RE: 48px vs 44px target sizing

> As 1 inch=25.4mm, so 6px per mm on the tablet, although that is 2D which I think means 2.5px per non-square mm.
> Therefore 20mm wide = 50px on that device.

I think this statement is looking the wrong way.  It may be that on that device 20mm = 50px -- but that doesn't mean 50px is going to be sufficient on other devices because it won't be 20mm.   If they say 20mm is what was needed then we'd need to figure out how many CSS pixels would 20mm be on a target device.

Just looking at an iPad, a 50px button may be sufficient -- but on an iPhone 6s the same button is considerably smaller -- likely too small for many.  I wonder if we could somehow create a relative size requirement with some minimum.  Like the size of a control in device independent pixels must be no smaller than 1/15 of the smallest viewport dimension with a minimum of 50 device independent pixels.

Jonathan

Jonathan 

-----Original Message-----
From: Alastair Campbell [mailto:acampbell@nomensa.com] 
Sent: Monday, September 05, 2016 11:37 AM
To: David MacDonald
Cc: public-mobile-a11y-tf@w3.org
Subject: Re: 48px vs 44px target sizing

> “Buttons in study were 20mm = 75px with spacing of   6.35mm = 24 px  (conversion here https://css-tricks.com/the-lengths-of-css/ ) This was based on research listed below.“

You have to be careful converting from physical to CSS units when you don’t have the digital sizing specified. 

They were using a 10.1 inch tablet, with 1200x800 resolution.  For the Galaxy Tab 10.1 the device-pixels = CSS pixels, so ~150 pixels per inch.  http://dpi.lv/#800×1280@10.1″ 

As 1 inch=25.4mm, so 6px per mm on the tablet, although that is 2D which I think means 2.5px per non-square mm.

Therefore 20mm wide = 50px on that device.

It is a shame they don’t actually specify the sizing from a programming point of view, but I think 50px is more realistic and wouldn’t look odd.

I’ve some (expensive?) reading to do, but presumably it is a sliding scale kind of thing? Obviously bigger is easier, but then you get to the point of having one button on a screen, where do we draw the line?

I suspect that if we specified more than the platform guidelines, we’re looking at a triple-A level criterial that might be better done on the user-agent side anyway... 

Cheers,

-Alastair

Received on Tuesday, 6 September 2016 16:24:43 UTC