[luatex] pre_output_filter: How to get the vertical list of the full page?

[Henri Menke]

On 05/03/2018 07:13 PM, Hans Hagen wrote:
> On 5/1/2018 11:50 PM, Henri Menke wrote:
>> On 02/05/18 03:17, Hans Hagen wrote:
>>> On 5/1/2018 12:16 PM, Henri Menke wrote:
>>>> Dear list,
>>>>
>>>> It seems as if the pre_output_filter only gives you the nodes in the
>>>> body block but without the \headline and \footline.  I would have
>>>> expected that those are included because \plainoutput performs
>>>>
>>>>     \shipout\vbox{\makeheadline\pagebody\makefootline}
>>>>
>>>> and I would have expected to get the vlist defined by the above \vbox
>>>> inside the callback.  Am I doing it wrong or am I using the wrong
>>>> callback?
>>> you can for instance use vpack_filter
>>
>> But how do I detect whether what is being packed is the main vertical
>> list?  According the manual `groupcode` should be <empty> for the main
>> vertical list but that just never happens.  With the following MWE
>>
>>      \directlua{
>>      callback.register("vpack_filter",
>>                        function(head,groupcode)
>>                            print('"' .. groupcode .. '"')
>>                            return head
>>                        end)
>>      }
>>
>>      Hello World!
>>
>>      \bye
> 
> build_page_filter

Sorry for being such a pain in the neck :(

How can I access the material to be pushed from within this callback?
Box 255 (the shipout box) is nil.  The reason I am asking this is
because I want to implement visual debugging like in ConTeXt.  Therefore
I want to walk the main vertical list on shipout and sprinkle it with
whatits to draw rules for boxes, glues, kerns, etc.

\directlua{
callback.register("buildpage_filter",
                  function() print(tex.box[255]) end)
}

Hello World!

\bye

> 
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>                                           Hans Hagen | PRAGMA ADE
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