Does the "Support for real world document and page size units, e.g. millimeters" mean any mm dimensions are also stored in the file as mm?
For all the SVG-base CNC apps this would be great news.
For all the SVG-base CNC apps this would be great news.
This is because everything inside SVG document lives in an imaginary infinite world where dimensions and distances are expressed in abstract "user units".
You use "viewBox" attribute on the outermost <svg> element to specify a rectangular fragment of that world for the purpose of rendering. You use "width" and "height" properties to specify the real-world size of that fragment.
Authoring tools might show you dimensions of SVG objects such as rectangles or paths in real-world units computed from "viewBox", "width" and "height" of the outermost SVG element, but under the hood the size of those objects should be stored in user units.
Not a big deal if you only use Inkscape. The real mess starts when going between different authoring apps. Then pretty soon a 100mm line in Inkscape turns into something longer or shorter. This is when you revert back to DXF, the bastard of all file formats.
I don't see how the viewBox's real world units can solve this. I think the geometries need to actually use real world units (which the SVG standard supports).
Not sure if this is what you meant, but some tiny rounding errors might occur during conversion if there are many transforms involved between the user space and the viewport space, but this can be avoided if user space values are stored with high enough precision.
In order to convert user units to viewport (physical) units and vice versa you only need the current transformation matrix, viewBox rect and viewport size. I have created a demo which shows how I can determine the line length in physical units: http://jsfiddle.net/9n5hf2jw/1/
The way how real world units are interpreted inside the SVG document is so messed up that even the SVG spec itself discourages their use:
Defining the size of a document in mm and then using mm units for shapes within it is going to give counterintuitive results, since they'll be converted to user units to resolve against the view box. (https://svgwg.org/svg2-draft/coords.html#Units)
When I setup up Inkscape to use mm it does set with/height in mm and then the viewBox to the same dimensions (without units). This way it is implying the user/px units (e.g. all unitless dimensions in the file) are basically mm.
Previous versions of Inkscape would always safe in user/px units with an implied conversion of 90dpi, meaning a scaling factor of (25.4/90) to go from values in the file to mm. CorelDraw always uses 96dpi, Illustrator always 72dpi. I am not saying this makes sense. It's just how different authoring apps do it. If you want to use the file in a CNC machine you have to make sense of what the app meant when it uses unitless dimensions. When a file was edited with two different apps it very often is not possible to make a smart guess by some basic heuristics.
BTW: the CNC app I am working on is LasaurApp, part of an open source laser cutter called Lasersaur - http://lasersaur.com