Blobby Modelling
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Blobby Modelling: Creating Organic Shapes through Implicit Surfaces, Slides of Fundamentals of E-Commerce
Blobby modelling is a technique used to create organic and non-linear shapes through implicit surfaces. It uses force functions to generate fields of force, which define the implicit surface in space. An in-depth explanation of how blobby modelling works, including several force functions and their implementations. It also discusses the advantages and disadvantages of using blobby modelling and introduces octrees as a method for finding the implicit surface. Examples of blobby modelling applications include organic forms, scientific modelling, and rapid prototyping.
Typology: Slides
2012/2013
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Blobby Modelling
What is it?
“Metaball, or ‘Blobby’, Modelling is atechnique which uses implicit surfaces toproduce models which seem more ‘organic’ or‘blobby’ than conventional models built fromflat planes and rigid angles”. --me
Examples--
“New Train” -Wyvill
Examples--
“Cabrit Model” - Wyvill
How does it work?
Each point in space generates a field of force,which drops off as a function of distance fromthe point.
A blobby model is formed from the shells ofthese force fields, the implicit surface whichthey define in space.
How does it work?
(Bourke 1997)
Several force functions work well. Examples: - “Blobby Molecules” - Jim Blinn - F(r) = a e -br 2
Here ‘b’ is related to the standard deviation of thecurve, and ‘a’ to the height.
How does it work?
(Bourke 1997)
Several force functions work well. Examples: - “Soft Objects” - Wyvill & Wyvill - F(r) = a(1 - 4r 6 /9b 6
- 17r 4 /9b 4
- 22r 2 / 9b 2 )
This function is basically the first few terms in the seriesexpansion of an exponential function.
‘a’ scales the function, and ‘b’ determines radius ofinfluence.
Advantage : rapid computation.
How does it work?
(Bourke 1997)
Force functions comparison:
How does it REALLY work?
Introducing : OCTREES.
An Octree is a recursive subdivision of spacewhich “homes in” on the surface, from largerto finer detail, and then uses similartechniques to Marching Cubes approximatethe implicit surface with polygons. - Octrees can display initial approximations ofthe surface immediately.
How does it REALLY work?
Because the octree is a cube in space, youevaluate the force function F(r) at each vertexof the cube.
This allows you to polygonalize the cube, inthe same manner as Marching Cubes. - To refine the polygonalization, you subdividethe cube into eight subcubes, discarding anychild whose vertices are all hot or all cold.
How does it REALLY work?
Recursive subdivision:
How does it REALLY work?
Recursive subdivision:
How does it REALLY work?
For each Octree with hot and cold corners,you can find the best-fitting polygons thatapproximate that surface. The edges of thepolygons pass through points linearlyinterpolated along the edges of the cube.
T = (0.5 - F(P1)) / (F(P2) - F(P1)) - P = P1 + T * (P2 - P1)
Pros and Cons
Benefits:
Very rapid general shapes - Allows rapid manipulation at multiple levels ofdetail - Surface complexity is not a function of datacomplexity - Enables a “poor man’s” solid geometry