Re: CfC: Changes to Understanding 1.4.3

Hi Gregg,

> PT is defined in the literature as a particular size in real space.    (I know it is also used in virtual space in a variable manner — but it does have a real world size definition)

I’ll assume you are correct about the literature, but in a web context, PT also has a definition in terms of CSS units. If you are talking to web designers/developers, they will assume you can use PT in the CSS and that is actually based on pixels.

If you use PT in HTML/CSS, 1pt = 1.33px. Which leads to your next question:

> 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 this is separate from the “device pixels”, devices have various device pixels per inch, display densities, ‘retina’ displays etc. https://alastairc.ac/2012/11/in-defence-of-pixels/


The browser maker decides how many device pixels you have per CSS pixel. It used to always be 1:1, now it can be 2:1, 3:1, 1.5:1, 2.3:1 etc.

You can use the size of the browser’s CSS pixel to work out what the distance of viewing should be!
https://alastairc.ac/2013/02/how-to-hold-your-ipad/


Whilst the definition & maths can be complex, the results are fairly simple: pixels are a good relative unit across devices, and it is what designers and developers are used to.

Cheers,

-Alastair