# [luatex] behavior of buildpage_filter ?

Hans Hagen pragma at wxs.nl
Thu Mar 10 20:33:14 CET 2016

```On 3/10/2016 7:38 PM, Frank Mittelbach wrote:
> Hi there,
>
> the way I read the documentation on the buildpage_filter callback it is
> supposed to be called whenever LuaTeX is ready to move material to the
> main vertical list.
>
> My understanding was that this means the cases that are documented in
> the TeX book as "...exercises the pagebuilder"
>
> However, it seems to be that the callback is run far more often and in
> cases where it is definitely clear that there is no way whatsoever to
> move something to the main vertical list, e.g. it seems to be called
> whenever a box is packaged by tex.linebreak regardless of what happens
> with the output of the linebreaking
>
> Of course in such a case the recent contributions are empty so nothing
> much happens but the following simple input
>
>
> \input{ltluatex}
>
> \directlua{
> function buildpage ( context )
>    print ( "************* buildpage at: " .. context .. " at " ..
> tex.inputlineno)
>    return true
> end
> callback")
> }
>
> \input tufte

indeed, this is a known issue and it's on my list to look into that (one
can check for the location, at least that's what i do as a way out for
now); we probably need an additional callback but not for the next
release (as one might need to adapt code due to a break in compatibility)

Hans

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