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CGT 581 – G Procedural MethodsFractalsBedrich Benes, Ph.D.Purdue UniversityDepartment of Computer Graphics Technology
© Bedrich Benes
Procedural Techniques• Model is generated by a piece of code.
• Model is not represented as data!
• The generation can take some time
• and of course the data can be pre‐calculated
© Bedrich Benes
Procedural TechniquesThree huge (vague) classes:• Fractals• Particle systems• GrammarsUsed when shape cannot be represented as a surface (fire, water, smoke, flock of birds, explosions, model of mountain, grass, clouds, plants, facades, cities, etc.)
Used for Simulation of Natural Phenomena
© Bedrich Benes
The Mandelbrot Set – the big logoThe Mandelbrot set• Discovered in 1970 by Benoit Mandelbrot
• It is a non‐linear deterministic fractal
• It is graph of a solution of a dynamic systemBenoit Mandelbrot
© Bedrich Benes
The Mandelbrot SetTake the equation
where:and are complex numbers and
Explore complex numbers from the complex plane Measure the speed of divergence of the i.e., measure when (predefined value)
© Bedrich Benes
The Mandelbrot Set
there are two kinds of points:for , (stable points)for certain and greater (unstable points)
© Bedrich Benes
The Mandelbrot Seta) stable points displayed in blackb) unstable points
for every point c in the plane <-2-2i>, <2+2i> doset z=0+i0set n=0while (n<MAX) and (|z|<2) do
z=z2+cend of whileif (n==MAX) Draw Point(Black)
else Draw Point(Color(n))end of for
© Bedrich Benes
The Mandelbrot SetZooming into the Mandelbrot set
© Bedrich Benes
The Mandelbrot SetZooming into the Mandelbrot set
© Bedrich Benes
The Mandelbrot SetZooming into the Mandelbrot set
© Bedrich Benes
The Mandelbrot SetZooming into the Mandelbrot set
© Bedrich Benes
The Mandelbrot SetZooming into the Mandelbrot set
© Bedrich Benes
The Mandelbrot Set
http://www.youtube.com/watch?v=0jGaio87u3A
© Bedrich Benes
DimensionsDimension ‐ how long is the coast of Corsica?A stick of 500m will give 700km (not very precise)A stick of 100m will give 1200km (?)
What if we use length equal to ?The length will be…
© Bedrich Benes
DimensionsHow long is the coast of Corsica?applied sticks of length timesthe total length is stick of the length and we need piecesif we need
© Bedrich Benes
Dimensions
In the case of Corsica we need to use formula
to get reasonable results…
What is this “D”? (Richardson)
© Bedrich Benes
DimensionsHow long is the coast of Corsica?
Having we get
with (i.e., getting the stick shorter) we get
→© Bedrich Benes
Dimensions
For a line segment we need to set it the case of Corsica needs to be non‐integer…
The D is so called Hausdorff dimensionSometimes called fractal dimensionFractals (usually) do not have dimensions
© Bedrich Benes
FractalsFractalsfractal is a set that is self‐similar,
it is a set that is copy of itselfhere is transformation scales downScale must be average contraction
Fractal is a set that has Hausdorff’s dimension greater than its dimension topological
© Bedrich Benes
FractalsVaguely
Fractal is a set that is geometrically complex and its structure is given by repetition of a certain shape at different scales.
© Bedrich Benes
Fractals Classification
Linear Deterministic
Non-linear Non-deterministic
Fractals
© Bedrich Benes
Fractals Classification• Deterministic (or exact) fractals• Non‐deterministic or random fractalsDepends on the type of self‐similarity (exact, random)
• Linear – rotation, scale, translation, shear• Non‐linear – the othersDepends on the used transformations
© Bedrich Benes
DimensionsSpecial casesIf we use just ONE transformation with fixed and repetitionsthe fractal dimension is
© Bedrich Benes
Linear Deterministic FractalsThe Cantor’s discontinuum(published in 1883)
(Georg Cantor - 1845 - 1918)
1) Take a line segment2) Sale it 1/3x3) Make two copies at the ends4) Do it recursively
© Bedrich Benes
Linear Deterministic FractalsWhat is the dimension of the Cantor’s discontinuum? scale
new piecesfor this we can use formula
this object is something between a point and a line
© Bedrich Benes
Linear Deterministic FractalsThe Koch’s snowflake
Scale , , Properties: does not have derivativeinfinite lengthzero areathe area inside is non‐zero and non‐infinity
(Helge von Koch 1870 –1924)
1) Take a line segment2) Erase the mid third (similar to Cantor)3) Create equilateral triangle4) Repeat this recursively
© Bedrich Benes
Linear Deterministic FractalsThe Sierpinsky gasket
not plane, not lineNote the similarity with Pascal triangle
1) Take a triangle2) Erase mid triangle3) Repeat this recursively
(Wacław Franciszek Sierpiński 1882-1969)
© Bedrich Benes
Linear Deterministic FractalsThe Menger sponge
Scale
Dust of points.
