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

From: Christoph Päper <christoph.paeper@crissov.de>
Date: Tue, 24 May 2011 12:44:42 +0200
Message-Id: <AC80D4F4-1A2C-4EDC-AFAA-769DE003D3F4@crissov.de>
To: www-style list <www-style@w3.org>
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.

Note that currently “calc(1/300in)” is invalid and reciprocal length units are not allowed at all in ‘calc()’. <http://dev.w3.org/csswg/css3-values/#the-calc-min-and-max-functions>

> 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’.

> Everything I know of that talks about image resolution uses dpi or
> dpcm only.  I don't think I've ever seen anything equivalent to dppx.

It’s kind of like a (inverse) scale factor, ain’t it?

   dppx   dpin   dpcm   um     pt     twip   px     %
  +------+------+------+------+------+------+------+------+
   1       96     37.8  264.6  0.75   15     1      100
   1.563  150     59.1  169.3  0.48    9.6   0.64    64
   2      192     75.6  132.3  0.375   7.5   0.5     50
   2.65   254    100    100    0.283   5.67  0.378   37.8
   3      288    113.4   88.2  0.25    5     0.333   33.3
   3.13   300    118.1   84.7  0.24    4.8   0.32    32
   4      384    151.2   66.1  0.188   3.75  0.25    25
Received on Tuesday, 24 May 2011 10:45:12 GMT

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