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Why doesn't 100% zoom in Adobe Reader draw things at the correct size?

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Chris Shearer Cooper

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Sep 30, 2010, 11:23:11 AM9/30/10
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I have a very simple PDF file that changes the effective resolution
from 72 dpi to 96 dpi (and the origin from bottom-left to top-left)
and then draws a line 96 units long:

q 0.750000 0 0 0.750000 0.000000 1456.500000 cm

q 1.000000 -0.000000 -0.000000 1.000000 48.000000 -48.000000
cm
0.0 0.0 1.0 RG 1 w 1 J 100 -125 m 196 -125 l S Q

Q

The line is 96 units long, apply the transformation and it should be
72 units long in the default coordinate system, which would be exactly
1 inch.

But when I view this PDF in Adobe Reader 8 on my screen (Windows XP)
at 100% zoom, the resulting line is almost 1.25" long.

Wassup?

Thanks,
Chris

bugbear

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Sep 30, 2010, 11:59:26 AM9/30/10
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Use a longer line. to get better test data.

The most obvious thing thats wrong is that you have dot-pitch set
wrong "somewhere".

I have *NO IDEA* where dot pitch is configured on XP,
or even if it *is* configured.

BugBear

Chris Shearer Cooper

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Sep 30, 2010, 12:17:08 PM9/30/10
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New PDF code:

q 0.750000 0 0 0.750000 0.000000 1456.500000 cm
q 1.000000 -0.000000 -0.000000 1.000000 48.000000 -48.000000
cm

0.0 0.0 1.0 RG 1 w 1 J 100 -125 m 580 -125 l S Q
Q

That line is 480 units long, times 0.75 is 360 units at 72 dpi = 5.00"

On-screen at 100% it's showing up as almost 6 1/8" long, at 72 dpi
that's about 441 units long.

Chris

Matti Vuori

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Sep 30, 2010, 12:46:18 PM9/30/10
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bugbear <bugbear@trim_papermule.co.uk_trim> wrote in
news:aNedncBGDrVDLjnR...@brightview.co.uk:

>
> I have *NO IDEA* where dot pitch is configured on XP,
> or even if it *is* configured.

As far as I know, it isn't... XP believes it is 96.

But Adobe Reader itself has a DPI setting... and there is no reason to
think that it is set correctly. Mine has two options: either a) system
default - 96 or b) a "modified resolution", which in my case is chosen and
set to 110 -- and I have no idea where that comes from; I have never set it
myself.

Anyway, Windows - or any Windows application - has no knowledge of the real
size of the screen so any DPI value is just guesswork and usually used just
to scale text and not to configure the technical system.

bugbear

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Oct 1, 2010, 4:51:01 AM10/1/10
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Matti Vuori wrote:
> bugbear <bugbear@trim_papermule.co.uk_trim> wrote in
> news:aNedncBGDrVDLjnR...@brightview.co.uk:
>> I have *NO IDEA* where dot pitch is configured on XP,
>> or even if it *is* configured.
>
> As far as I know, it isn't... XP believes it is 96.
>
> But Adobe Reader itself has a DPI setting

Where? (no really, I don't know)

BugBear

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