Thanks for the feedback. Two comments
- we have experimental code for all the options, so the work in
implementing them does not differ greatly. The long-term maintenance
costs of having different options is a real consideration, though.
R-devel currently has the options of B (default) and C, but is work in
progress.
- I estimate that letter-by-letter placement increases the file size for
text strings by a factor of 6, so this would be significant in a plot with
a lot of annotations, but not one in which points were labelled by single
letters.
On Sun, 12 Oct 2008, Duncan Murdoch wrote:
On 12/10/2008 11:36 AM, 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.
I think we can make this sort of change in 2.9.0.
2) Is kerning worth pursuing?
I think that is up to you and other people who might do the work; I don't
think I'll contribute to it.
3) If so, is elegant looking output more important than exact layout?
I suppose it matters how bad the exact layout looks, but I think your comment
above that exact layout will produce much larger files is of more concern.
We're sure to get complaints if "much larger" is noticeable. Other concerns
are whether text searches in .pdf or .ps files get confused by the
difference.
4) If we allow kerning, should it be the default (or only) option?
If we do it, I think we should make it the default. Whether it is optional
depends on how much work that would be (so it is mainly up to the
implementor).
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
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