It's because svg was designed as a web format and according to the wiki: "Eventually, after long arguments, the CSS working group dictated that one inch would be fixed to 96 pixels regardless of screen resolution"

Yea, that's the insanity we are stuck with. Pixels being units of physical measurement.

The cure is to never, ever under ANY circumstances uses pixels as a unit of measurement. It was always a bad idea, but since svg was meant to contain css it's kinds what we are stuck with if we want to be/stay compatible with web standards... even the ones that make no goram sense. :)

Note: Inkscape still correctly will output your vectors at any dpi you wish, but pixels should always be calculated based on a physical measurement (metric or imperial), and pixels calculated as a dpi.

This is not a problem that will go away. The CSS working group will no doubt decide to haphazardly change it again because "retina" displays are so popular, in which case it will probably be 300px / inch.

It doesn't matter though. If you're using pixels to calculate witdth, you will have to expect that the width will change based on the pixel size of your output, so best thing is to not do that again in the future.

Here's the scoop on units in Inkscape: http://wiki.inkscape.org/wiki/index.php/Units_In_Inkscape

-C

On Fri, May 12, 2017 at 9:46 PM, Mick Ralph <mick.ralph@...3070...> wrote:

As the background to my problem, I use Inkscape to prepare drawings of sides and ends, etc, to be made into model railway carriages. The designs are then opened in the Designer Edition (DE) of Silhouette Studio for cutting out of styrene sheet on the Silhouette Portrait cutting machine. DE can open SVG files, as created with Inkscape.

For Inkscape 0.48 and 0.91, this works fine and the parts are cut to the same size as the Inkscape drawing, although it is necessary to ensure that the document properties are set to "default units" of mm and "custom size" "units" of px; if custom size units are left as mm, then the actual size of a part decreases from (say) 110mm to about 88mm. I created a new default template with these properties to ensure that the Inkscape drawings would always cut at the right sizes.

When Inkscape 0.92 was released I downloaded it and I have just used it to draw out another carriage. I then opened the svg file in DE and without examining it, sent it to the cutter. The parts seemed too small. I then realised that the default template was no longer in mm/px, but was now in mm/mm. When I altered the document properties back to mm/px, I opened the file in DE and examined the size of the parts. I now find that the part which should have been 110mm is now about 117mm.

This is clearly useless for making scale models. I opened in v 0.92 a file which was originally created in Inkscape 0.91 and received the message that the original file was drawn at 1 inch = 90 px, but the new version uses 1 inch = 96 px. As a user rather than a developer, I do not understand the reason for this (nor do I need to understand it) - I just need to know that if I draw a line 100mm long it will cut at that length, as in the earlier versions.

As I can see no option in v 0.92 to allow compatibility with earlier versions, it seems that I will have to reload v 0.91 and use that to create my drawings.

Is there a compatibility option to allow drawings to be made at 90px per inch in v 0.92?

Mick Ralph

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