Re: CfC: Changes to Understanding 1.4.3

On 26/04/2016 16:36, Alastair Campbell wrote:

> /> is PX also defined as a real size?       It seems to me that it is
> highly dependent on pixels per inch — but perhaps PX has a defined size
> in the real world now just like PT did./
>
> Yep, and it has a definition that is more useful for the various
> contexts: an angle;  1px = 0.0213 degrees
> https://www.w3.org/TR/css3-values/#reference-pixel
>
> "The reference pixel is the visual angle of one pixel on a device with a
> pixel density of 96dpi and a distance from the reader of an arm’s
> length. For a nominal arm’s length of 28 inches, the visual angle is
> therefore about 0.0213 degrees. For reading at arm’s length, 1px thus
> corresponds to about 0.26 mm (1/96 inch)."

Note that the reference pixel is an idealised unit in real space. Device 
and UA manufacturers don't religiously follow this, there will still be 
variations. Also, the reference pixel is defined using idealised viewing 
distance, where actual viewing distance is of course variable.

But, at this point, it's the only reliable anchoring point that can be 
used. Treating 1 CSS pixel as being equal to the reference pixel, and 
trusting that for most sane devices and user agents, the manufacturers 
have opted for a sensible resolution in relation to their physical 
screen size and expected average viewing distance that results in an 
acceptably close match to the reference pixel.

And because reference pixel includes the concept of viewing distance, it 
obviously means that the size of a reference pixel on the screen itself, 
if you held a ruler against it, will vary...but that variation is the 
result of the device / UA's assumption about viewing distance.

And, as per CSS definition 
https://drafts.csswg.org/css-values/#absolute-lengths all other units 
("pt", "cm", "mm", "in", "pc", "q") are anchored on the CSS (reference) 
pixel.

P
-- 
Patrick H. Lauke

www.splintered.co.uk | https://github.com/patrickhlauke
http://flickr.com/photos/redux/ | http://redux.deviantart.com
twitter: @patrick_h_lauke | skype: patrick_h_lauke

Received on Tuesday, 26 April 2016 19:10:10 UTC