W3C home > Mailing lists > Public > public-low-vision-a11y-tf@w3.org > March 2016

Re: New view of horizontal scrolling

From: Gordon Legge <legge@umn.edu>
Date: Thu, 10 Mar 2016 17:03:06 -0600
To: Wayne Dick <wayneedick@gmail.com>, public-low-vision-a11y-tf <public-low-vision-a11y-tf@w3.org>, legge <legge@umn.edu>
Message-ID: <56E1FD2A.3020107@umn.edu>
Hi Wayne,

I'm sorry for being very slow in responding to your message. It's been a 
very busy week.

First, I'm happy with the text in your blog post. Your term "barely 
legible" acknowledges that legibility is a continuum, not a dichotomy. 
Your reference to "sustainable" recognizes that readability of text 
requires more than good legibility.

Regarding the relationship between "legibility" and the problem of 
magnified text that requires scrolling:

In everyday reading, eye movements are used to move through text. 
Normally sighted readers make about 4 eye movements per second, jumping 
roughly word by word. This is the easiest and most familiar way of 
navigating through text. This method of reading can be maintained with 
magnified print, as long as the text remains visible on the screen.

When people with low vision need very large print, they must often use a 
magnifier of some sort e.g., hand-held magnifier, stand magnifier, 
electronic magnifier or screen magnifier. Moving the magnifier over text 
can impose additional demands on reading e.g., manual scanning of a hand 
magnifier, head scanning sometimes, or manual dexterity with a mouse for 
magnified computer text. These additional demands of page navigation can 
result in slower and more effortful reading.

In the research literature on this issue, people have sometimes referred 
to the "page navigation problem.' They typically distinguish this from 
legibility per se. Although there is certainly some interaction, 
legibility depends primarily on perceptual constraints, and page 
navigation is limited by motor constraints.

In short, I agree with your analysis, except I would distinguish 
problems of legibility from problems with page navigation. Of course, 
good legibility often means high magnification for people with low 
vision, and high magnification often can result in problems with page 

I hope these comments help.


On 3/2/2016 12:37 PM, Wayne Dick wrote:
> Horizontal scrolling May be a legibility issue.
>     “Legibility” refers to perceptual properties of text that influence
>     readability. Text which is hard to read because of obscure
>     vocabulary, or complex syntax or meaning may be incomprehensible,
>     but still highly legible. Legibility depends on both local and
>     global properties of text. Local properties refer to characteristics
>     of individual letters or pairs of letters such as font, print size,
>     and letter spacing. Global properties refer to layout
>     characteristics such as line length, line spacing, and page format.
>     [Legge, Psychophysics of Reading (ch 4)]
> G. Legge identifies two types of legibility issues: Local and Global.
> Local covers issues like letter spacing, font face etc. Global covers
> issues like line length, line spacing etc. The key to Legge's definition
> is that legibility is perceptual. He carefully rules out issues of
> understanding. Now if line length is a global legibility issue then
> isn't a line that runs off the view port a global legibility issue?
> Maybe we have been framing the issue wrong by focusing on the solution,
> horizontal scrolling, and not the need, legibility interference by lines
> that exceed the available space.
> Gordon, am I off the wall. Could you send a reply to all so the Low
> Vision Task Force can consider your comments. Also if you have the time
> could you comment on my refinement of legibility at
> https://github.com/w3c/low-vision-a11y-tf/issues/31 .
> Thank You,
> Wayne
Received on Thursday, 10 March 2016 23:03:33 UTC

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