Ja Min Jeong
2013/03/22
Korea University
Computer Graphics Lab.
Jing Liao · Jinhui Yu
Visual Computer 2012
Korea University Computer Graphics Lab.
Ja Min Jeong | 2013/03/22 | # 2 KUCG |
Abstract
• For animating cracks and fractures
Input • a 2D hand drawn object
• Minimum user intervention
Generate cartoon cracks and fractures animations procedurally • Generate the Voronoi textures on the 2.5D object model
‗ Visual abstraction of cartoon cracks
• Cracking gaps are widened progressively until Voronoi cells split apart
• Fall onto ground according to simplified physical rules
Korea University Computer Graphics Lab.
Ja Min Jeong | 2013/03/22 | # 3 KUCG |
Overview
Result
Algorithm Overview
Input Image and User Intervention
Korea University Computer Graphics Lab.
Ja Min Jeong | 2013/03/22 | # 4 KUCG |
1. Introduction
• Animating cartoon crack
Requires animators to draw many broken pieces in a frame
Animate them with multiple frames in a visually convincing manner
Hand-drawn crack and fracture effects
Korea University Computer Graphics Lab.
Ja Min Jeong | 2013/03/22 | # 5 KUCG |
1. Introduction
• Some approaches of physically based modeling of crack fractures on 3D objects
These methods focus on realistic effects
Cannot be used straightforwardly in cartoon animation because of inconsistency in visual style
We have to seek for different solutions to animate crack fractures in cartoon style, using just 2D painted objects and the background
Korea University Computer Graphics Lab.
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1. Introduction
• Pursuing this goal therefore imposes the following challenges different from those in 3D realistic crack fracture animation
1. It is very difficult to reconstruct 3D models from 2D cartoon drawings in general
2. Introduce 3D information such as thickness to broken shapes
3. Object shadows on the background
Korea University Computer Graphics Lab.
Ja Min Jeong | 2013/03/22 | # 7 KUCG |
2. Related work
• Physical approach Crack patterns
• Hirota et al. ‗ Crack in surface ‗ Crack in volume
• Gobron and Chiba ‗ Cellular automata
• Paquette et al. ‗ Paint cracking and peeling were also simulated using a two-layered
model on a 2D grid
• Federl and Prusinkiewicz ‗ Wedge-shaped finite elements ‗ Model cracks formed by drying mud and tree bark
• Iben and O’Brien ‗ Finite elements method ‗ Generate cracks from a stress field
Korea University Computer Graphics Lab.
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2. Related work
• Physical approach
Simulate objects that are being broken or torn apart • Terzopoulos et al
‗ elastic deformation with mass–spring systems
• Terzopoulos and Fleischer ‗ plastic deformation and fracture
‗ spring stretches beyond its elastic limit, it breaks
• Norton et al & Mazarak et al ‗ similar approaches by attaching voxels together with springs
‗ local pressure or force exceeds a designated yield limit
Korea University Computer Graphics Lab.
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2. Related work
• Physical approach
Mass–spring systems, Finite Elements Method • Brittle fracture
• Ductile fracture
• Elasto-plastic materials & Interactive fracture
• Fracture and deformation of voxelized surface meshes
Other algorithms • Generating fracture on elastic and plastic materials with
the virtual node algorithm
• A membrane-bending model for thin shell objects
• A meshless framework
Korea University Computer Graphics Lab.
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2. Related work
• Non-physical approach • Physical models
‗ be slow and difficult to control
• Non-physical models ‗ Quicker and easy to control
Voronoi-based methods ‗ Crack : Voronoi boundaries ‗ Progress of a crack : Voronoi network
Raghavachary Mould Wyvill et al.
Korea University Computer Graphics Lab.
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3. Surface and volume approximation
• Our system the input object is drawn on 2D
• Just breaking the drawn object into 2D broken pieces could not show the volumetric appearance
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3. Surface and volume approximation
• Construct the front and back half of the input object to approximate its surface area instead.
• This region is assigned the color of the original image on the non-shadow region
Fig. 3 2.5D model of the input object
FH : front half BH : back half
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3. Surface and volume approximation
• Two images of FH and BH are stored in two layers with depth order assigned as a 2.5D model
• Hand-drawing many broken pieces involved in cartoon crack fractures
• Reduces the animator’s workload Interactive segmentation and construction of FH and BH
• Volumetric appearance Introduce thickness to FH and BH (Sect. 6.)
Fig. 7 Modeling 3D fragment
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Ja Min Jeong | 2013/03/22 | # 14 KUCG |
4. Voronoi textures
• Voronoi diagram to simulate crack textures appearing in hand-drawn crack fractures
• VP(pi) : defined to be the set of all points q in the plane for which pi is among the closest point to q in P
P : Points N : Number
VP(pi) : Voronoi polygon VD(pi) : Voronoi Diagram
Fig. 4 Voronoi diagram (solid) and Delaunay triangulation (dot)
Korea University Computer Graphics Lab.
