Writer's A4 page pixel dimensions

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Dave Koelmeyer Dave Koelmeyer
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Writer's A4 page pixel dimensions

Hi All,

I'm creating full A4-sized page background images in GIMP to insert into
A4-format Writer documents. I've noticed that when these images are
added to Writer and anchored to page, things are not perfectly flush as
I would expect.

After a bit of tinkering I believe this is because Writer doesn't treat
an A4-format page as having exactly 2480x3508 pixel dimensions (at 300 DPI).

For example, my GIMP working files use the A4 300 DPI preset, which is
2480x3508 pixels. If I create a new blank Writer document using A4 page
size, then export this blank document as a PNG and set the export DPI
setting to 300, Writer instead displays this as 2481x3507 pixels. I see
similar ever-so-slightly different results when attempting to import an
A4 PDF exported from Writer into GIMP at 300 DPI, again off by a matter
of pixels.

Everything I've read points to 2480x3508 definitely being the dimensions
for A4 (e.g. http://www.a4papersize.org/a4-paper-size-in-pixels.php).
Can someone please help me understand why there is a difference in Writer?

Cheers,
Dave

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Dave Koelmeyer
http://blog.davekoelmeyer.co.nz
GPG Key ID: 0x238BFF87


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Re: Writer's A4 page pixel dimensions

Dave Koelmeyer wrote:

> Hi All,
>
> I'm creating full A4-sized page background images in GIMP to insert into
> A4-format Writer documents. I've noticed that when these images are
> added to Writer and anchored to page, things are not perfectly flush as
> I would expect.
>
> After a bit of tinkering I believe this is because Writer doesn't treat
> an A4-format page as having exactly 2480x3508 pixel dimensions (at 300 DPI).
>
> For example, my GIMP working files use the A4 300 DPI preset, which is
> 2480x3508 pixels. If I create a new blank Writer document using A4 page
> size, then export this blank document as a PNG and set the export DPI
> setting to 300, Writer instead displays this as 2481x3507 pixels. I see
> similar ever-so-slightly different results when attempting to import an
> A4 PDF exported from Writer into GIMP at 300 DPI, again off by a matter
> of pixels.
>
> Everything I've read points to 2480x3508 definitely being the dimensions
> for A4 (e.g. http://www.a4papersize.org/a4-paper-size-in-pixels.php).
> Can someone please help me understand why there is a difference in Writer?

Probably differences in rounding. An A4 page isn't exactly 2480 x 3508
pixels at 300 dpi. A4 is 210mm x 297mm.
   210mm / 25.4mm/in * 300dpi ~= 2480.3 pixels
   297mm / 25.4mm/in * 300dpi ~= 3507.9 pixels
Rounding that to 2481 x 3507 does seem a bit odd, but it looks like
there may be some intermediate rounding in the calculation:
   210mm / 25.4mm/in ~=  8.27in;  8.27in * 300dpi = 2481 pixels
   297mm / 25.4mm/in ~= 11.69in; 11.69in * 300dpi = 3507 pixels

How are you determining the pixel sizes that LibreOffice is using for
your image anyway? Image sizes are specified in terms of the printed
size (mm, inches, etc.) rather than in pixels. Unless your display is
300dpi, the image on screen will be scaled anyway.

If you're finding that the size in the image properties can't be set to
210mm x 297mm because of the slight difference in pixel aspect ratio,
you could try unticking the "keep ratio" box so that the size can be set
to 210mm x 297mm.

If you've got LibreOffice set to work in inches, you might setting it to
cm or mm when using ISO paper sizes avoids the intermediate rounding.
Tools > Options > LibreOffice Writer > General > Measurement unit.

Mark.


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Dave Koelmeyer Dave Koelmeyer
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Re: Writer's A4 page pixel dimensions

Hi Mark,

On 20/03/16 02:07, [hidden email] wrote:

> Dave Koelmeyer wrote:
>> Hi All,
>>
>> I'm creating full A4-sized page background images in GIMP to insert into
>> A4-format Writer documents. I've noticed that when these images are
>> added to Writer and anchored to page, things are not perfectly flush as
>> I would expect.
>>
>> After a bit of tinkering I believe this is because Writer doesn't treat
>> an A4-format page as having exactly 2480x3508 pixel dimensions (at
>> 300 DPI).
>>
>> For example, my GIMP working files use the A4 300 DPI preset, which is
>> 2480x3508 pixels. If I create a new blank Writer document using A4 page
>> size, then export this blank document as a PNG and set the export DPI
>> setting to 300, Writer instead displays this as 2481x3507 pixels. I see
>> similar ever-so-slightly different results when attempting to import an
>> A4 PDF exported from Writer into GIMP at 300 DPI, again off by a matter
>> of pixels.
>>
>> Everything I've read points to 2480x3508 definitely being the dimensions
>> for A4 (e.g. http://www.a4papersize.org/a4-paper-size-in-pixels.php).
>> Can someone please help me understand why there is a difference in
>> Writer?
>
> Probably differences in rounding. An A4 page isn't exactly 2480 x 3508
> pixels at 300 dpi. A4 is 210mm x 297mm.
>   210mm / 25.4mm/in * 300dpi ~= 2480.3 pixels
>   297mm / 25.4mm/in * 300dpi ~= 3507.9 pixels
> Rounding that to 2481 x 3507 does seem a bit odd, but it looks like
> there may be some intermediate rounding in the calculation:
>   210mm / 25.4mm/in ~=  8.27in;  8.27in * 300dpi = 2481 pixels
>   297mm / 25.4mm/in ~= 11.69in; 11.69in * 300dpi = 3507 pixels
>
> How are you determining the pixel sizes that LibreOffice is using for
> your image anyway? Image sizes are specified in terms of the printed
> size (mm, inches, etc.) rather than in pixels. Unless your display is
> 300dpi, the image on screen will be scaled anyway.
>
> If you're finding that the size in the image properties can't be set
> to 210mm x 297mm because of the slight difference in pixel aspect
> ratio, you could try unticking the "keep ratio" box so that the size
> can be set to 210mm x 297mm.
>
> If you've got LibreOffice set to work in inches, you might setting it
> to cm or mm when using ISO paper sizes avoids the intermediate
> rounding. Tools > Options > LibreOffice Writer > General > Measurement
> unit.

A belated thanks for your very useful information (I have only just
found time to look at this in more detail). I think your observation of
intermediate rounding does indeed explain this. I can confirm that I see
this behaviour when Writer's measurement units are set to inches. When
the same image 2480x3508 resolution image is inserted as a full page
background with centimetre or millimetre measurement units set as you've
suggested, it then does appear to be flush.

Thanks again! :)

Cheers
Dave

--
Dave Koelmeyer
http://blog.davekoelmeyer.co.nz
GPG Key ID: 0x238BFF87


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