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

From: Prof Brian Ripley <>
Date: Thu, 16 Oct 2008 10:03:11 +0100 (BST)

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,
> Professor of Applied Statistics,
> 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,        
Professor of Applied Statistics,
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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