W3C home > Mailing lists > Public > www-style@w3.org > January 2010

Re: Making pt a non-physical unit

From: Brad Kemper <brad.kemper@gmail.com>
Date: Thu, 14 Jan 2010 15:22:43 -0800
Message-Id: <98112E07-3362-4972-B57A-7FC2CA333554@gmail.com>
To: "robert@ocallahan.org" <robert@ocallahan.org>
Cc: www-style <www-style@w3.org>

On Jan 14, 2010, at 12:48 PM, "Robert O'Callahan"  
<robert@ocallahan.org> wrote:

>> How about the following definition for CSS pixel:
>> For print media intended to be read "at hand", a CSS pixel should  
>> be rendered as 1/96 of an inch.
> Agreed, but for all print media.
> Handling exceptionally sized media by treating them as "always  
> having some default scale" would be OK, but less that optimal from a  
> spec point of view, IMHO, because it amounts to the spec having  
> nothing to say about what units should map to in such media, when we  
> could provide useful guidance for user-agents and authors.

I'm not sure I understand. I'm saying the default scale **for print**  
would be 100% (1in = one inch). If, while printing, I (the user) set  
it print it at 200% (from the Print dialog box) and everything would  
scale. My experince is that you can always tell a printer to print  
something at some exact size (if the print driver is any good). I  
don't think we need to consider printers that just duplicate whatever  
pixels are on the screen (you wouldn't be using the print media CSS  
for that).

Or I would print to disk at 100%, then give the file the printing  
company to scale. Or I could print to disk at 1200% and give the file  
to the printing company to output at full size.

> Define "truemm":
>> "truemm" is defined to correspond to physical millimeters in all  
>> media. For extremely small or large output surfaces, or when the  
>> physical media characteristics are unknown, the user agent may use  
>> an approximation.
> I don't want two different kinds of mm.
> Do you agree that we need some kind of physical unit? If so, what  
> should it be?

I think it's a nice idea, in the same way that unicorns are a nice  
idea. But practically speaking, for non-print media, the does not seem  
to be a very reliable cross-platform way to know the physical size of  
the pixels (and therefore of mm) and and even less so to know the  
viewing distance (for some devices you wouldn't know if it was a half  
inch or a half mile).

So for print, there are exact phyical units, and the size of a CSS  
pixel is derived from that (divide inches by 96). For all others,  
there is some exact definition of what a pixel is, and physical  
dimensions are calculated from that (monitor pixels times 96 equals  
1in). I'd rather have all non-print media be consistent, and all print  
media be consistent, than to define a "true" mm that is sometimes,  
occasionally, somewhat more accurate than "regular" mm, just because  
we failed to make mm accurate in the past  (nor 10 years from now to  
have a 'really-true-mm' that is more reliable and accurate than 'true- 
Received on Thursday, 14 January 2010 23:23:31 UTC

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