[poppler] Supporting PDF 1.6 UserUnit

[Brad Hards]

On Sun, 10 Jul 2005 13:42 pm, Leonard Rosenthol wrote:
>          UserUnits do NOT effect the resolution of the image - they
> effect the scaling of the user space coordinates in order to enable
> larger then 32K pixels per dimension.
OK, so if I'm following this, then if the UserUnit is 2, then the userspace is 
36 dpi. 
If the cropbox is 3600 x 7200, then with  the default (72) it would be 50 x 
100 inches, but UserUnit 2 represents a "drawable" area of 100 inches x 200 
inches?

>          It is only currently generated by the Visio & AutoCAD
> PDFMakers when the output is larger than 32Kx32K.
OK, so it represents a user space that is nominally larger than 444 inches (10 
metres) each side. Probably we'll be drawing that scaled anyway...

> >However a quick scan of the poppler source tree suggests that this might
> > not be a fairly widespread change, so maybe we should get to it soon?
>
>          Depends on how you intend to support this.
>
>          What is your thinking about what it means to have a UserUnit
> present with respect to rendering?  Do you see it simply as a
> measurement?  Do you plan to use the value as a scale/transform
> ratio?  Do you plan to use that to effect the size of the output?
I'd really like some help about now :-)

From I see it changes the size of the cropbox, and the dimensions of 
everything within it. Now the change to the size of the cropbox in user space 
will that should be reflected in a change to the transformation to the device 
space (since presumably we'll be scaling).
So if an object would have been drawn 100 units x 50 units on a cropbox that 
is 150 units x 100 units, then setting UserUnit = 3 will produce an object 
that is 3 times bigger on a cropbox that is 3 times bigger. For any fixed 
transform from userspace to device space, the output will be 3 times bigger. 
If when we render it onto a physical sheet with a "fit to page" scaling, it 
will be the same size though.

So the only case where it matters is for things like engineering drawings, 
where if the original sheet is A1, then if we print at "100%" but don't take 
into account a UserUnit not equal to 1, then we can render (and print) the 
drawing at the wrong size.

Am I on the right track?

Brad
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