CSc Computer Graphics Algorithm Lecture 20 Overview of 3D Modeling Techniques Ying Zhu Georgia State University

Outline ●The 3D computer graphics process ●The 3D modeling techniques –Surface models ● Polygon mesh models ●Subdivision surfaces ●Parametric curves and curved surfaces –Solid models –Procedural models

The 3D computer graphics process for movies ●The modelers create 3D models using modeling tools ●The animators create animations using animation tools –Eventually create frame-by-frame animation ●The animation sequence is sent to the rendering software for rendering, one frame of an image at a time –The images are saved into one or more video files ● Post-production, video editing, etc.

The 3D computer graphics process for games ●The modelers create 3D models –3D models are saved in model files ●Animators create animation sequences for each 3D object –Saved in a library of motion sequences ●Programmers write game programs that load and display the 3D models and animation sequences based on current game play –Game engine and game AI determines which model and which animation sequence to load

What is 3D modeling? ●3D modeling may include several activities –Edit the shape of individual objects –Edit the object surface material properties –Apply textures (texture mapping) –Apply bump-maps –Prepare a 3D model for animation ●E.g. fit an object with a skeleton for later animation ●Overall, 3D modeling is the process of creating a scene

How to do it? ●Use 3D scanning ●Use modeling and animation tools –E.g. Blender, 3DS Max, Maya, etc. ●Use scene description language –E.g. POV-Ray ●Use 3D graphics API (E.g. OpenGL, DirectX, etc.) –Procedural modeling ●Reconstruct 3D objects from a set images

Hybrid approach ●Get a real object or create a miniature object ●Scan the real object or the miniature object to obtain a 3D digital model ●Use modeling tools to edit and refine the 3D digital model

3D scanner High accuracy, long range scanner – Range up to 800m – Accuracy up to 5mm – Measurement rate up to pts/sec – Field of view up to 80 x 360 – Applications: topography, mining, architecture, etc. Image courtesy of Riegl USA

3D scanner Head and face scanner – Motion range: 0 – 360 degrees (cylindrical), 0 – 350mm (Y stage travel) – Applications: sculpture, VR, animation, medicine, etc. – Output file format: 3D Studio, PLY, OBJ, ASCII, etc. Image courtesy of Cyberware

How does 3D scanning work? A light scanner uses projects a laser stripe across the object to be scanned. The projected stripe is viewed by a camera so that height variations in the object can be seen as changes in the shape of the line. Each viewed stripe forms a profile which is built up from several hundred measured points.

How does 3D scanning work? Multiple profiles are captured and positioned in 3D space each second, resulting in a high accuracy high density 3D point cloud. This 3D point data can processed to become a polygon mesh Images courtesy of 3dscanner.com

Surface reconstruction case study ●The Digital Michelangelo Project. ●30 faculty, staff, and students from Stanford University and the University of Washington spent the academic year in Italy scanning the sculptures and architecture of Michelangelo. ●The motivations are to advance the technology of 3D scanning, to place this technology in the service of the humanities, and to create a long-term digital archive of some important cultural artifacts. ●More information at:

The Digital Michelangelo Project The largest dataset was of the David – 2 billion polygons and 7,000 color images. For each statue and map fragment, researchers must – clean up the raw scans – align them with each other – merge them to form a seamless mesh – fill holes in this mesh – map color photographic data onto it. Perhaps the biggest surface reconstruction project in history

The Digital Michelangelo Project ●The resulting meshes contain hundreds of millions of polygons, so even a simple task like displaying them on a computer is challenging. ●Need new ways to compress, transmit, and render large polygon meshes efficiently. ●At present, researchers have built a full-resolution model Michelangelo's St. Matthew and several medium-resolution models of the head of David. ●Scanview demonstration

Models from 3D scanning The Stanford 3D scanning repository – nrep/ nrep/ GA Tech large geometric models archive – _models/ _models/

Reconstruct objects from images ●How to reconstruct objects from 3D medical images (MRIs, CT images, etc.)? –Hand tracing contours of the object and then connect the contours to create 3D surface model –The marching cubes algorithm (Lorensen & Cline, SIGGRAPH 1987) ● –Many other algorithms ●This subject will be covered in my “Scientific Visualization” course

Polygon Mesh Models The polygon mesh data structure is the most common and oldest modeling method for computer graphics. "Perspective Study of a Chalice", by Paolo Uccelloin , Florence, Italy

Polygon Mesh Method ●A mesh is a collection of vertices and polygons that define the shape of an object ●Meshes usually contain only triangles because it's easier to deal with triangles than general polygons ●Benefits –Creating complex objects is simple. There are no limits to what a polygon mesh surface can do. ●Drawbacks –Need to store large numbers of points in order to get a smooth surface. –Also, if you want to animate that surface, you have to determine a transformation for each of those points.

Issues with polygon mesh model ●Model complexity vs. rendering speed ●You need more and smaller polygons to model fine details of an object ●But the more polygons you have in a scene, the slower the rendering ●There is usually an upper limit (called polygon budget) for the number of polygons per scene –Especially for real-time graphics applications that requires 30+ frame/sec frame rate

Subdivision surfaces ●The process of taking a low resolution polygon mesh and refining it to produce another shape that is more visually attractive (usually smooth) ●Subdivision Surfaces are defined by an initial polygonal mesh, along with a subdivision (or refinement) operation. ●Given a polygonal mesh, subdivision algorithm will generate a new mesh that has a greater number of polygonal elements, and is smoother

Subdivision surfaces

Parametric curves and surfaces ●Use mathematical formulations to describe curves and surfaces –E.g. a NURBS surface

Solid modeling ●Solid modeling describes the volume of space occupied by a solid ●Necessary for many applications where objects must be maintained as solids. –Finite element analysis –CAD/CAM –Mass property determination ●Every solid is defined and stored by its boundary, consists of faces, and includes the topology for each face

Procedural modeling ●Some irregular or complex objects have dynamic topologies ●Some objects have no solid surface –E.g. Smoke, water, etc. ●Usually we need to write programs to generate these objects –E.g. particle systems, fractals, and L-systems

Modeling Natural Phenomena Water (by Dr. Ron Fedkiw, et al.) –

Fire

Smoke

Cloud Cloud (by Dr. David Ebert) –

Plants and Flowers Plants and flowers (by Dr. Przemyslaw Prusinkiewicz, et al.) – on/materials/HyperGraph/model ing/procedural_modeling/plants _flowers/artificial_life_of_plants. htm on/materials/HyperGraph/model ing/procedural_modeling/plants _flowers/artificial_life_of_plants. htm

Terrain Fractal method for terrain modeling (by Dr. Ken Musgrave) –

Summary ●The 3D computer graphics process ●The 3D modeling techniques –Surface models ● Polygon mesh models ●Subdivision surfaces ●Parametric curves and curved surfaces –Solid models –Procedural models

Reference The Digital Michelangelo Project Download and try Blender

Next lecture ●Polygon mesh