is useful.

Paul

d <- volcano

cols <- grey(t(d)/max(c(d)))
xy <- expand.grid(x=seq(0, 1, length=ncol(d)), y=seq(0, 1,
length=nrow(d)))

pdf("comparison.pdf", width=10, height=10/2*ncol(d)/nrow(d))
pushViewport(viewport(layout=grid.layout(1, 2)))

pushViewport(viewport(layout.pos.r=1, layout.pos.c=1))
grid.rect(xy$y, rev(xy$x), 1/(nrow(d)), 1/(ncol(d)), ?gp=gpar(col=NA,
fill=cols))
grid.text("grid.rect")
upViewport()

pushViewport(viewport(layout.pos.r=1, layout.pos.c=2))
cols.mat <- matrix(cols, ncol=ncol(d), byrow=T)
grid.raster(t(cols.mat))
grid.text("grid.raster")
dev.off()

Of course grid.raster provides a much better output in terms of file
size, speed, visualisation artifacts, and interpolation. My question
though: is it necessary to have a distinct grob for raster output?
Couldn't "raster" be an option in grid.rect when the width and height
are constant?

Alternatively, it might be useful to provide a function that converts
a rectGrob into a rasterGrob,

rect2raster <- function(g){

?with(g,
? ? ? rasterGrob(matrix(gp$fill, ncol=length(unique(x))),
mean(x),mean(y)))
}

This way, much of the existing code relying on grid.rect (e.g in
lattice or ggplot2) could easily be adapted to work with grid.raster
in favorable cases.

Best regards,

baptiste



2009/12/1 Paul Murrell <p.murrell at auckland.ac.nz>:

Hi

This is for developers of extension packages that provide extra *graphics
devices* for R.

In the *development* version of R, support has been added to the graphics
engine for sending raster images (bitmaps) to a graphics device. ?This
consists mainly of two new device functions: ?dev_Raster() and dev_Cap().

The R_GE_version constant (in GraphicsEngine.h) has been bumped up to 6
as a
marker of this change.

This means that, at a minimum, all graphics devices should be updated to
provide dummy implementations of these new functions that just say the
feature is not yet implemented (see for example the PicTeX and XFig
devices
in the 'grDevices' package).

A full implementation of dev_Raster() should be able to draw a raster
image
(provided as an array of 32-bit R colors) at any size, possibly
(bilinear)
interpolated (otherwise nearest-neighbour), at any orientation, and with
a
per-pixel alpha channel. ?Where these are not natively supported by a
device, the graphics engine provides some routines for scaling and
rotating
raster images (see for example the X11 device). ?The dev_Cap() function
should return a representation of a raster image captured from the
current
device. ?This will only make sense for some devices (see for example the
Cairo device in the 'grDevices' package).

A little more information and a couple of small examples are provided at
http://developer.r-project.org/Raster/raster-RFC.html

Paul
--
Dr Paul Murrell
Department of Statistics
The University of Auckland
Private Bag 92019
Auckland
New Zealand
64 9 3737599 x85392
paul at stat.auckland.ac.nz
http://www.stat.auckland.ac.nz/~paul/

--
Dr Paul Murrell
Department of Statistics
The University of Auckland
Private Bag 92019
Auckland
New Zealand
64 9 3737599 x85392
paul at stat.auckland.ac.nz
http://www.stat.auckland.ac.nz/~paul/