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Презентация на тему Lemke’s Algorithm: The Hammer in Your Math Toolbox?

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First, a Word About Hammersrequirements for this to be a good ideaa way of transforming problems into nails (MLCPs)a hammer (Lemke’s algorithm)lots of advanced info + one hour = something has to givemajority of lecture is
Lemke’s Algorithm: The Hammer in Your Math Toolbox?Chris Hecker definition six, inc.checker@d6.com First, a Word About Hammersrequirements for this to be a good ideaa Hammers (cont.)by definition, not the optimal way to solve problems, BUTcomputers are What are “advanced game math problems”?problems that are ammenable to mathematical modelingstate Prerequisiteslinear algebravector, matrix symbol manipulation at leastcalculus conceptswhat derivatives meancomfortable with math notation and concepts Overview of Lecturerandom assortment of example problems breifly mentioned5 specific example problems A Look Forwardlinear equations Ax = b linear inequalities Ax >= blinear Applications to Games graphics, physics, ai, even uicomputational geometryvisibilitycontactcurve fittingconstraintsintegrationgraph theorynetwork floweconomicssite allocationgame theoryIKmachine learningimage processing Applications to Games (cont.)don’t forget...The Elastohydrodynamic Lubrication Problem Solving Optimal Ownership Structures“The Specific Examples #1a:  Ease Cubic Fittingwarm up with an ease curve Specific Examples #1a:   Ease Cubic Fitting (cont.)x(t)=at3+bt2+ct+d,   x’(t)=3at2+2bt+c Specific Examples #1a:   Ease Cubic Fitting (cont.)d = 0, a+b+c+d Specific Examples #1a:   Ease Cubic Fitting (cont.)or,x(0) = Specific Examples #1b:  Cubic Spline Fittingsame technique to fit higher order Specific Examples #1b:  Cubic Spline Fitting (cont.)a0b0c0d0a1b1c1d1...x0x100x1x200...=xi(ti)=xi    xi(ti+1)=xi+1 Specific Examples #2:  Minimum Cost Network Flowwhat is the cheapest flow Specific Examples #2:  Minimum Cost Network Flow (cont.)min cost: minimize cTxthe Specific Examples #3:  Points in Polyspoint in convex poly defined by Specific Examples #3:  Points in Polys (cont.)closest point in two polys Specific Examples #3:  Points in Polys (cont.)how do we stack x1,x2 Specific Examples #3:  Points in Polys (cont.)more points, more polys! min Specific Examples #4:  Contactmodel like IK constraints a = Af + Specific Examples #5:  Joint Limits in CCD IKhow to do child-child What Unifies These Examples?linear equations Ax = b linear inequalities Ax >= QP is a Superset of Mostquadratic programming min ½xTQx + cTx s.t. Karush-Kuhn-Tucker Optimality Conditions get us to MLCPfor QPform “Lagrangian” L(x,u,v) = ½ Karush-Kuhn-Tucker Optimality Conditions (cont.)L(x,u,v) = ½ xTQx + cTx - uT(Ax - This is an MLCPxvuQ  -DT -ATD   0 Modeling Summarya lot of interesting problems can be formulated as MLCPsmodel the Solving MLCPs (where I hope I made you hungry enough for homework)Lemke’s Playing Around With MLCPsPATH, a MCP solver (superset of MLCP!)really stoked professional References for Lemke, etc.free pdf book by Katta Murty on LCPs, etc.free Specific Examples #5:  Constraints for IKcompute “forces” to keep bones together Specific Examples #5:  Constraints for IK (cont.)multiple bodies gives coupling...a1a2A11 A12A21
Слайды презентации

Слайд 2 First, a Word About Hammers
requirements for this to

First, a Word About Hammersrequirements for this to be a good

be a good idea
a way of transforming problems into

nails (MLCPs)
a hammer (Lemke’s algorithm)
lots of advanced info + one hour = something has to give
majority of lecture is motivating you to care about the hammer by showing you how useful nails can be
make you hunger for more info post-lecture
very little on how the hammer works in this hour

“If the only tool you have is a hammer, you tend to see every problem as a nail.”
Abraham Maslow


Слайд 3 Hammers (cont.)
by definition, not the optimal way to

Hammers (cont.)by definition, not the optimal way to solve problems, BUTcomputers

solve problems, BUT
computers are very fast these days
often don’t

care about optimality
prepro, prototypes, tools, not a profile hotspot, etc.
can always move to optimal solution after you verify it’s a problem you actually want to solve


Слайд 4 What are “advanced game math problems”?
problems that are

What are “advanced game math problems”?problems that are ammenable to mathematical

ammenable to mathematical modeling
state the problem clearly
state the desired

solution clearly
describe the problem with equations so a proposed solution’s quality is measurable
figure out how to solve the equations
why not hack it?
I believe better modeling is the future of game technology development (consistency, not reality)

Слайд 5 Prerequisites
linear algebra
vector, matrix symbol manipulation at least
calculus concepts
what

Prerequisiteslinear algebravector, matrix symbol manipulation at leastcalculus conceptswhat derivatives meancomfortable with math notation and concepts

derivatives mean
comfortable with math notation and concepts


Слайд 6 Overview of Lecture
random assortment of example problems breifly

Overview of Lecturerandom assortment of example problems breifly mentioned5 specific example

mentioned
5 specific example problems in some depth
including one that

I ran into recently and how I solved it
generalize the example models
transform them all to MLCPs
solve MLCPs with Lemke’s algorithm

