SSC logo Scottish Sensory Centre
university of edinburgh

Adapting Video for VI Learners

Designing for VI needs


  • Primary design
    Linear Supplements
    Nonlinear Supplements

    Primary design:


    • Visually impaired people watch an average of 24 hours of TV per week.
    • 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 loss.


    Colour -

    • 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).

    Contrast -

    • 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 with.
    • avoid complex backgrounds.
    • consider carefully before using multiple windows - especially transient ones.
    • avoid rapid changes of scene and unnecessary cross cuts and zooms.
    • avoid overlaying (ghosting) material.

    Movement -

    • 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

    Text -

    • 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.

    Designing supplementary material:

    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 times.

    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 description.
    • 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.