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From Luca Furini <lfur...@cs.unibo.it>
Subject Re: letter-spacing
Date Wed, 01 Mar 2006 14:30:09 GMT
Jeremias Maerki wrote:

> Still trying to fix my problem with letter-spacing and fixed width
> spaces. Do I understand that correctly that XSL-FO's view of
> letter-spacing is different than, say, PDF's? PDF's character spacing 
> (PDF 1.4, 5.2.1) is designed so it advances the cursor for each (!)
> character by the Tc value.

Yes, I remember that when I was working on letter spacing it took me a 
while to understand what was wrong with the resulting pdf! :-)

> letter-spacing="1pt":
> 
> |_t__e__x__t_  _t__e__x__t_  _t__e__x__t_|

At the moment, fop has

   |t__e__x__t  t__e__x__t  t__e__x__t|

in other words there are letter spaces only between letters, and not 
between a letter and a space.

The recommendation states that "The algorithm for resolving the adjusted 
values between word spacing and letter spacing is User Agent dependent." 
(7.17.2 in the candidate recommendation), so I think this is not a wrong 
behaviour: it just assumes that word spaces have a higher precedence than 
letter spaces.

Another little difference: each letter space depends on the preceding 
letter size, instead of depending on both the preceding and following 
letters sizes; but this has some visible effect only when a word is 
composed of letters having different sizes.

> PDF's character spacing would work like this, I think (although the last
> character space needs to be eliminated by the layout manager [1]):
> 
> |t__e__x__t__  __t__e__x__t__  t__e__x__t|(__) <-- [1]

This is why the word spacing adjustment stored in the textAreas is not the 
computed one, but is specifically modified in order to counterbalance the 
2 letter spaces that the pdf will add.

> If I'm right here (not really sure, that's why I'm asking), it would
> mean that we should probably stop using the Tc feature from PDF and
> instead control the glyph positioning ourselves like we already do in
> PostScript.
> 
> WDYT?

As long as we have just two character categories (letter / spaces) the two 
pdf operators were enough.

Now, with fixed width spaces too, which should be unaffected by the both 
word spacing (such being different from spaces) and letter spacing 
(differing from normal letters), two operators are too few.

I don't think we need to set the horizontal positioning of each character 
or word, but just fix the placement of a character sequence following a 
fixed width space, removing the letter spaces wrongly added by the Tc 
operator, alternating character sequences and horizontal adjustments in 
the TJ array.

HTH

Regards
     Luca


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