Some properties of our visual system. Designing visualisations. Gestalt principles

Transcription

1 Designing visualisations Visualisation should build both on the perceptual abilities of the human and the graphical conventions that have developed over time. Also the goal of the visualization should be taken into consideration. Some properties of our visual system Some properties of the visual field are perceived pre-attentively. These include line orientation, colour, stereo depth and closure. Sudden changes are more visible than gradual. Motion is easily detected. We segment the image into background and figure. Symmetry is best perceived along the vertical axis. Gestalt principles Proximity Proximity Similarity Direction Closure... Similarity Direction seen as 1

2 Everyday experience Light from above Commonly used angles Usual way of observing objects 2

3 viewer Colour interaction Ishizaki After Josef Albers 3

4 The proper use of colour Colour warnings To show values The environment affects colour perception To group items Hue changes introduce perceived brightness changes To draw attention We can only keep track of a small number of colour categories Colour perception differs between individuals We are differentially sensitive to different hues We experience colours through conventions More warnings Luminance contours Use your available colour/grey scale space efficiently Do not use overlapping colour scales for different attributes Data are important, coordinate axes and grids should not stand out Luminance and chrominance ramps Histogram equalisation 4

5 Group and value The human visual system has better resolution for brightness than for hue Colour sensitivity Emphasize the data 5

6 Missing data Not available (invalid data) Not applicable (out of measurement range) These are distinct reasons, but in both cases some data points are missing. This should ideally be marked somehow. Both measurements and computation contain an element of uncertainty. It is often a good idea to indicate this uncertainty in a visualisation. Uncertainty Format conversions When converting an on-screen visualisation to some other format, you have to consider the resolution in space and colour of the other medium. To print Print media has (much) higher spatial resolution than computer screens, but cannot display as many colours. One can either work in a print-compatible colour range from the start or remap colours; this will of course affect any colour maps. To video Video has lower resolution and smaller colour gamut than computer screens. In addition video cannot display small details well. Lying with visualisations There are many ways in which a visualisation can make it more difficult to see what is going on in the data. Some of these methods are used on purpose, but mostly people are just clueless. 6

7 Broken axes Avoiding broken axes Inconsistent scales Inconsistent axes Ways to mislead perception There are many ways in which wellmeaning visualisations may mislead a viewer. 3D barcharts are common, but distort information by perspective and occlusion. 3D glyphs 7

8 Some advice regarding good pictures computer graphics, visualization -Try to select what is of interest for the user and help the user to focus on that. Things outside focus may be presented with less precision and should not disturb the focus of the user. -Don't present information that is not possible to read/perceive - the computing time can be used for other things - Avoid confusing information/layout Blue is used both for sunni and shia (diagram and map)! -Adjust pictures (and text) to the resolution, presentation medium and the culture -Use redundancy to make it easier for the user to perceive information -Different presentation forms serve different purposes: a sketch can be used to illustrate the content, a realistic picture gives more faith -Use well known effects concerning everyday scenes (lighting, shadows, right angles,...). Avoid illusions -Scales for sorted information should be used with care (brightness and saturation is better than hue). 8