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

From: Brad Kemper <brad.kemper@gmail.com>
Date: Sun, 15 Aug 2010 12:37:10 -0700
Cc: public-fx@w3.org, www-style@w3.org
Message-Id: <44EDF69F-A196-489E-A61F-9C886F090361@gmail.com>
To: "Dr. Olaf Hoffmann" <Dr.O.Hoffmann@gmx.de>

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. 

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



> 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. 
Received on Sunday, 15 August 2010 19:37:49 GMT

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