[texhax] LuaLaTeX: increase font expansion in narrow columns

[Arno Trautmann]

Paul Isambert wrote:
> Alleged shepherd Philip TAYLOR wrote:
>> "A man who would letter-space lower-case text
>> would probably steal sheep" : F W Goudy.
>
> Stealing sheep is better than starving to death. See attached file.

Do you also have an example (font expansion) vs. (font expansion + 
stolen sheep)? I find it hard to judge what the effect of sheep is in 
that example.

Sticking to font expansion, I wondered if the following could work in 
theory:

• as changing the expansion is not possible, change the font in narrow 
lines, i.e. load the font twice with different expansions.
• in post_linebreak_filter, collect all subsequent narrow lines
• pack them back into one hlist, change the font to the one with larger 
expansion, break this hlist into a paragraph with the same number of 
lines as before.
• put that list of hlists back into the vertikal list.

I don't have to mention that I am not able to even try to translate this 
into code, do I? …

Also, related to this question: Is there any technical documentation on 
how pdfTeX (or, if it differs in that case, LuaTeX) breaks lines into 
paragraphs using font expansion and character protrusion? I mean 
something like the corresponding chapter in the TeXbook – I'd like to 
understand at which point and how font expansion is taken into account 
in the line breaking.

cheers
Arno