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Re: Making pt a non-physical unit

From: Ambrose LI <ambrose.li@gmail.com>
Date: Tue, 12 Jan 2010 14:02:20 -0500
Message-ID: <af2cae771001121102y76e1a425od319fb9537ac8aca@mail.gmail.com>
To: Boris Zbarsky <bzbarsky@mit.edu>
Cc: "Linss, Peter" <peter.linss@hp.com>, www-style W3C Group <www-style@w3.org>
2010/1/12 Boris Zbarsky <bzbarsky@mit.edu>:
> On 1/12/10 1:15 PM, Linss, Peter wrote:
> The point (no pun intended) is that there is such a reason: doing it is not
> compatible with most of the web content out there.
>
>> I do see a use case for a device pixel unit, but it still scares me as
>> it's ripe for abuse and will likely be misunderstood. The best use case I
>> can come up with for it is a hairline border, where I want it to be the
>> thinnest possible line that the output device can render. If there's really
>> a compelling need for that, I think I'd rather have a "hairline" unit than
>> device pixel.
>
> Indeed, since it may well be that the thinnest possible line the output
> device can render is actually invisible to human eyes.  For example, a
> 1-pixel line on a 2400-dpi printer is nominally 0.01mm == 10um wide;
> http://en.wikipedia.org/wiki/10_micrometres and
> http://en.wikipedia.org/wiki/1_micrometre have some things in this range
> (e.g. very very fine human hairs, cotton/silk/nylon fibers, 1/10 the
> thickness of a typical sheet of paper, two red blood cells next to each
> other, that sort of thing).  I suspect that most papers wouldn't actually
> show that line at that width, but if they did it would be below even the
> theoretical maximum visual acuity of human eyes (.35mm at 1m viewing
> distance according to <http://en.wikipedia.org/wiki/Eye#Acuity>) if held at
> normal reading distance.
>
> I might be ok with a "hairline" width for this use case, with the UA trying
> to do something sane with it, I guess.

FWIW, in PostScript this "thinnest possible width" is represented by 0
(points). Whether it can be seen or not, it can be represented.
-- 
cheers,
-ambrose
Received on Tuesday, 12 January 2010 19:02:52 GMT

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