Ja Min Jeong | 2013/03/22 | # 15 KUCG |
4. Voronoi textures
• Algorithms for constructing the Voronoi diagram
Adopt the simple algorithm by Lischinski • The incremental construction of the Delaunay triangulation
and the Voronoi diagram
Synthesize a Voronoi diagram on two layers of FH and BH
The VD(P ) constructed above only models underlying structures of the cartoon crack fractures
Korea University Computer Graphics Lab.
Ja Min Jeong | 2013/03/22 | # 16 KUCG |
5. Modeling cracking
• Cracking is a process that usually starts from some positions where the object is hit cracks advance on the object (crack advancing) Gaps of cracks widen progressively (gap widening)
Fig. 5 Algorithm of dynamic cracking texture
1. Several Voronoi vertices are chosen by the user as activating seeds
2. The cracking line zigging along its corresponding edge at a certain speed
3. When a cracking line reaches an inactivated vertex 1. Became a new activating seed
2. Trigs more cracking lines
Korea University Computer Graphics Lab.
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5. Modeling cracking
• Cracking propagation process terminates All cracking lines have reached the contours
Vertices activated previously
• Crack widening The earlier emerged crack lines are wider than those new ones
Model this by assigning age to the cracking line width • The older, The wider
• Cracks animating starts FH BH
Fig. 6 Cracking texture animated in three different frames
line widening W0 : the initial width W : speed t : time.
L0 : the initial length L : speed t : time.
Korea University Computer Graphics Lab.
Ja Min Jeong | 2013/03/22 | # 18 KUCG |
6. Modeling fracturing
• Second phase begins When cracking lines transverse the whole object Object shatters into small fragments which then fall on other objects
such as floor and ground
• During the falling period Fragments may interact with each other under the gravity Causing rotation, Collision and Piling up Fragments are just 2D shapes of Voronoi cells
• Can not be used realistic rotations, collision and piling up of fragments
• Add thickness to each fragment Construct a 3D fragment by copying a Voronoi cell Move it along the direction of the cell normal to a distance denoted by d
• equals the thickness of the fragment specified by the user
Fig. 7 Modeling 3D fragment
Korea University Computer Graphics Lab.
Ja Min Jeong | 2013/03/22 | # 19 KUCG |
6. Modeling fracturing
• Thickness of the fragments Should be increased so that they appear reasonably bigger
than the thinner fragments derived from some objects such as bowls
Simply increased the thickness of fragments • Center : d, Size : 1.5d
• All fragments Are made of the same materials and the same density Fall as the solid bodies to the ground plane under gravity Elastic collision or inelastic collision according to the material
defined
• Implementation Fragments falling, colliding and piling up Physx library
Korea University Computer Graphics Lab.
Ja Min Jeong | 2013/03/22 | # 20 KUCG |
6. Modeling fracturing
• No 3D information available For calculating the collision between falling fragments
and the ground Set up a virtual 3D ground plane
• Physx Set some mechanical characteristics
• Density of the object • Gravity • Restitution parameter • Friction parameter
The position of every fragment in each frame Colliding detection
• If you want to speed up : using bounding box
Korea University Computer Graphics Lab.
Ja Min Jeong | 2013/03/22 | # 21 KUCG |
7. Treatment of shadows
• Shadows of broken objects
Cracking phase • Objects just begin to crack
• Objects still remain their shapes
• No shadow animation is required
Fracturing phase • Objects break down into pieces
• Shadows of broken pieces vary as they fall
• Shadow animation can be done with the traditional shadow map algorithm
Korea University Computer Graphics Lab.
Ja Min Jeong | 2013/03/22 | # 22 KUCG |
8. Result
• 2.5 GHz Pentium PC with 2048 MB of RAM.
• Microsoft Visual C++ and OpenGL Libraries.
Fig. 8 Cracking and fracturing of a bowl
Korea University Computer Graphics Lab.
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8. Result
Fig. 9 Cracking and fracturing of a house
8 Frame
Korea University Computer Graphics Lab.
Ja Min Jeong | 2013/03/22 | # 24 KUCG |
8. Result
Fig. 10 Cracking and fracturing of a cartoon character
Material : Mud
Korea University Computer Graphics Lab.
Ja Min Jeong | 2013/03/22 | # 25 KUCG |
9. Conclusion
• Present a theme of modeling cracking and fracturing effects for cartoon animation
• The first attempt to model cracking and fracturing effects for 2D cartoon objects
• Limitation
Cannot be seen from arbitrary views in 3D • only reconstruct the 2.5D model for cartoon objects