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A Compact Random-Access Representation for Urban Modeling and Rendering

A Compact Random-Access Representation for Urban Modeling and Rendering. Zhengzheng Kuang Bin Chan Yikhou Yu Wenping Wang. 1 Introduction. Problem – Rendering urban “City- Scapes ” requires high storage capacity, transmission speed and rendering capability.

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A Compact Random-Access Representation for Urban Modeling and Rendering

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  1. A Compact Random-Access Representation for Urban Modeling and Rendering ZhengzhengKuang Bin Chan Yikhou Yu Wenping Wang Glen Hoffing

  2. 1 Introduction • Problem – Rendering urban “City-Scapes” requires high storage capacity, transmission speed and rendering capability. • Solution – A highly memory efficient representation for modeling and rendering urban buildings composed mainly of rectangular block structures. Glen Hoffing

  3. 1 Three Goals of This Paper • Develop an efficient compression format for building representation (NUT) • Develop an efficient method for rendering NUTs on the GPU within the existing pipeline • Develop a semi-automatic tool to convert from mesh format to NUT format Glen Hoffing

  4. NUTPerformance – Figure 1 NUT Representation offers two orders of data compression (three orders in these examples) Glen Hoffing

  5. 3 What is it about Buildings that lend themselves to Data Compression? • Orthogonality (many right angles) • Many repetitions of the same elements (windows, floors, etc) • Element alignments are often horizontal or vertical • Surfaces are usually homogeneous (marble pattern, latex paint) Glen Hoffing

  6. 3 Modeling Building Structures • Hierarchical approach – Figure 2 • Locate rectangular regions of a building and identify the sets of parallel lines that align them • Recursively repeat this for the identified regions, until the cells at the lowest level represent areas with the same texture (material properties) • Allow child cells to be freely translated and rotated with respect to their parent cells. Glen Hoffing

  7. 3.1 What is a NUT? • NUT = Non-Uniform Texture • A completely image-based (pixel) structure without any mesh structures • Can be directly transmitted to client GPU for rendering without any prior conversion • Can be transmitted over Internet and rendered in high resolution at real-time frame rates Glen Hoffing

  8. 3.1 Generating NUTs • Result of hierarchical modeling is a collection of cells of varying size, each containing a single uniform texture • Because the textures are highly repetitive across cells, a limited number of texture maps is required. • Key– what is non-uniform in NUTs is not the texture of a cell, but the size of a cell. Don’t confuse the authors’ use of texture with a texture map. Their definition of a non-uniform texture applies to the pixel map, where in the NUT model different pixels represent physical areas of different size. Glen Hoffing

  9. Figure 5 Explains It All • Image is of a window structure for a building (5a) • To represent this conventionally without loss of resolution would require a 21x30 pixel grid (5b) • To represent this as a NUT would require a 7x7 pixel grid (5c). A visual representation of the compression achieved is shown in 5d (compare 5b to 5d). Note that the pixels are of varying size. The pixel representation of a NUT requires the ability to contain that sizing information. Glen Hoffing

  10. 4 NUT Representation • NUTs consist of NUT headers (Figure 7) and NUT data(Figure 8) • 4.1 NUT headers contain information on how to render NUT data, such as size, color and rotation angles. • 4.2 NUT data consist of 5 different 3D geometric shapes. With rotational permutations, there are a total of 69 shapes to choose from. Glen Hoffing

  11. 4 Hierarchical NUT Images (HNI) • The NUTs in a hierarchy are compiled into a 2D 32-bit HNI. The upper left 256x256 pixels (sized this way so the locating coordinates are 8-bit) are the metadata region that contain the NUTs. • Multiple buildings can be encoded in a single HNI to share common sub-blocks. Glen Hoffing

  12. 4.3 Texture Mapping • NUT model provides limited support of ordinary texture mapping • Since the NUT was originally generated to represent a region of a single material, each NUT is allowed only one texture, which is applied to all faces of the NUT Glen Hoffing

  13. 5 Modeling Tool (Auto-Conversion from mesh to NUT) • Three-step process (takes several minutes to several hours) • 1) Solid conversion – Identifies (x,y,z) coordinates of axis-aligned planes. Non-axis aligned mesh faces remain as mesh. • 2) Block extraction – User interaction required to select basic building blocks (e.g., windows, balconies) within candidate planes. Once identified, similar structures can be automatically identified at other locations. • 3) Simplification – Identified blocks of the same type are labeled with a unique id. Simplified grids of like items are marked and generated as NUTs. This mesh to NUT conversion is usually pretty quick. Glen Hoffing

  14. 6 Rendering from NUT Models • NUT models can be efficiently rendered without mesh reconstruction. • Ray-casting is run in a pixel-shader as with other GPU ray-casting algorithms. • HNI ray-casting algorithm iterates over two main steps: • 1) NUT traversal – Walks the hierarchical NUT structure within the HNI. • 2) Redirection landing – This is the additional processing that must take into account the different geometry of adjacent NUTs as the light ray reflects from one NUT to the next. Glen Hoffing

  15. 6.2 Examples • Figure 15 – 53 story apartment building with closeups of windows • Figure 16 – Car park and house from same model as Figure 15 • Figure 17a,b – distant and closeup view of European city model • Figure 17c – European style house modeled entirely in NUT. • Figure 17d – Church model is a hybrid NUT/mesh • Figure 17e – Detailed scene with shops and cafes. Entirely NUT • Figure 17f – Hospital modeled using NUT demonstrating rotational redirection Glen Hoffing

  16. 7 Comparisons • The authors compare their method to standard mesh models (7.1) grammar-based façade (GBEF) models (7.2) and distant viewing algorithms (7.3). • The GBEF comparison was difficult to make directly, but the authors estimate improvements in both storage and frame rates. They not that GBEF is only intended to model facades, while NUT is able to model whole buildings. Glen Hoffing

  17. 7.4 Limitations • Limitations to NUT model have been found: • 1) Odd-shaped structures such as the Sydney Opera House are not efficiently modeled wit NUTs • 2) Common building structures such as honeycombs and sky domes are more effectively modeled with meshes. Glen Hoffing

  18. 8 Conclusions • NUTs provide an effective method for modeling buildings and populating libraries of buildings for collaboration over the web. • Rendering the NUT structures requires computationally intensive ray casting. Although standard ray casting “keeps up”, there is little horsepower left in the GPU for other tasks. An adaptive ray caster optimized for NUTs would be “very useful”. • The mesh-to-NUT converter only works for mesh models, which are in turn very labor intensive. The authors plan to work on a tool that converts directly from photos or laser scans. Glen Hoffing

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