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Re: [css21][css3][svg] SVG and unit-less length values

From: Dr. Olaf Hoffmann <Dr.O.Hoffmann@gmx.de>
Date: Mon, 16 Aug 2010 10:50:32 +0200
To: public-fx@w3.org, www-style@w3.org
Message-Id: <201008161050.33174.Dr.O.Hoffmann@gmx.de>
Brad Kemper:
> On Aug 15, 2010, at 4:04 AM, Dr. Olaf Hoffmann wrote:
> > 2. There are a few use cases (already mentioned, technical drawings,
> > maps etc), that require absolute units, at least absolute units for width
> > and height of the root svg element. Primarily the lengths have to fit
> > precisely for printers for example to give such a technical drawing to a
> > mechanical work shop to produce a real object, else SVG is not usable
> > for such applications and authors need still another format or still have
> > to draw everthing manually on millimetre paper without the option to
> > reuse and to archive this on their computers - and will maybe never
> > switch to a standard like SVG, if such W3C formats continue to have
> > problems with such simple 'real use cases'.
> >
> > This is what really happens every day in my job - I have to ask:
> > Is SVG advanced enough for technical drawings? For scientific graphs?
> > Typical answer currently is: No!
>
> For print, an inch is a inch and a cm is a cm. In that case the size of a
> CSS "px" unit is derived from the actual physical inch (a single device
> pixel would be far to small, and would vary greatly between output
> devices). So I think your statements above about printers and precision are
> unfounded.

Indeed, this direction is useful - but this should not depend on the output
device. What a cm is, is defined by other people in precision. 
Therefore such a definition will always define, what 'px' means in relation to
cm.

However this does not cover the need to have a unit to allow authors
to display raster images on monitors without interpolation (what is typically
not easy in SVG either - not sure, how to improve this).

>
> For other non-print output devices (e.g. video monitors), having a pixel
> that is an integer of device pixels is usually more important (at 100% zoom
> at least), and the accuracy of physical measurements such as inches is less
> reliable, and less important than maintaining proportions between different
> measurement systems. 
> So the calculation goes in the other direction (inches 
> derived from pixels). 

It is surprising, that this is so hard to understand. Because cm is already
defined by other people, there is no other direction.

> I should note too, that the UA is free to scale/zoom 
> all the units together, and most have this feature. 

Sure, I cannot see a problem with this, this is due to a user interaction.
If the user wants to zoom into a document, it is obvious, that the
sizes change, there is no problem. Absolute units apply only to
something that is not scaled due to a user interaction.

> This generally does a 
> good job of maintaining the designers proportions between elements. So it
> is entirely OK to choose a zoom level that would result in fractional CSS
> px (as the iPhone does), and thus there will be a zoom level at which a CSS
> inch is exactly the same measure as a physical inch. 

Sure, this will help people.

> A UA may even chose to 
> have an initial zoom level that it assumes will create an actual size inch,
> if the implementor truly believes that the output size is reliable and
> accurate, and this is perfectly fine with CSS.
>

This should be left to the preferences of the user of course, else the
user will be confused with a text like: 'See on the right a  true to scale
of our new product.' If the user changed the scaling on his own, the
interpretation should be obvious. If the user-agent cheats on his own
decision, this is no help for the user to interprete text and image ;o)

> > possible in many circumstances. Obviously for these situations authors
> > and the audience should not be cheated by stupid programs and
> > stupid recommendations.
>
> Enough with the invective already! Maybe you don't share our priorities,
> but that doesn't make our decisions stupid.

I think, it is not incentive, as already explained with some example, these
are facts. 
I do not think, that someone here is stupid, but if the decisions lead to
situations that a cm defined by other people has a different meaning in
CSS leads always to the same conclusion about the relevance of such
a recommendation - there is no choice for me as physicist - to cheat about
elementar units is stupid and the rules of conduct of scientific organisations
require to disclose intentional and unintentional cheating like this to avoid
as much confusion as ever possible.
Your might think as a designer (?), to have a choice here - I as a physicist
have not.

Doug Schepers:
>I agree with Brad here.  There's been an unusual amount of sniping on 
>this thread, and it's not productive.  We're all working toward the same 
>goal of sanity and interoperability.

No, as explained above, there is no choice for me.
This is not meant as affront to anyone. 
This is an essential information about what is possible here and what not.
There are many options to solve the problem, to redefine the meaning of
a cm is surely no option to be taken seriously.

Olaf
Received on Monday, 16 August 2010 08:51:07 GMT

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