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Re: [css3-images] Is the dppx unit needed?

From: Tab Atkins Jr. <jackalmage@gmail.com>
Date: Tue, 24 May 2011 08:52:20 -0700
Message-ID: <BANLkTi=0kE=URZd7Z9jzrz+cTv=sYGeHtw@mail.gmail.com>
To: Christoph Päper <christoph.paeper@crissov.de>
Cc: www-style list <www-style@w3.org>
On Tue, May 24, 2011 at 3:44 AM, Christoph Päper
<christoph.paeper@crissov.de> wrote:
> Tab Atkins Jr.:
>
>> I'm rewriting the section on the <resolution> type in the Images spec
>> <http://dev.w3.org/csswg/css3-images/#resolution-units> to actually
>> explain what the unit does right now.  While writing an example, I was
>> struck by the fact that the dppx unit seems unnecessary,
>
> Actually all reciprocal length units are unnecessary since we could just use length units. We should rather introduce ‘um’ for micrometres, and perhaps ‘twip’ for “twentieth of an point”, i.e. 1in/1440 or 17.63(8) µm.

While we could use length units, in general resolution is expressed as
an inverse-length.  I would prefer not to put the onus on authors to
translate 300dpi to an explicit length if I don't have to.


>> given that we now have a guaranteed 96:1 ratio of 'px' per 'in'.
>
> How does this matter? We also have (and always have had) a guaranteed 2.54:1 ratio of ‘cm’ per ‘in’.

Previously, the ratio between 'px' and 'in' was, theoretically at
least, variable depending on the pixel size of the screen.  Thus, you
couldn't express the default resolution of images in dpi, because it
would vary based on screen pixel size, so you *needed* dppx.  Now that
we have a guarantee that 1dppx = 96dpi, there's less need for it.

I assume that non-american authoring tools express resolutions in
dpcm, given that nobody else uses inches.  That unit was in the spec
before I became one of the editors, and I trust that Fantasai had a
good reason to add it.

~TJ
Received on Tuesday, 24 May 2011 15:53:07 GMT

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