How to calculate the print size zoom value for our monitor

[nikto]

Hi, I was wondering if there is a way in Inkscape to visualize graphics in real size dimensions when you work in "cm" o "mm"as display units.
I know Inkscape has no zoom to print size features like Photoshop because a vector software is not so tied to resolution as a raster software needs to be but... As a graphic designer I think it is wrong.
Graphic designers work for printing at least as much they work for web ... and when you work for printing with very complex graphics you constantly need to check your work in real print size dimensions. Not just to verify measures but above all to get a concrete perception of your work in real dimensions. Just as could do those who work for display/web dimensions by zooming to 100%.
It's damned easy to spend three hours working on details that you will discover invisible on a press....
So resolution, dpi and real mensions are not a problem for vector graphics, but they definitely are for a graphic designer.

If an utopian solution would be a new Inkscape feature to setup a "Zoom to Print Size" or "Zoom to Real Dimensions" based on the specific pixel resolution value of our monitor as in Photoshop - At least I think we certainly could get a manual, temporary but precise solution just using math to define our "zoom % value" to achieve the real prints size dimensions for our monitor . But I need your help beacuse math is a problem for me.......... 

I think we have all the data we need to calculate our castom zoom value for print size:
- we know inkscape works in 90 dpi by defoult
- we know the exact resolution of our monitor that we can calculate from this site http://pxcalc.com/ (mine is 90,05 dpi)

So why should not be possible to calculate our zoom to print size "% value"?
Any math suggestion? Thanks

[nikto]

for now I have done this really rough trick:
I drew a 30 cm wide line, parallel to the x axis, in inkscape .. And with a zoom value of 100% I have measured it on the monitor with a ruler!! hahahah yes ... the result is 31.9 cm. So Inkscape with a 100% zoom value shows me a 30cm line as a 31.9 cm line.
To define the approximate zoom value to obtain that 31,9 as a 30 cm I made this calculation:
31,9 (cm) : 100(%) = 30 (cm) : X(%)
And the result is X= 94,04388714733542 and given that Inkscape zoom values don't have the decimals I have approximated it as a 94%.
Now if a drow in cm and I use a Zoom value of 94% I get a quite accurate visualizzation of my "print size dimensions"

It would be very nice to have a more professional way to do that, but until then I think this is a pretty good solution.
In fact, for me it is not essential to have pinpoint accuracy, the important thing is to see at least 1 cm as 1 cm. And not as a 1,3 cm...

[brynn]

Well, I had most of this typed out when I saw you already replied.  But I'll post anyway, because there's some important info.

I can totally understand what you mean!

First I should tell you that Inkscape developers will probably not see your comments here.  For some reason, they don't like to participate in bb style forums.  For some reason, they like mailing lists (https://inkscape.org/en/community/mailing-lists/).  Sometimes you can catch them on IRC (https://inkscape.org/en/community/discussion/).

If you want to make a formal request for this (which I think is a good idea) here is info for that.  I know this is a feature request, but the Inkscape project uses the bug tracker for that.  https://inkscape.org/en/contribute/report-bugs/

More towards answering your specific question -- I'm not sure if I know the proper formula.  I don't mind doing the math, but need the formula to do it.

But before that, I need to tell you something, that is probably going to complicate things for you.  In the new stable version 0.92, the native dpi has been changed to 96 DPI.  So version 0.91 and earlier, it's 90 dpi.  But from 0.92 on, it's 96.  The reason for that is to be more compliant with SVG standards, which call for 96 dpi.

Next, you report your monitor resolution as 90.05, which is confusing to me.  I usually see resolution as n x n (number x number).  For example, my monitor is 1366 x 768.

Oh I see.  You mean on the site you referenced, your DPI was calculated to be 90.05?  (mine is calculated to be 104.47).

Well, the first thing that comes to mind, is for Inkscape version 0.91 and earlier, you monitor's DPI is almost exactly the same as Inkscape's.  So no change needed, right?  (0.05 dots would hardly be noticable, would it?)

