[poppler] Unit of the result from pageSize() and pageSizeF()

[Ross Moore]

Hi all,

On 25/11/2011, at 8:55 AM, Albert Astals Cid wrote:

>> Yes, I've read that, but I doesn't get it. Does it mean 1/72th points/inch
>> or what? They first said, the unit are points and the example tells
>> something from inch.
> 
> 1 point = 1/72th on an inch
> Thus an A4 has 595x842 points ~ 8,26x11,69inches ~ 21x29,7cm

Well, that is actually not true.
So if you are getting small errors, using  72 pts = 1 inch,
then try instead:  72.27pt = 1 inch.
This difference can easily be noticeable in large images.

In 1886 the point was defined to be  1/0.013837 in, which is
quite close to 1/72 in  but actually almost exactly 
   1/72.27 =  0.0138370001... . 

Furthermore, in 1959, the inch became exactly 2.54 cm  when 
previously it had been a little more than this 
  ( 1/0.3937 cm  =  2.540005... cm ).
But this change is virtually imperceptible in practice.



So far as PDF is concerned, the ISO 32000 Spec. says:

  §8.3.2.3   User Space

  ... 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.


 viz. "not exactly the same"  and  "no universal definition".


>> The coordinates yes. But what unit have the parameters xres and yres, which
>> are used to calculate the resolution and the size of the image.
> 
> It's dpi, thus if you use 72 you'll get an image of 595x842 pixels for an A4

However if you are laying out pieces measured in points on a grid 
with spacing in cm, then you may well find small gaps.

> 
> Albert
> 
>> 
>> But still thanks for your answer!
>> 
>> Regards,
>> Joachim


Hope this helps,

	Ross

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Ross Moore                                       ross.moore at mq.edu.au 
Mathematics Department                           office: E7A-419      
Macquarie University                             tel: +61 (0)2 9850 8955
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