I've now implemented B and C in R-devel, with C as the default.
On Sun, 12 Oct 2008, Prof Brian Ripley wrote:
Ei-ji Nakama has pointed out (from another Japanese user, I believe) that
postscript() and pdf() have not been handling kerning correctly, and this is
a request for opinions about how we should correct it.
Kerning is the adjustment of the spacing between letters from their natural
width, so that for example 'Yo' is usually typeset with the o closer to the Y
than 'Yl' would be. Kerning is not very well standardized, so that for
example R's default Helvetica and its URW clone (Nimbus Sans) have quite
different ideas of the amount of kerning corrections for 'Yo'. This matters,
because not many people actually see Helvetica when viewing R's PostScript or
PDF output, but rather a similar face like Nimbus Sans or Arial, or in the
case of Acrobat Reader, a not very similar face. Kerning is only a feature
of some proportionally spaced fonts and so not of Courier nor CJK fonts.
The current position (R <= 2.8.0) is that string widths have been computing
using kerning from the Adobe Font Metric files for the nominal font, but the
strings have been displayed without using kerning (at least in the viewers we
are aware of, and the PostScript and PDF reference manuals mandate that
behaviour, if rather obscurely). This means that in strings such as 'You',
the width used in the string placement differs from that actually displayed.
For postscript(), this doesn't have much impact, as centring or right
justification ('hadj' in text()) is done by PostScript code and computes the
width from the actual font used (and so copes well with font substitution).
It might affect the fine layout in plotmath, but using strings which would be
kerned in annotations is not common.
For pdf() the effect is more commonly seen, as all text is set
left-justified, and the computed width is used to centre/right-justify.
There are several things we could do:
A. Do nothing, for back compatibility. After all, this has been going on
for years and no one has complained until last month.
B. Ignore kerning, and hence change the string width computations to match
the current display. This is more attractive than it appears at first sight
-- as far as I know all other devices ignore kerning, and we are increasingly
used to seeing 'typeset' output without kerning. It would be desirable when
copying graphs by e.g. dev.copy2eps from devices that do not kern.
C. Insert kerning corrections by splitting up strings, so e.g. 'You' is set
as (Y)-140 kc(ou): this is what TeX engines do.
D. Compute the position of each letter in the string and place them
individually.
C and D would give visually identical output when the font used is exactly as
specified, and hopefully also when a substitute font is using with the same
glyph widths (as substituting Nimbus Sans for Helvetica, at least for some
versions of each), but where the substitute is a poor match, C ought to look
more elegant but line up less well. D would produce much larger files than
C.
We do have the option of not changing the output when there is no kerning.
That would be by far the most common case except that some fonts (including
Helvetica but not Nimbus Sans) kern between punctuation and a space, e.g. ',
'. I'm inclined to believe that most uses of ',' in R graphical output are
not punctuation (certainly true of R's own examples), and also that we
nowadays do not expect to see kerning involving spaces.
Ei-ji Nakama provided an implementation of C for pdf() and D for postscript()
(thanks Ei-ji, and apologies that we did not have a chance to discuss the
principles first). I'm inclined to suggest that we should go forwards with
at most two of these alternatives, and those two should be the same for
postscript() and pdf() -- my own inclination is to B and C.
So questions:
1) Do people feel strongly that we should preserve graphical output from past
versions of R, even when there are known bugs? I can see the need to
reproduce published figures, but normally this would also need using the same
version of R.
2) Is kerning worth pursuing?
3) If so, is elegant looking output more important than exact layout?
4) If we allow kerning, should it be the default (or only) option?
To see that sometimes there can be a large effect, try in postscript() or
pdf()
xx <- 'You You You You You You You You'
plot(0,0,xlim=c(0,1),ylim=c(0,1),type='n')
abline(v=0)
text(0, 0.5, xx, adj=0)
abline(v=strwidth(xx))
x2 <- strsplit(xx, "")
w <- sapply(x2, strwidth)
abline(v=sum(w))
The leftmost of the right pair of lines is the computed width, the rightmost
the (normal) displayed width.
Unless there are cogent reasons to bring this forward to 2.8.1, any changes
would be as from 2.9.0.
Brian Ripley
--
Brian D. Ripley, [EMAIL PROTECTED]
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 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
--
Brian D. Ripley, [EMAIL PROTECTED]
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 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
______________________________________________
R-devel@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-devel