For Inkscape version 0.92 and later, as far as I remember from school (40 to 50 years ago) you would just take 90.5 and divide by 96.....or maybe it's the other way around 96 divided by 90.5.  And that's the conversion factor.  So whichever way is correct, you set Inkscape to zoom by that amount.

The answer to the former is 0.9380208333333333, while the answer to the latter is 1.066074403109384.

So no matter which one is the correct answer, Inkscape can only zoom by whole numbers.

Here's where I saw you posted.

So I got a slightly different number than you.  Note that I didn't move the decimal point yet, but mine is probably more accurate.  Although as you said, since inkscape can only zoom in whole numbers, it comes out the same.

I guess I'm curious what kind of project you're working on where 6% zoom makes that much difference.  Just pure curiosity, don't feel obligated to answer.

[Lazur]

In the preferences (Shift+Ctrl+P) there is a slider you can adjust the scale.
With the help of a ruler adjust the zoom correction factor (in %) in the interface settings.

[Moini]

(which doesn't always work, depending on selected units, also see https://bugs.launchpad.net/inkscape/+bug/1482877)

[brynn]

Oh wow, I never have noticed that before!

[nikto]

Thaks to all! Yes the most comfortable thing would be to use the "zoom factor correction" in preferenxce> interface as reported by Lazur but sadly, as anticipated by Moini, it does not work. At least for me. Nothing changes in my visualizzation zoom factor when i setup the zoom correction factor.
Until there is a more efficient solution I will use my manual value of 94% because it is the easiest way I have

[Moini]

You need to hit 1 to get the correct zoom (as is described in the comments on the report). This is how it's supposed to work (yes, I agree that's strange - but not a bug, only a weird feature).

The issue described in the report can be worked around by setting the unit in the zoom preferences and the unit for 'display (or document unit' in the document settings to the same type.

[nikto]

No unfortunately it doesen't work. To minimize the error I measured as reference the maximum width of the ruler that I could view in "zoom correction factor" window. The better value for the "zoom correction factor" was 93.80 -  after that I hitted "1" as asked.
I sow the page refreshing but the scale was wrong. A 10 cm line misured by monitor was now 10,6 cm.
There is something strange.
As I said if I use my method I use as zoom value of 94 and a 10 cm line is measured on monitor as 10 cm.

[Moini]

And is the unit, in the preferences, on the zoom ruler's unit dropdown in Edit -> Preferences -> User Interface, set to 'cm', and the unit in File -> Document properties, top right corner 'Display units' also set to cm?
(or any other unit, only both must be identical).

If those are identical, hitting 1 should give you the natural size on the screen.

The 'units must be identical' thing is a bug. The 'hit 1 to get the 1:1 zoom level' is intentional.

[nikto]

Ah OK! My fault!!! I had completely misunderstood that tool..
I was convinced it was for "calibrate" and "synchronize" the 100% zoom value to the "real dimension size".
So my measuring was wrong because I was using the 100% value......
But no, "zoom correction value" is the procedure to find out and set up - which - zoom value gives that result to allow us to recall it by hitting "1"
Ok ok ok.... yes useful! Thanks!!

[Moini]

I'd say there's a high probability that most people (mis)understand the feature like you did (and I did, too, before V explained it to me).
I'd probably also rather have used this option as an additional factor for the zoom size (but I'm not a developer and can't do it, and don't know if that would have other, undesired ramifications).

[nikto]

I think the only ugly thing is that "zoom factor correction" value could not be setted as defoult or blocked and it must be redone for every new project.
This is absurd because it's obvious it needs to be changed only if we change the monitor...

[Moini]

Mmmh. It should be saved in the global preferences. Only if your default new project uses a different unit in its template, then it doesn't work.
So if you change the unit to the default document unit you use, then it should work with hitting 1.