An image read by sight can usually be seen in great detail all at once. As a result the detail can be seen in the context of the entire image. For instance, when looking at a portrait painting the subtle detail of the face such as wrinkles at the eyes can be seen. However a sighted person is also aware of the other details in the portrait including the clothing and posture.
The tactile sense is considerably less sensitive than the visual sense, and touch works in a more methodical manner than vision. Only a small part of the tactile image is felt at a time, followed by the next adjacent section. All of these sections need to be pieced together mentally by the reader like a jigsaw puzzle to view the whole image.
Therefore it is key for a visual graphic to be re-designed thoughtfully to make sense in a tactile form so blind and partial sighted readers fully appreciate the message being conveyed by the image. Unecessary fine detail must be removed because it will be felt but not understood and may confuse the blind reader. Instead clearly defined shapes will be understood more easily by the blind reader.
A good tactile graphic is really best started from scratch. If transcribing a mainstream image time spent on a little planning is well worth the investment. It will help to ensure the final tactile image works well, is easy to use and may eliminate wasteful reworking time.
Points to consider:
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Identify the key message in the original visual image. What is the essential information that needs to be conveyed in the tactile image? This will highlight all of the elements which can be excluded from the tactile image without losing important information.
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Detail which may add interest for a sighted viewer of a mainstream image but is not actually conveying any important information needs to be removed from a tactile image.
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It is important that the designer considers visual concepts like perspective, occlusion and transparency which are best removed as they can distort information trying to be conveyed.
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Usually the tactile image will be larger than its mainstream equivalent. It should not, however, be too large. This would make the various well spaced out components difficult to relate to each other.
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With sight we can perceive very small items, like a full stop in printed material. It is easy for a sighted person to distinguish the difference between quite small symbols - a couple of millimetres across. The difference between the symbols could be quite subtle - a square and a pentagon but still differentiable. However for a blind or partially sighted person the smallest tactual perception possible is roughly the size of a finger tip. An open square and circle 6 mm across are tactually very difficult to tell apart so a designer will need to verify that the symbols selected for their image are appropriate.