[NTG-context] Minimum METAPOST code produces problematic PDF

[Gerben Wierda]

The minimum is a relative one:

It is page 650 in the (normative) Annex C Implementation Limits for PDF readers and writers:

Additionally, conforming writers should adhere to the following constraints, and conforming readers should accommodate PDF files that obey the constraints. 
The minimum page size should be 3 by 3 units in default user space; the maximum should be 14,400 by 14,400 units. In versions of PDF earlier than 1.6, the size of the default user space unit was fixed at 1⁄72 inch, yielding a minimum of approximately 0.04 by 0.04 inch and a maximum of 200 by 200 inches. Beginning with PDF 1.6, the size of the unit may be set on a page-by-page basis; the default remains at 1/ 72 inch. 

In other words: you need to change the unit per page so the page is at least 3x3 and maximally 14400x14400.


> On 9 Jul 2020, at 12:10, Hans Hagen <j.hagen at xs4all.nl> wrote:
> 
> On 7/9/2020 9:08 AM, Taco Hoekwater wrote:
>>> On 9 Jul 2020, at 08:32, Hans Hagen <j.hagen at xs4all.nl> wrote:
>>> 
>>> On 7/8/2020 8:14 PM, Gerben Wierda wrote:
>>>> I found out this in the PDF spec:
>>>> The minimum page size should be 3 by 3 units in default user space; the
>>> 
>>> I never noticed that ... what section/paragraph in the spec?
>> It’s in the implementation limits for Acrobat, not in the normative section.
> 
> Great. That used to be 1 bp so now it's more. As it happens, we do have a check for this in context so we can adapt it ... but, as this 3 bp is kind of arbitrary and because 3 bp becomes a float and thereby ends up slightly less than the integer 3 we can best play safe and revert to the usual tex joke, so i now set the minumum to
> 
>  \luaexpr{math.pi}\onebasepoint
> 
> which then gives:
> 
>  /MediaBox [ 0 0 3.141539539 3.141539539 ] because after all, any rediculous minimum will do and that way a user can still figure out that it's a value set by context.
> 
> Hans
> 
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