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From Luca Furini <lfur...@cs.unibo.it>
Subject Re: [Bug 29124] - New line breaking algorithm
Date Thu, 12 Aug 2004 10:27:38 GMT

Simon Pepping wrote:

> > - word spacing and letter spacing are now fully implemented, they can
> > both have MinOptMax values; but I am still thinking about how to
> > differentiate a user-defined zero value from a default zero value ...
>
> You cannot. A default value is a user-defined value supplied by the
> system to save the user the trouble of always having to enter a
> value. It is just a convenience, and you cannot attach a different
> meaning to it.

You are right: default values must be respected no less than expressed
values, I asked the wrong question.

The point is that the XSL recommendation states that the default
word-spacing value is "normal", meaning "The normal inter-word space, as
defined by the current font and/or the UserAgent", not zero.
At the moment, the SpaceVal variable in the TextInfo object used by the TLM
has
     .getSpace().min == .getSpace().max == 0
even if the word-spacing property was not set in the fo document.

So, the right question is: how can the TLM see if the word-spacing property
value is "normal"?

> > - text-align-last is partially implemented; text-align-last = "justify"
> > works only if text-align = "justify" too; this is because Knuth's
> > algorithm doesn't  provide for a different alignment for the last line.
>
> TeX uses glue to achieve this, \parfillskip. It is the special amount
> of glue appended to the last line. In the TeXbook, p. 99, Knuth
> describes it as 'the special trick that allows the final line of a
> paragraph to be shorter than the others'. Setting \parfillskip to 0
> removes this ability. Usually \parfillskip has infinite
> stretchability.

I fear this trick works only with justified text.
Knuth's book suggests a way to implement right/left and center alignment
which is not just "justify text and then ignore the computed adjustment":
this different strategy involves using special sequences of elements
representing spaces.
For example, with left (or right) aligned text each space generates the
sequence:
    glue(width 0, stretch X, shrink 0)
    penalty(value 0)
    glue(width word-space-width, stretch -X, shrink 0)
If the line is not broken after the first glue element, the overall sretch
of these elements is 0: so, the total available stretch of each line is
always X, regardless of the number of spaces, while with the justified-text
strategy the more spaces are in a line, the more stretch that line will
have.
The computed adjustment ratio refers to this constant value, and it is
completely useless if we want to justify the last line. We could use the
computed difference to calculate the space adjustment, but we don't know how
many spaces there are in the line.
Setting \parfillskip to 0 does not avail, as it just forces the algorithm to
find lines all with the same width.

Maybe, with text-align = "left", "right" or "center" and text-align-last =
"justify" we should use the justified-text strategy with stretchable and
unshrinkable spaces.

Regards,
    Luca



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