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Animations for 3D Solid Charts in a Virtual Globe – Techniques, Use cases, and Implementation
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Animations of 3D objects are an integral part of 3D animated movies and 3D computer games. Many concepts, such as key frames or procedural animations, originate from these two domains to simulate smooth and realistic movements. With the rise of virtual globe engines, real-time animations of 3D models receive attention also in cartography. While methods have been elaborated for animating 2D charts on maps (Mitdbø 2003) and in statistics (Heer & Robertson 2007), animations for 3D solid charts are rather the exception. In Microsoft's Power Map for Excel, for instance, stacked 3D bars are scaled along the height axis of the virtual globe when the user initializes or changes properties of the chart. Kennelly (2015) demonstrated a revolving helix chart which represents annual temperature data in the modelling software Blender. As these are only two single examples, a coherent framework for animating charts within the 3D mapping space is still missing. We therefore like to extend cartographic theory by giving an overview of animation techniques for 3D solid charts, by outlining realistic usage scenarios, and by discussing advantages and disadvantages when implementing them in a virtual globe. Based on a taxonomy of 3D charts (Schnürer 2015), we present different animation techniques which can be applied to the chart as a whole or to single chart parts. Animations include shape-preserving transformations like translation, rotation, or scaling. When shapes are altered, animations involve deformation processes such as shearing, twisting, or bending. By deformations, it is furthermore possible to add parts to or remove parts from a chart, e.g. by extrusion or by compression. A special deformation is the morphing of distinct chart shapes, e.g. from cube to sphere. The animation technique split segments chart parts, whereas the technique merge joins chart parts together. Surface and outline properties of charts can also be animated by modifying color, transparency, glow, or texture. As an environment variable, the position of the light source can be changed, e.g. to spotlight a certain chart part. Leaving a trace of previous states is thinkable as a visual aid. Lastly, different animation techniques can be combined, e.g. translation and rotation to rearrange chart parts from a grid to a helix alignment. Common parameters of these techniques are the animation duration and timing functions, e.g. linear or ease in. Animations can be initiated after an event, e.g. a mouse click on a chart part, or at a certain point of time, e.g. in a story map. Use cases of chart animations are manifold: Translation can help to expand 3D pie chart sectors, rotation of a helix chart avoids camera movement, and scaling may change depicted attribute values of stacked pyramid frustums. Categorical representations of shapes can be altered by morphing; other deformations may serve as indicators for conflicting data. Changing the color or adding glow can help to gain the map reader’s attention or to highlight certain chart parts. We implement representative examples of each use case in the Cesium virtual globe. For this, we describe 3D solid charts implicitly by minimum distance functions, which are not only well-suited for shape-preserving transformations, but also for constructive solid geometry operations and animating deformation processes. Charts are rendered by sphere tracing - also known as raymarching - in the fragment shader, including perspective projection, depth information, a lighting model, transparency, and querying individual chart parts. By implementing animations for 3D solid charts, we intend to raise the map reader’s interest in exploring multivariate data and, at the same time, to increase the overall user experience in a virtual globe Show more
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Organisational unit03466 - Hurni, Lorenz
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