[Rd] Strange behaviour of image (PR#510)
Prof Brian Ripley
Prof Brian Ripley <email@example.com>
Fri, 7 Apr 2000 17:51:47 +0100 (BST)
> Date: Fri, 7 Apr 2000 17:04:17 +0100 (BST)
> From: Jonathan Rougier <J.C.Rougier@durham.ac.uk>
> Hi Peter,
> On 6 Apr 2000, Peter Dalgaard BSA wrote:
> > J.C.Rougier@durham.ac.uk writes:
> > > The following commands
> > >
> > > z <- matrix(seq(0, 1, len=1000))
> > > image(z, 1, z, zlim=c(0, 1), col=gray(1:10/10))
> > >
> > > appear to violate the image functionality as described on the
> > > help page. As I understand it, by specifying a zlim of c(0, 1)
> > > and a colour vector of length 10, the colours should correspond
> > > to the intervals (0, 0.1), (0.1, 0.2), ..., (0.9, 1.0), this
> > > being 10 equally-spaced intervals on (0, 1). It is clear from
> > > the image that this has not occurred. In fact the intervals are
> > > apparently (0, 1/18), (1/18, 3/18), ..., (15/18, 17/18), (17/18,
> > > 1). Have I misunderstood something?
> > I don't see that anything else is being promised...
> > If someone actually thought about this, then the choice might be
> > related to the behavior at peaks and troughs. (since a circle with
> > radius r looks wide compared to the region between two circles with
> > diameter n*r and (n+1)*r)
> I'm not quite sure I follow. My feeling is that the displayed behaviour
> of image is unreasonable, and, I hazard, not what most people who use
> image expect. I think the default behaviour of image should be to give
> colour bands of equal widths, and, if I may append to the wish list, I
> woud like to be able to specify a "breaks" argument to have unequal widths
> should I so choose.
I side with Jonathan.
(1) The prototype does as JCR describes: given 64 colours it allocates
1/64th of the space to each.
(2) Setting the break points seems a good idea to me (but can always be
faked by a non-linear monotonic transformation of z, as I have done
I suggest we do add a `breaks' arg and that we do change the default
at the level of the R code, perhaps with an oldstyle parameter.
Brian D. Ripley, firstname.lastname@example.org
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272860 (secr)
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