[luatex] Interfere with page breaking to balance pages

[Patrick Bergner]

Thank you all for your comments, advises and explanations.

The tex.box.AtBeginShipoutBox approach is nice, although I'm a bit
ashamed I did not come up with it myself. The shipout cancellation also
works (the back slash had to be escaped though).

For the record, my \AtBeginShipout hook now reads something like

[code]
balancepages = function( page )
  if page < 2 then return end
  if page % 2 == 0 then
    tex.setbox( 'global', 142, node.copy_list( tex.box.AtBeginShipoutBox ) )
    tex.print( '\\AtBeginShipoutDiscard' )
  else
    tex.setbox( 'global', 143, node.copy_list( tex.box.AtBeginShipoutBox ) )
    tex.print( '\\AtBeginShipoutDiscard' )
    -- page balancing to be done here
    tex.shipout( 142 )
    tex.shipout( 143 )
  end
end
[/code]

Now let's see how far I get with the actual page balancing.

______________________

On general page breaking callbacks: I'm with Paul in that something like
a pre_shipout_filter with the possibility to cancel/delay shipout would
be desirable. Basically what pre_linebreak_filter does on paragraph level.

Patrick


Am 18.10.2012 10:11, schrieb Taco Hoekwater:
> On the original problem: TeX's page breaking is more than a little
> tricky to get 'into' from the Lua end because there are a bunch
> of interwoven C functions that mutually depend on each other's
> calculations. In fact, that is why the current interface is so
> weird: it closely follows what the internals do.
> 
> When we exposed the current situation, it was not at all
> clear what a 'good' interface would look like (and personally, I
> am still not sure). So, any constructive thoughts on that would
> be welcome.
> 
> A better, cleaner solution would be nice to have, but whatever
> form it takes, it will require some digging and a bit of a rewrite in
> the internal code. And to make sure at the same time that something
> important does not break is tricky, which is also part of why it has
> not been done yet.
> 
> Best wishes,
> Taco