# $Id: xyz2img.q,v 1.4 2006-02-10 19:01:07 edzer Exp $
"xyz2img" <-
function (xyz, zcol = 3, xcol = 1, ycol = 2, tolerance = 10 * .Machine$double.eps)
{
if (ncol(xyz) < 3)
stop("xyz object should have at least three columns")
z = xyz[, zcol]
x = xyz[, xcol]
y = xyz[, ycol]
xx = sort(unique(x))
yy = sort(unique(y))
nx = length(xx)
ny = length(yy)
nmax = max(nx, ny)
difx = diff(xx)
if (diff(range(unique(difx))) > tolerance)
stop("x intervals are not constant")
dify = diff(yy)
if (diff(range(unique(dify))) > tolerance)
stop("y intervals are not constant")
dx = mean(difx)
dy = mean(dify)
xmin = min(xx)
xmax = max(xx)
xrange = xmax - xmin
ymin = min(yy)
ymax = max(yy)
yrange = ymax - ymin
row = round((x - xmin)/dx) + 1
col = round((y - ymin)/dy) + 1
zz = rep(as.numeric(NA), nx * ny)
zz[row + nx * (col - 1)] = z
zz = matrix(zz, nrow = nx, ncol = ny)
list(x = seq(xmin, xmax, dx), y = seq(ymin, ymax, dy), z = zz)
}
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