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*To*: "Walter Schmidt" <walter.schmidt@arcormail.de>*Subject*: Re: track kerning*From*: Lars Hellström <Lars.Hellstrom@math.umu.se>*Date*: Tue, 6 Jun 2000 21:48:10 +0200*Cc*: "tex-fonts" <tex-fonts@math.utah.edu>*In-Reply-To*: <393d345c393db9eb@mail.arcor-ip.de> (added bymail.arcor-ip.de)

At 19.26 +0200 0-06-06, Walter Schmidt wrote: >Hi, > >apologizing that my question is a bit off-topic -- it is not >special for TeX. > >I found the following piece of information in an afm file: > >StartTrackKern 3 >TrackKern -1 6 0.10 144 -2.09 >TrackKern -2 6 0.05 144 -4.02 >TrackKern -3 6 0.00 144 -5.96 > >I know that this means, that letterspacing should vary with >font size. However, what does this mean _quantitatively_? >How do I have to read this information? On p. 27-28, Adobe technical note 5004 (AFM specification) says: Normally track kerning is provided in different degrees of tightness. Within a track (a degree of tightness), the amount to decrease (or possibly increase) the amount of space between characters increases (or possibly decreases) with the point size of the font (for example, for tight track kerning, the amount to decrease the space between characters at 6 point might be 0.1 points and at 72 point it might be 3.78 points). These distances are measured along the width of the characters: parallel to the x-axis for horizontal writing directions and parallel to the y-axis for vertical writing directions. The data itself begins with the keyword TrackKern and is followed by the track kerning information TrackKern degree min-ptsize min-kern max-ptsize max-kern The degree is an integer where increasingly negative degrees represent tighter track kerning and increasingly positive degrees represent looser track kerning. min-ptsize, min-kern, max-ptsize, and max-kern are all numbers. Since the track kerning is a linear function, the minimum and maximum cut-off values (point sizes) are provided, along with the amount to track kern by, at the point size. The kerning amounts are given relative to the point size. From those four values, the track kerning function can be derived. The track kerning function is a linear function. The equation for the line is determined from the data provided and, therefore, the track kerning values for any point size can be determined. The track kerning values for any point size below/above the minimum/maximum point size are constant (the minimum kerning amount/maximum kerning amount). I must say it looks a bit confusing, though, since what follows appears to claim that min-kern and max-kern are in absolute points. They do give formulae for how the track kerning varies with at-size, but the asymptotic behaviour of those formulae gets absurd. (Maybe a case of programmer not thinking too hard of what the formula implemented really means.) Lars Hellström

**References**:**track kerning***From:*"Walter Schmidt" <walter.schmidt@arcormail.de>

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