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How does italic correction work?

  1. Where are the correction values looked up?
  2. Why is it not automatically included?
  3. What is the effect of multiple \/ or \/ in odd places such as between letters or after a space?

(There is a related question about italic correction as described in the TeXbook.)

Community
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Lover of Structure
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1 Answers1

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This is a property of the font. Every character of the font has four associated lengths specified in the tfm (font metric) file. Height, Width, Depth and Italic Correction.

  1. When you use \/ TeX extracts the dimension from the font metric file, scales it by any scaling that has been applied to the font and then inserts that much horizontal space.

  2. Often you need aesthetic judgement around punctuation as to whether it is needed. (But LaTeX \textrm and similar command do apply it automatically and get it mostly right.)

  3. Well the correction spacing is applied, and also any ligatures or kerns that would be between those letters are lost. (Originally I added most of the time here).

Stephen
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David Carlisle
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  • most of the time? When are ligatures not lost? – Martin Schröder Jan 15 '13 at 20:46
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    @MartinSchröder 20 years ago I'd have constructed an example by now, something in the back of my brain made me add that rider but I can't get it to fail, I thought it was if tex did a third emergencystretch hyphenation pass and the italic correction was zero it got lost and so theligature was re-constructed, but I don't have the conditions quite right in my head at present:-) – David Carlisle Jan 15 '13 at 20:55
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    @MartinSchröder my mind was playing tricks on me, the TeX2 Book suggested using {} to break ligatures, that does fail after hyphenation and \/ is a suggested fix TeX(3) book says In fact, the latter idea-to insert an italic correction-is preferable because TeX will reinsert the ff ligature by itself after hyphenating shelf{}ful (Appendix~H points out that ligatures are put into a hyphenated word that contains no explicit kerns,and an italic correction is an explicit kern.) But the italic correction may be too much (especially in anitalic font); shelf{\kern0pt}ful is often best. – David Carlisle Jan 15 '13 at 21:44
  • @DavidCarlisle If "shelf" and "ful" are on different lines, how would TeX reinsert/place an ff-ligature? (I'm not quite following this point.) – Lover of Structure Jan 16 '13 at 06:13
  • @DavidCarlisle By the way, applying multiple \/ (like \/\/\/\/) seems to have the same effect as applying just one \/. – Lover of Structure Jan 17 '13 at 11:15
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    Yes, \/ looks at the previous item (in the list being typeset) and if that is a character it looks up the italic correction, for a second \/ the previous item will be a kern from the first one, so later ones don't see a character and so do nothing. – David Carlisle Jan 17 '13 at 11:25
  • @DavidCarlisle Just checking with you about your phrase "TeX will reinsert the ff ligature by itself after hyphenating shelf{}ful", which I'm not quite following. – Lover of Structure Jan 19 '13 at 00:06
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    @LoverofStructure Don Knuth's phrase not mine that was a quote from the TeXBook, if you go shelf{}ful TeX initially puts two f into the horizontal list not an ff ligature but when considering hyphenation TeX takes apart ligatures (so it can look up the word in its hyphenation trie) and re-constitutes the ligatures if a hyphenation doesn't happen, but it "forgets" that the ligature was disabled and reconstitutes the ff ligature if shelfful is considered as a possible break point but not broken. – David Carlisle Jan 19 '13 at 00:12