[R] graphics::rasterImage and grid::grid.raster, doubly upside down images

[Abby Spurdle]

Dear list,

I've been writing R-based software for image and spatial data. And
I've decided to support a dual implementation with both base graphics
and grid graphics, and the possibility of other graphics systems
later.

Also, I've decided to plot raster images with the vertical axis
flipped. Which means that each image also needs to be flipped.
(In rasterImage, ybottom/ytop are swapped, and in grid.raster, height
is negative).

This appears to work fine.
However, the help files don't explicitly say that height/etc can be
negative. And then go on to warn users about possible problems.

Given that both functions support scaling, I'm assuming that negative
scaling should be fine. But I thought I'd run it through the list, for
feedback/suggestions on any possible problems that one is likely to
encounter plotting raster images like this.

Examples follow:
(Requires the png package).

#required packages
library (png)
library (grid)

#base graphics wrapper function
base.wrapper <- function (x, y, colm, interpolate)
{   rasterImage (colm, x [1], y [2], x [2], y [1],
        interpolate=interpolate)
}

#grid graphics wrapper function
grid.wrapper <- function (x, y, colm, interpolate)
{   grid.raster (colm, mean (x), mean (y), x [2] - x [1], y [1] - y [2],
        default.units="native", interpolate=interpolate)
}

#create raster object
im <- readPNG (system.file ("img", "Rlogo.png", package="png") )
im <- as.raster (im)

#base graphics example
plot.new ()
plot.window (0:1, 1:0)
base.wrapper (0:1, 0:1, im, FALSE)
axis (2)

#grid graphics example
grid.newpage ()
pushViewport (viewport (xscale=0:1, yscale=1:0, width=0.75, height=0.75) )
grid.wrapper (0:1, 0:1, im, FALSE)
grid.yaxis ()