Minimum spanning trees for valley and ridge characterization in digital elevation maps

Texture synthesis employs neighbourhood matching to generate appropriate new content. Terrain synthesis has the added constraint that new content must be geographically plausible. The profile recognition and polygon breaking algorithm (PPA) [Chang et al. 1998] provides a robust mechanism for characterizing terrain as systems of valley and ridge lines in digital elevation maps. We exploit this to create a terrain characterization metric that is robust, efficient to compute and is sensitive to terrain properties. Terrain regions are characterized as a minimum spanning tree derived from a graph created from the sample points of the elevation map which are encoded as weights in the edges of the graph. This formulation allows us to provide a single consistent feature definition that is sensitive to the pattern of ridges and valleys in the terrain Alternative formulations of these weights provide richer characteristicmeasures and we provide examples of alternate definitions based on curvature and contour measures.We show that the measure is robust, with a significant portion derived directly from information local to the terrain sample. Global terrain characteristics introduce the issue of over- and underconnected valley/ridge lines when working with sub-regions. This is addressed by providing two graph construction strategies, which respectively provide an upper bound on connectivity as a single spanning tree, and a lower bound as a forest of trees. Efficient minimum spanning tree algorithms are adapted to the context of terrain data and are shown to provide substantially better performance than previous PPA implementations. In particular, these are able to characterize valley and ridge behaviour at every point even in large elevation maps, providing a measure sensitive to terrain features at all scales.The resulting graph based formulation provides an efficient and elegant algorithm for characterizing terrain features. The measure can be calculated efficiently, is robust under changes of neighbourhood position, size and resolution and the hybrid measure is sensitive to terrain features both locally and globally.<br /

Fast, Realistic Terrain Synthesis

The authoring of realistic terrain models is necessary to generate immersive virtual environments for computer games and film visual effects. However, creating these landscapes is difficult – it usually involves an artist spending many hours sculpting a model in a 3D design program. Specialised terrain generation programs exist to rapidly create artificial terrains, such as Bryce (2013) and Terragen (2013). These make use of complex algorithms to pseudo-randomly generate the terrains, which can then be exported into a 3D editing program for fine tuning. Height-maps are a 2D data-structure, which stores elevation values, and can be used to represent terrain data. They are also a common format used with terrain generation and editing systems. Height-maps share the same storage design as image files, as such they can be viewed like any picture and image transformation algorithms can be applied to them. Early techniques for generating terrains include fractal generation and physical simulation. These methods proved difficult to use as the algorithms were manipulated with a set of parameters. However, the outcome from changing the values is not known, which results in the user changing values over several iterations to produce their desired terrain. An improved technique brings in a higher degree of user control as well as improved realism, known as texture-based terrain synthesis. This borrows techniques from texture synthesis, which is the process of algorithmically generating a larger image from a smaller sample image. Texture-based terrain synthesis makes use or real-world terrain data to produce highly realistic landscapes, which improves upon previous techniques. Recent work in texture-based synthesis has focused on improving both the realism and user control, through the use of sketching interfaces. We present a patch-based terrain synthesis system that utilises a user sketch to control the location of desired terrain features, such as ridges and valleys. Digital Elevation Models (DEMs) of real landscapes are used as exemplars, from which candidate patches of data are extracted and matched against the user’s sketch. The best candidates are merged seamlessly into the final terrain. Because real landscapes are used the resulting terrain appears highly realistic. Our research contributes a new version of this approach that employs multiple input terrains and acceleration using a modern Graphics Processing Unit (GPU). The use of multiple inputs increases the candidate pool of patches and thus the system is capable of producing more varied terrains. This addresses the limitation where supplying the wrong type of input terrain would fail to synthesise anything useful, for example supplying the system with a mountainous DEM and expecting deep valleys in the output. We developed a hybrid multithreaded CPU and GPU implementation that achieves a 45 times speedup

Assessment of Terrain Database Correlation Using Line-Of-Sight Measurements

The uncountable number of tools for the creation of synthetic terrains poses as a challenge for simulation interoperability. The permutations of tools, elevation maps, and software settings leads to combinations of poorly correlated virtual terrains. An important issue in distributed simulations is the lack of line-of-sight correlation. For example, in military networked simulations, consistent intervisibility between simulated entities is crucial for a fair-fight, especially when simulations include direct-fire weapons. The literature review presented in the Chapter Two discusses a multitude of interoperability issues caused by discrepant terrain representations and rendering engines noncompliant to any standard image generation process. Furthermore, the literature review discusses past research that strived for measuring (or mitigating) the correlation issues between terrain databases. Based on previous research, this thesis proposes a methodology for analysis of line-of-sight correlation between a pair of terrain databases. All the mathematical theory involved in the methodology is discussed in the Chapter Three. In addition, this thesis proposes a new method for measuring the roughness of a visual terrain database. This method takes into account the 3D dispersion of the vectors normal to the polygons in the terrain\u27s mesh. Because the vectors normal to the polygons are conveniently stored in most visual databases, the roughness calculation suggested here is fast and does not require sampling the terrain\u27s elevation. In order to demonstrate the proposed method, twin terrain databases and a tool were created as part of this thesis. The goal of this tool is to extract data from the terrain databases for statistical analysis. The tool is open source and its source code is provided with this thesis. The Chapter Four includes an example of statistical analysis using an open source statistic software. The line-of-sight correlation analysis discussed here includes the terrain\u27s geometry only (terrain\u27s culture is not addressed). Human factors were not taken into consideration