[Rd] RFC: Kerning, postscript() and pdf()

From: Prof Brian Ripley <ripley_at_stats.ox.ac.uk>
Date: Sun, 12 Oct 2008 16:36:56 +0100 (BST)

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:

  1. Do nothing, for back compatibility. After all, this has been going on for years and no one has complained until last month.
  2. 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.
  3. Insert kerning corrections by splitting up strings, so e.g. 'You' is set as (Y)-140 kc(ou): this is what TeX engines do.
  4. 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)
x2 <- strsplit(xx, "")
w <- sapply(x2, strwidth)

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,                  ripley_at_stats.ox.ac.uk
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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