Video for VI Learners
Designing for VI needs
- Primary design
- Visually impaired people watch an average of 24 hours of TV per
- 7 out of 10 people registered as visually impaired can use text
if it is clear and large enough.
- Most visually impaired people lose their sight rather than being
born with the condition: around 17 out of 20 have had progressive
- choose distinctive colours, especially for on-topic objects.
Try to avoid colours with counter-intuitive meanings (green
is a pretty universal 'safety' colour, red is 'hot', 'danger',
blue 'cold', and so on).
- avoid colour combinations which produce on-screen colour artifacts
like edge creep
- check to see if your colour choice works in greyscale.
- don't rely on colour alone: instead, give multiple cues (such
as shape, audio or spoken cues, on-object labels, or fixed object
locality on screen).
- aim for good, even contrast.
- avoid glare spots.
- don't use oversaturated colours.
Scene complexity -
- make background / foreground distinctions clear. Ask yourself
if the usual techniques (such as using tight depth of field
to defocus background) work for people with poor vision to start
- avoid complex backgrounds.
- consider carefully before using multiple windows - especially
- avoid rapid changes of scene and unnecessary cross cuts and
- avoid overlaying (ghosting) material.
- avoid multiple action points. A person with restricted fields
may miss one part of what is going on
- help users who need to pause on still frames by avoiding movement
blur: use a high speed frames per second if possible
- help VI viewers orient themselves. Provide secondary cues
(cursors, audio) where on-screen action is transient or indistinct
- use clear fonts at an appropriate size and with good contrast.
If you aren't able to offer user resizing and re-colouring,
then any choices you make will be a compromise. A starting point
for video viewed on a 20 inch screen from a normal living room
distances might be text about 1" high with clear space all round
and good interline spacing. However, if the text is to be viewed
up close (on a monitor, for instance) you can reduce the text
size - halving the viewing distance to the screen will allow
halving of text height (but remember that the original was a
compromise - don't treat such guidelines as sacrosanct).
- avoid text effects: text which hops about, changes font or
size, uses highly stylised fonts, fades, overlays other text,
or presented in non-standard reading order.
- restrict scrolling speeds.
- if you provide open subtitles, settle on a fixed cue location.
- use san serif fonts and try to use lower case - the word outlines
are more distinctive.
If your medium is linear (DVD)
then all the techniques we have described in the add-on section still
apply. You can plan for supplementation, bearing in mind what channels
will be available to you on the delivery medium you choose -
- Take conventional photographs at the same time as you shoot the video
- you can use them in handouts.
- Keep A4 versions of diagrams for the same reason.
- You can help audio subtitlers by leaving gaps for their supplementary
explanations and descriptions. Don't explain one aspect while showing
another: this leaves no room for descriptive manoeuvres.
- Create script notes as part of the teaching pack, annotated with elapsed
However, as the primary designer you can go much further. You can consider
providing special versions for VI viewers in ways which add-on modifiers
would find difficult. Properly planned for, there is no great incremental
cost to producing a second variant of the teaching material with extended
audio descriptions, high quality stills, data bursts and supplementary
views of difficult material on multichannel streams
If you are designing non-linear
materials then your ability to support learners with special needs
is massively extended, and as with the linear materials, need not cost
much more to provide. Because you can incorporate extra materials without
resorting to tricks such as borrowing audio channels for subtitling, you
can develop a single package to serve all users. Planning involves -
- Collecting the same supplementary materials described above, together
with extra video where this helps, and incorporating these assets into
the main stream as branching options.
- Choosing one or more 'hot-spot' selection metaphors and associated
cues which match the needs of your target range of viewers. Partially
sighted learners will be able to choose from on-screen branch options,
provided they are in a fixed location and audio cued. Blind users might
be given multiple options through a spoken menu in complex situations:
for supplementary audio descriptions a simple sound prompt and mouse
button response could be used to pause the main flow and start the supplementary
- Adding text markers to the video, audio and picture assets, and incorporating
any notes or scripts as screen viewable text files. Because these text
markers and text files are ASCII resources (rather than text incorporated
into video or pictures as images), they will enable teachers and students
to create printed hard copy of relevant sections and to search both
for basic information and supplements.