The scope of 3D representations in cartography is expanding rapidly. Truly 3D auto stereoscopic displays are replacing pseudo 3D depictions. New techniques are here to ensure that 3D is no longer limited to the representation of landform characteristics
Principle of perception of true-3D/flip lenticular display
Space and spatial are considered to be an inherent, even axiomatic feature of both subject and function of cartography (Koch 2002). This includes ‘thematic spatial models’ (Bollmann 1985) like the ones derived from 3D terrain models in the early 1970s by Spiess (Spiess 1974). Also, the ‘data relief’, of theoretical or mathematical constructed surfaces in particular, can in most instances be spatially perceived in a significantly better way than with conventional two-dimensional or pseudo-three-dimensional cartographic depictions, especially if there are relatively small elevation differences (Koch 2002; Rase 2003).
Untrained map users have great difficulties in deriving a 3D mental geo-relief model and a “value landscape” (cf. Buchroithner & Knust 2013) of any type of thematic data out of a 2D depiction (Buchroithner 2007; Bröhmer et al. 2013). Until recently, the separation of the stereo-partners corresponding to the left and right eye has frequently been realised by means of anaglyph glasses, polarisation glasses or shutterglasses. The use of these glasses — over longer periods — finds only little acceptance among map users. The lenticular foil automatically separates the images so that no viewing aids have to be used.
Planar true-3D visualisations can be parallax-3D or full- 3D. There exist both glasses-based and glasses-free true- 3D visualisation techniques. Glasses or other visual aids are needed to separate the stereo partners, i.e. one image for the left eye and one image for the right eye. This separation is either timebased or filter-based. One well known glasses-based method with temporal image separation is the visualisation in combination with active shutter glasses. Most of the present-day 3D visualisation devices work with this method. They show the left and the right half-image alternately and in high frequency (normally 120 Hz) and the shutter glasses synchronously shut and open the left and the right glass. Due to the high frequency the eyes do not notice that the glasses are shut alternately. Each eye perceives only the images which are allotted to it, because the glass is only transparent in the moments the correct halfimages are shown on the display and it is opaque when the halfimages for the other eye are shown.
The filter-based image separation uses anaglyphs (e.g. red and cyan filters) or displays that use passive polarisation glasses. The half-images that have been especially pre-processed for this method are simultaneously visualised for the left and the right eye and filtered by the glasses which have different optical characteristics (different colour or polarisation direction). Other auto-stereoscopic visualisation methods are normally based on two or more stereo-mates, but they can also consist of a single image or model. The two methods often used are the lenticular foil technique and the parallax barrier technique. The lenticular foil technique uses vertically arranged half-cylindrical lenticules to geometrically deflect the half-images to the left and the right eye. The parallax barrier technique uses a strip mask which makes each one half-image visible to the eye and hides the other one. Both the techniques make only one of the stereo-mates visible to each eye. Digital lenticular foil or parallax barrier displays are available with or without tracking unit which helps the stereo-mates to adapt to the viewer’s position.
Cameras detect the viewer’s eyes and thus detect the motion of the user. The visualised 3D content can be adapted by changing the image or the display according to the changed viewing position. A possibility which compensates motion in XY-direction and also in Z-direction was investigated by Fraunhofer Heinrich Hertz Institute, Interactive Media-Human Factors Berlin, Germany. Björn Schmidt investigated the lenticular foil technique for use in printed maps (Dickmann and Schmidt 2011; Schmidt 2012). He also developed, with VLR method, a new algorithm for interlacing the stereo mates to enhance the quality of a printed lenticular display.
The most realistic true-3D visualisation can be created with volumetric imaging techniques. Volumetric visualisations include solid landscape models, subsurface engravings and globes. There can also be non-solid visualisations like the holograms. Both solid and non-solid visualisations have parallaxes in all directions of the XY-plane, making them full-3D.
Solid landscape models or city models can be created manually or by machine. Rapid prototyping methods like 3D-printing or milling are perfect examples of machinegenerated models. Another possibility is the transformation method where a planar thermoplastic foil is transformed by heat and pressure into a form which corresponds to the 3D relief model (Rase 2012). Solid terrain models experienced a sort of renaissance over the last decade. “Producing a solid landscape relief is comparable to composing music: the finest details and nuances can only be produced by men and not by machines. My slogan is: Do not give away the most creative work to machines – to create a landscape. I want to do it myself,” says Toni Mair, the renowned relief artist. His statement describes best why handmade relief models are still produced today. But this method is very time-consuming. A recent approach at TU Dresden tries to combine automated DTM-based milling and high-level artistic handicraft.
Solid globes also allow volumetric visualisation. These can be traditional globes made of brass or wood covered with paper or thermoplastic material. Another type is the tactile hyperglobes which are volumetric and tactile. They combine the benefits of a solid globe with those of a digital globe image. The cartographic image is visualised digitally with the help of several projectors onto the acrylic glass. Riedl (2010, 2012) distinguishes further between the hologlobe (volumetric and digital) and the digital hyperglobe (pseudo-3D). There can be more potential spherical displays (Hruby 2011).
Holograms are another method for autostereoscopic volumetric imaging. They are available as hardcopy (‘traditional’ holography) and also as softcopy (digital holography). The requirement of very high calculating resources to create a digital hologram is still a major problem. Real Technologies Inc. in Dresden, Germany has introduced some promising innovations to reduce these resources to a minimum with the help of sub-holograms. The company is convinced that this will lead to “commercial real-time holographic 3D imaging in the near future”. The so-called Holographic Relief Maps made by a mapping unit in Ankara, Turkey is a good example of printed cartographic holograms. It is a combination of topographic and thematic map data with a DEM and aerial photographs.
A computer generated image of the largest autostereoscopic true-3D display of geodata worldwide with a height of almost 3 metres and a width of close to 7 metres showing the mighty Dachstein Southface in the Austrian Alps
Motion parallax adds value
An important prerequisite for 3D vision is the understanding of different speeds at which two objects at different depths are viewed by an observer. Objects closer to the observer move relatively faster sideward than the objects that are further away (if the observer moves laterally). The images on the observer’s retina behave accordingly. This effect is simulated by the lenticular technique. The viewer can use it to ‘shift’ objects on a static map by moving his head in order to see which object is located above the other. Thus it is possible to overlay map symbols, cartographic labeling and even map inserts in different spaces.
Empirical studies carried out by TU Dresden and Bochum University in Germany demonstrated that the use of the motion parallax achieves a higher information transfer. Initial findings of the study prove that the changeable central perspective caused by the lenticular autostereoscopic threedimensionality allows the observer to discern superimposed (vertically staggered) map symbols in the horizontal direction. The advantage towards 2D maps is the possibility to perceive map symbols laying on the map basis behind diagrams or other map symbols needing big space requirements. However currently, it is important to comply with comparatively big minimum dimensions for map symbols.
Today, digital autostereoscopic three-dimensionality is available on devices such as autostereoscopic TV sets, digital picture frames, smartphones, portable game consoles, tablet computers, and notebooks. They use the same technique as 3D monitors. Offering detachable masks with such devices is the latest trend. The user can simply remove a mask from the device and go back to watching 2D content. A lot more is in store but the ‘classical’ cartographic 3D methods will continue to be used. Both the classical planar 3D visualisation methods as well as advanced autostereoscopic developments will have their special fields of application.
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