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Re: Making pt a non-physical unit

From: Robert O'Callahan <robert@ocallahan.org>
Date: Fri, 15 Jan 2010 12:40:19 +1300
Message-ID: <11e306601001141540p5b8fda1eua20ab5168146e9a0@mail.gmail.com>
To: Brad Kemper <brad.kemper@gmail.com>
Cc: www-style <www-style@w3.org>
On Fri, Jan 15, 2010 at 12:22 PM, Brad Kemper <brad.kemper@gmail.com> wrote:

> 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.


I'm saying the spec could give some guidance for what the software should do
by default if the user doesn't change any settings. With my text, the spec
would encourage a billboard printer to make 1px a certain size based on an
approximate expected viewing distance of billboards.

I realize that this is contrived and doesn't really matter very much since
in practice people are going to scale such content quite carefully.


>  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).
>

In fact, for screens it is often quite easy to tell what the physical size
of a pixel is with high accuracy. All OSes support APIs for this, and with
many devices the APIs even work.

Estimating viewing distance is harder, but for "truemm" we don't need to do
that.

Rob
-- 
"He was pierced for our transgressions, he was crushed for our iniquities;
the punishment that brought us peace was upon him, and by his wounds we are
healed. We all, like sheep, have gone astray, each of us has turned to his
own way; and the LORD has laid on him the iniquity of us all." [Isaiah
53:5-6]
Received on Thursday, 14 January 2010 23:40:52 GMT

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