1) Take a cube2) Erase mid cross of cubes3) Repeat this recursively
(Karl Menger 1902 - 1985)
© Bedrich Benes
Linear Deterministic FractalsThe Peano curve
space filling curve
1) Take a line segment 2) Substitute it by the lines from the image3) Repeat recursively on each line segment
(Guiseppe Peano 1858-1932)
© Bedrich Benes
Non‐Linear Deterministic FractalsQuaternionsif we apply the formula
in quaternion space(hypercomplex numbers ‐ 4D)
© Bedrich Benes
Non‐Linear Deterministic Fractals
© Daniel White © Bedrich Benes
Linear Non‐Deterministic FractalsInvolve random numbers and linear transformationsMotivation:What would you hear if you increase the speed of playing music? Noise?The speed change is scaling
In other words:How large is this stone?10cm?1m?10m?
© Bedrich Benes
Linear Non‐Deterministic Fractals
© Bedrich Benes
Linear Non‐Deterministic FractalsThe way to model random features in natureis capturing the self‐similarity with stochastic processes
The key for these modelsare noise functionsNoise is (usually) self‐similar
© Bedrich Benes
Brownian MotionBrownian motion (Bm)Particles of pollen in water
They move because of random hits of molecules of water
This is also special case of the random walk
Bm app
© Bedrich Benes
Brownian Motion1D Bm simulation:Random hits have Gaussian random number distribution
© Bedrich Benes
Brownian MotionHow to get Gaussian Random numbers?
the rand() function generates
suppose we have of such numbers the Gaussian random number is:
© Bedrich Benes
N=1 N=2 N=3 N=4 N=5 N=10
Brownian Motiondetermines how steep the curve is
Gauss app
© Bedrich Benes
Brownian MotionWhite noise is a random function with a constant power density
Discrete samples are uncorrelated with and
Gaussian white noise is one of them
© Bedrich Benes
Brownian Motion1D Bm simulation (contd.):
Let’s have that moves a point in the directionit has Gaussian random numbers distribution
© Bedrich Benes
Brownian Motionthe Bm (Brownian function) is
we accumulate the perturbations
W(t)
X(t)The fractal dimension is D=1.5
Bm app
© Bedrich Benes
Fractional Brownian Motion (fBm)fBmif we scale the Bm in the axis by coefficient we have to scale in the y axis by ,where is so called then the fractal dimension of the curve is
this curve is called Fractional Brownian Motion or fBm
(this is NOT Fractal Brownian Motion)
© Bedrich Benes
Fractional Brownian Motion (fBm)fBm• fBm has i.e., it is Bm• fBm has • fBm has higher D means “wilder”
H=0.5 autocorrelation is 0H<0.5 autocorrelation is positiveH>0.5 autocorrelation is negative
© Bedrich Benes
Fractional Brownian Motion (fBm)
© Bedrich Benes
Linear Non‐Deterministic Fractals
• In order to generate nice fractals, we need fBm
• The above described function is good
• But it does not provide adaptive results
© Bedrich Benes
The Midpoint Displacement Algorithm• the MDP is the key algorithm for fractals
1) Take a line segment2) Find its midpoint (center)3) Move it randomly in the direction4) Apply this step to all lines recursively
© Bedrich Benes
The Midpoint Displacement Algorithm• The random numbers distribution has its and .• Scale down the by two in every iteration.• Divide the Gaussian random numbers by two
© Bedrich Benes
The Midpoint Displacement Algorithm• If we divide in each step by
is the Hurst exponent
• we will get fBm with dimension
© Bedrich Benes
The Midpoint Displacement AlgorithmProperties• easy to implement• it is interpolation of two points, so it is also called
Fractal interpolation
• just division by two and random numbers call• quite realistic
© Bedrich Benes
The Midpoint Displacement AlgorithmTwo dimensional fBmworks with 2D arraysit is called fBm surfacedimension is
,
where so the
© Bedrich Benes
The Midpoint Displacement Algorithm2D fBm on quadrilaterals (diamond‐square)having four points P0,P1,P2,P3 1) Evaluate the midpoint2) Evaluate points on the edges3) Evaluate points in the middle4) Ad 2) etc...
