Undefined ratio between px and inch
Hi all,
On the user maillist there has been a discussion on a (imo) very serious flaw in the units (px, cm, in, m, ...) in Inkscape.
In short, all units except 'px' are a 'lie'. Inkscape stores lengths in 'px' unit, while the SVG standard nowhere specifies the ratio between 'px' and one of the other units Inkscape features (like inch, cm, m, ...). Inkscape relates inch and other units to 'px' by assuming 90 px = 1 in. This is not defined in the SVG standard nor in the SVG file Inkscape creates.
Although SVG permits the use of several other units besides 'px', for example inches, Inkscape luckily uses only px units to store graphics (correct me if I am wrong). This means that all objects will scale the same, whatever the ratio between 'px' and 'inch' is chosen on interpretation. Unfortunately, the absolute length information of objects is lost. These two rects have the same dimensions in Inkscape, but not in some other viewers (e.g. firefox): <rect width="90" height="90"/> <rect width="1in" height="1in"/>
My concern is the following. Inkscape now chooses 90 px = 1 in. But this information is not stored in the SVG. If someone desides to have Inkscape follow the CSS default (96 px = 1 in) and changes Inkscape's code, this would mean that all old files will be opened with wrong dimensions. For webgraphics, perhaps not a big deal. For my book cover, for my technical drawings, etc... this is a very very big deal.
So we have to come up with an idea to explicitly define the lengths of the units we expose to the user. Jon has suggested using the 'viewBox' attribute. In think that <svg width="2in" height="2in" viewBox="0 0 180 180"> would explicitly define Inkscape's measure of 90 px = 1 inch (viewbox coords are expressed in px). Perhaps there are better ideas to solve this.
Ciao, Johan
On Tue, Jan 11, 2011 at 9:09 AM, <J.B.C.Engelen@...1578...> wrote:
Hi all,
On the user maillist there has been a discussion on a (imo) very serious flaw in the units (px, cm, in, m, ...) in Inkscape.
This is a very old issue, and your summary of it is correct. Adding a viewbox was the solution proposed back then, but never implemented. If we now decide to pursue it, the first thing is, of course, not to break something that worked before. We need to try adding this viewbox declaration to old files to make sure they work as before both in Inkscape and in the most important "other software" (such as Firefox). Or, if we will be adding this only to the new document templates, we should check if this will alter the behavior in some important and/or unpleasant ways.
One question I have: assuming the viewbox will match the size of the page, will it clip the content to the page, preventing users from drawing anything in the margins?
-----Original Message----- From: bulia byak [mailto:buliabyak@...400...] Sent: Tuesday, January 11, 2011 15:20 To: Engelen, J.B.C. (Johan) Cc: inkscape-devel@lists.sourceforge.net Subject: Re: [Inkscape-devel] Undefined ratio between px and inch
On Tue, Jan 11, 2011 at 9:09 AM, <J.B.C.Engelen@...1578...> wrote:
Hi all,
On the user maillist there has been a discussion on a (imo) very serious flaw in the units (px, cm, in, m, ...) in Inkscape.
This is a very old issue, and your summary of it is correct. Adding a viewbox was the solution proposed back then, but never implemented. If we now decide to pursue it, the first thing is, of course, not to break something that worked before. We need to try adding this viewbox declaration to old files to make sure they work as before both in Inkscape and in the most important "other software" (such as Firefox). Or, if we will be adding this only to the new document templates, we should check if this will alter the behavior in some important and/or unpleasant ways.
One question I have: assuming the viewbox will match the size of the page, will it clip the content to the page, preventing users from drawing anything in the margins?
This is also what I am concerned about. I think the viewBox solution is more of a "smart hack" than a solution. I would much rather have a simple attribute that only defines the px unit. Something like "pxtoinchratio=90", or "dpi=90". The viewbox attribute does more than that it seems, which complicates things.
-Johan
On 1/11/11, J.B.C.Engelen wrote:
Hi all,
On the user maillist there has been a discussion on a (imo) very serious flaw in the units (px, cm, in, m, ...) in Inkscape.
Also, http://www.inkscapeforum.com/viewtopic.php?f=22&t=7942
Alexandre Prokoudine http://libregraphicsworld.org
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bulia byak