SVG units are defined by document height/width (defaults to "px" units)
and the viewbox numbers. From that information, Inkscape (and python script inkex.py) calculates the SVG unit.
This is extremely confusing. I cannot even begin to determine how we are ever going to be able to explain this mess to a user, given the fact that we cannot even agree amongst ourselves as to what is being discussed here. First of all, the concept of "SVG unit" has never been defined anywhere, or at least I have never seen a meaningful definition of it. Secondly, there is no practical motivation for moving in this direction given that there was absolutely nothing wrong with the previous system of units. It is true that the name was unfortunate. It should not have been called default units, it should have been called document units, which is more meaningful. It is also true that there were some bugs, but those bugs are still there now and are not going to go away just by defining a new concept. Furthermore, not only are the bugs still there, but on top of that we are not in a position to be able to fix them because we are in a state of suspended animation while we try to figure out what to do. On top of that we will be defining yet another concept of unit, namely a SVG Unit on top of the existing display unit, to add to the interminable plethora of units that we already have. So, just for the record, here is my concept of how the previous system used to work, and also why I like it. The previous system worked with three types of variable. Two of these variables were independent variables and could be modified by the user using the gui. Both of these variables were geometrical in nature and had physical units associated with them, so they could be defined very clearly. The physical units were also defined very clearly and precisely as physical lengths. The first of these variables was the page size, which had units associated with it, and was specified in the page width and height in the XML root. The second variable was the viewbox which had lengths which were given in the root while the units of the viewbox were given in the namedview section using inkscape:document-units. It is unfortunate that the viewbox units are specified in a different section of the XML than the actual coordinate values, this is very confusing. However that deficiency would be very easy to fix, just a matter of relocating a variable to a better location. The point that I am trying to make is that this is a very physical picture of the representation and it is very easy to explain to anyone, no matter who they may be. It is also very flexible, because the units of the page size can be specified independently of the units of the viewBox. As for the reason why there are two independent quantities here, you can use the concept of a source document and a target document and you can visualize the process of projecting the original page size as a source onto the viewbox as a target. This is very visual and very unambiguous and very precise. Then the third variable of course is the relationship between page size and viewbox, or between source and target. This is a dimensionless ratio which is a dependent variable, not an independent variable, and would normally not be immediately accessible to the user, nor would the user normally need to have access to this particular variable. It might be useful to the programmer, not the user. To the best of my knowledge this model is sufficiently flexible to do everything that we need to do. It does not contain any redundant information, and it does not rely on any non-physical concepts. This, I believe, is the model that was in operation until about a month ago or so, and it was working perfectly well, apart from the fact that there were a few outstanding issues that needed to be resolved concerning coordinate transformations. Now it appears that this model has been abandoned in favor of a new system which has yet to be defined, for reasons that are still not clear to me. On top of that, all development work has apparently ground to a halt while we discuss this issue interminably, to no avail. I am utterly at a loss for words, I am speechless, notwithstanding the above outburst.
Alvin
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