© Bedrich Benes
The Midpoint Displacement Algorithm• 2D fBm on quadrilaterals (diamond‐square)
MDP app
© Bedrich Benes
Random FaultsAnother method for generating fBm1. take an object2. divide it into two parts3. increase the elevation of one and decrease the
other one (change color, etc.)4. apply this step many times5. in each decrease the intensity of changes 2x6. the limit of this process is fBm
© Bedrich Benes
Random Faults
Cuda random faults
© Bedrich Benes
Random FaultsImplementationData: 2D arrayAlgorithm:Do this many times:
1) Generate random line (get two random points)2) For each point in the 2D array
I. Check if it is on the left or right (dot product)II. Increase/decrease the value
© Bedrich Benes
Random FaultsVariationsUse any 2D object (circle, square)Randomly alter the area inside/outside
© Bedrich Benes
Random Faults on a Sphere1) Take sphere and divide it randomly into two hemispheres
2) Assign to each hemisphere different color
3) Apply the step 1 recursively, but decrease the intensity in each step with
© Bedrich Benes
Random Faults on a Sphere
© Bedrich Benes
Perlin NoiseKen Perlin 1985, 2002Perlin, K. (1985). An image synthesizer. ACM Siggraph Computer Graphics, 19(3), 287‐296.
Perlin, K. (2002). Improving noise. ACM Trans. Graph. 21, 3 (July 2002), 681‐682.
Academy Award for Technical Achievement 1997
© Bedrich Benes
Perlin Noise• Properties:
• Statistically invariant to rotation and translation• Continuous• Perlin Noise is 1D, 2D, 3D, … nD, • Limited frequency spectrum• Repeatable (returns the same value for the parameters)
double noise(double t) //1D casedouble noise(double x, double) //2D casedouble noise(double x, double, y, double z)//3D case
© Bedrich Benes
Perlin Noise• Limited frequency spectrum means we can chose
the maximum desirable detail
• Returns value from
• It is seeded by the input parameter
© Bedrich Benes
Perlin Noise ‐ Algorithm• Initialization
• Divide the space to equally distributed cells with integer coordinates
• Each cell has a predefined pseudorandom gradient
© Matt Zucker
© Matt Zucker
© Bedrich Benes
Perlin Noise ‐ Algorithm• Call:
• By calling with non‐integer coordinates , ,• Find the corresponding cell , , , ,• Calculate the vector from, , to each vertex• The influence of each vertex
is given by its distance• Sum the contributions by
using sigmoid weighting function3 2• For large tables hash function is used
© Matt Zucker
© Matt Zucker
© Bedrich Benes
Perlin Noise
© Bedrich Benes
Summing Noise Functions
where
is called persistenceis called noise frequency
© Bedrich Benes
Summing Noise Functions• Maintains the overall shape• Add details and higher frequencies
© Bedrich Benes
Summing Noise Functionsoctaves=1 octaves=2
octaves=7octaves=3
© Bedrich Benes
Summing Noise Functions
© Paul Bourke
© Bedrich Benes
Turbulence• Start with a simple color ramp
© Bedrich Benes
Turbulence ‐ marble• Perturb it by Perlin noise
defines the influence (amplitude) of each octave
3 6 10 octaves
© Bedrich Benes
Turbulence ‐ wood• Subtract the integer part of Perlin noise:
© Bedrich Benes
Hypertexture• Defines properties close to the object surface• It is a procedural definition, just perturbs the space• Can be displayed by raycasting
© Bedrich Benes
Hypertexture
© Ken Perlin © Bedrich Benes
libnoise
http://libnoise.sourceforge.net/index.html
© Bedrich Benes
Diffusion Limited Aggregation – DLA1) Particles (molecules) are floating in the water2) When a particle approaches
an condensation centerit is aggregated
3) The process is repeated for many particles
The important part is the way the particles traveli.e., random walkCorals, lightings, frozen ice on a window, etc.
© Bedrich Benes
Diffusion Limited Aggregation – DLA
© Bedrich Benes
Diffusion Limited Aggregation – DLA
© Bedrich Benes
Diffusion Limited Aggregation – DLA
Corals app