Re: Pixel, Points and Spatial Measures

If this problem was easy, it would have been solved. There is a lot we
can do to bridge the gap between web size and spacial size. We are
making guidelines and giving techniques.

The mean critical print size for text on  paper and computer has been
calculated. Web developers will want to use this because people won't
read there site if they deviate to much from that value. That is a
well established fact. It is generally about .347cm on paper and
.416cm for computer screens. We recommend in techniques that
developers use the pixel equivalent these for their base  font... And
we give them tables to calculate these values.

I can look up common platforms and compute the pixel size needed to
achieve critical print size. We then put it in a bunch of tables as
guidelines for platform types with guidance on how to use them. Once
we have spread sheets with the proper formulas, mostly trigonometric,
we can update our tables to match common platforms in use with
technique updates. Will it be perfect, no. Will it be better than what
we do. Yes a lot.

>From this foundation, percentage has meaning. Without it, percentage
is meaningless as an effective accommodation.

I really think we should avoid the word point in our standard.
Developers are not the only professional users of WCAG.  Disability
lawyers don't think of point as being .75 pixels, and imprecise
measurements will not help medical arguments in court.

Patrick, we are introducing new accessibility methods. LVTF was very
careful to identify bedrock needs. They are no more disruptive than
the original 2.0 methods were, but they will require developers to
think about other factors. Some of them will be difficult just like
programmatic determinism was difficult.

This problem really is solvable.

Wayne



On Fri, Jan 13, 2017 at 6:59 AM, Wayne Dick <wayneedick@gmail.com> wrote:
> On Fri, Jan 13, 2017 at 6:59 AM, Wayne Dick <wayneedick@gmail.com> wrote:
>> On Fri, Jan 13, 2017 at 1:52 AM, Alastair Campbell
>> <acampbell@nomensa.com> wrote:
>>> Wayne wrote:
>>>> In web jargon 12pt = 16px, a flexible measure... We will ultimately talk to web developers in pixels because pixel is the unit of web measurement. I propose omitting the use of point size as a measure in our discussions and speak only in pixels for screens size
>>>
>>> That I certainly agree with, on the call last Tuesday someone made the mistake of saying points and meaning pixels, which are 25% off in terms of size. As pixels are the unit designers and devs use, they general mistake PT as meaning pixels anyway, which leads to mis-calculation of text size when doing colour contrast checks.
>>>
>>>
>>>> what a person can see is determined by the angle it subtends on the retina.
>>>
>>> And worth noting that the CSS pixel is defined as an angle. [1]
>>>
>>>
>>>> I propose that in font size issues for low vision we use spatial measures, and give conversion tables for developers. We can give our developers conversion tables in the techniques.  I can do it with centimeters and inches.
>>>
>>> It would be misleading to use spatial measures as the basis for the SC for web content. When a browser opens on a device, it decides what size a “CSS pixel” is.
>>>
>>> *Every other measurement unit is based on that CSS pixel*.
>>>
>>> Inches, CMs, EMs, REMs, PT, everything is based on the CSS pixel. (Percentages and VW/VH are based on viewport size, in CSS pixels.) The browser and OS convert the CSS pixels to device pixels for display (which is why stuff doesn’t get smaller on high-res screens, and occasionally you can get rounding errors).
>>>
>>> As I’ve said before [2,3,4] and I’ll repeat: CSS Pixels are the only relevant, semi-consistent measure for sizing web content across devices.
>>>
>>> As Patrick said, you could convert the other way as a rough guide, but we have to test in CSS pixels.
>>>
>>> -Alastair
>>>
>>>
>>> 1] https://www.w3.org/TR/CSS21/syndata.html#img-pixel1
>>> 2] https://alastairc.ac/2012/04/relative-pixels/
>>> 3] https://alastairc.ac/2013/02/how-to-hold-your-ipad/
>>> 4] https://alastairc.ac/2012/11/in-defence-of-pixels/
>>>

Received on Friday, 13 January 2017 16:10:32 UTC