[gdal-dev] Confusion regarding GeoPDF composition.xml units

[Nyall Dawson]

On Tue, 22 Sep 2020 at 16:36, Andrea Giudiceandrea
<andreaerdna at libero.it> wrote:
>
> Nyall Dawson wrote
> > Actually it's a bit more complex than this, because regardless of the
> > DPI specified for the page, the width/height have to be specified in
> > size in inches * 72! (i.e. there's a hardcoded 72 dpi assumption
> > somewhere).
>
> Hi,
> I don't know how composition.xml works, but if you wonder why in PDF page
> the width/height have to be specified in "size in inches * 72" you will find
> a hint in PDF 32000-1:2008 [1] especially at paragraphs 7.7.3.3 Page
> Objects, Table 30 and 8.3 Coordinate Systems.
>
> "The length of a unit along both the x and y axes is set by the UserUnit
> entry (PDF 1.6) in the page dictionary (see Table 30). If that entry is not
> present or supported, the default value of 1⁄72 inch is used. This
> coordinate system is called default user space"
>
> "Note 2: The default for the size of the unit in default user space (1⁄72
> inch) is approximately the same as a point, a unit widely used in the
> printing industry. It is not exactly the same, however; there is no
> universal definition of a point."
>
> "UserUnit: A positive number that shall give the size of default user space
> units, in multiples of 1⁄72 inch. The range of supported values shall be
> implementation-dependent. Default value: 1.0 (user space unit is 1⁄72
> inch)."
>
> See also [2] and [3].

Thanks Andrea!

So then my question would be why the PDFs attached to
https://github.com/qgis/QGIS/issues/33465 are showing incorrect page
sizes in Acrobat Reader, when it appears as though GDAL is correctly
generating them!

Have you any ideas why this could be occurring?

Nyall


>
> Hope this will help.
>
> Best regards.
>
> Andrea Giudiceandrea
>
>
> [1]
> https://www.adobe.com/content/dam/acom/en/devnet/acrobat/pdfs/PDF32000_2008.pdf
> [2] https://stackoverflow.com/a/21164534/8779560
> [3] https://github.com/qgis/QGIS/issues/33465#issuecomment-644566633
>
>
>
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