Слайд 7 A Look Forward
linear equations Ax = b
linear inequalities Ax

A Look Forwardlinear equations Ax = b linear inequalities Ax >=

>= b
linear programming min cTx s.t. Ax >= b, etc.
quadratic programming min

½ xTQx + cTx s.t. Ax >= b Dx = e
linear complimentarity problem a = Af + b a >= 0, f >= 0 aifi = 0

Слайд 8 Applications to Games graphics, physics, ai, even ui
computational geometry
visibility
contact
curve

Applications to Games graphics, physics, ai, even uicomputational geometryvisibilitycontactcurve fittingconstraintsintegrationgraph theorynetwork floweconomicssite allocationgame theoryIKmachine learningimage processing

fitting
constraints
integration
graph theory
network flow
economics
site allocation
game theory
IK
machine learning
image processing


Слайд 9 Applications to Games (cont.)
don’t forget...

The Elastohydrodynamic Lubrication Problem
Solving

Applications to Games (cont.)don’t forget...The Elastohydrodynamic Lubrication Problem Solving Optimal Ownership

Optimal Ownership Structures
“The two parties establish a relationship in

which they exchange feed ingredients, q, and manure, m.”

Слайд 10 Specific Examples #1a: Ease Cubic Fitting
warm up with

Specific Examples #1a: Ease Cubic Fittingwarm up with an ease curve

an ease curve cubic x(t)=at3+bt2+ct+d x’(t)=3at2+2bt+c
4 unknowns a,b,c,d (DOFs) we get

to set, we choose: x(0) = 0, x(1) = 1 x’(0) = 0, x’(1) = 0


1

x

t

0

0

1


Слайд 11 Specific Examples #1a: Ease Cubic Fitting (cont.)
x(t)=at3+bt2+ct+d,

Specific Examples #1a:  Ease Cubic Fitting (cont.)x(t)=at3+bt2+ct+d,  x’(t)=3at2+2bt+c x(0)

x’(t)=3at2+2bt+c
x(0) = a03+b02+c0+d = d =

0
x(1) = a13+b12+c1+d = a+b+c+d = 1
x’(0) = 3a02+2b0+c = c = 0
x’(1) = 3a12+2b1+c = 3a + 2b + c = 0

Слайд 12 Specific Examples #1a: Ease Cubic Fitting (cont.)
d

Specific Examples #1a:  Ease Cubic Fitting (cont.)d = 0, a+b+c+d

= 0, a+b+c+d = 1, c = 0, 3a

+ 2b + c = 0
a+b=1, 3a+2b=0
a=1-b => 3(1-b)+2b = 3-3b+2b = 3-b = 0
b=3, a=-2
x(t) = 3t2 - 2t3

Слайд 13 Specific Examples #1a: Ease Cubic Fitting (cont.)
or,
x(0)

Specific Examples #1a:  Ease Cubic Fitting (cont.)or,x(0) =

=

d = 0
x(1) = a + b + c + d = 1
x’(0) = c = 0
x’(1) = 3a + 2b + c = 0

0 0 0 1
1 1 1 1
0 0 1 0
3 2 1 0

x(0)
x(1)
x’(0)
x’(1)

a
b
c
d

0
1
0
0

=

=

Ax = b, a system of linear equations

(can solve for any rhs)


Слайд 14 Specific Examples #1b: Cubic Spline Fitting
same technique to

Specific Examples #1b: Cubic Spline Fittingsame technique to fit higher order

fit higher order polynomials, but they “wiggle”
piecewise cubic is

better “natural cubic spline”
xi(ti)=xi xi(ti+1)=xi+1 x’i(ti) - x’i-1(ti) = 0 x’’i(ti) - x’’i-1(ti) = 0
there is coupling between the splines, must solve simultaneously


x0





x1

x2

x3

t0

t1

t2

t3

4 DOF per spline
2 endpoint eqns per spline
4 derivative eqns for inside points
2 missing eqns = endpoint slopes


Слайд 15 Specific Examples #1b: Cubic Spline Fitting (cont.)

a0
b0
c0
d0
a1
b1
c1
d1
.
.
.
x0
x1
0
0
x1
x2
0
0
.
.
.
=







xi(ti)=xi

Specific Examples #1b: Cubic Spline Fitting (cont.)a0b0c0d0a1b1c1d1...x0x100x1x200...=xi(ti)=xi  xi(ti+1)=xi+1 x’i(ti) -

xi(ti+1)=xi+1 x’i(ti) - x’i-1(ti) = 0 x’’i(ti) - x’’i-1(ti)

= 0









































.
.
.

Ax = b, a system of
linear equations










Слайд 16 Specific Examples #2: Minimum Cost Network Flow
what is

Specific Examples #2: Minimum Cost Network Flowwhat is the cheapest flow

the cheapest flow route(s) from sources to sinks?
model, want

to minimize cost cij = cost of i to j arc bi = i’s supply/demand, sum(bi)=0 xij = quantity shipped on i to j arc x*k = sum(xik) = flow into k xk* = sum(xki) = flow out of k
flow balance: x*k - xk* = -bk
one-way streets: xij >= 0


Слайд 17 Specific Examples #2: Minimum Cost Network Flow (cont.)
min

Specific Examples #2: Minimum Cost Network Flow (cont.)min cost: minimize cTxthe

cost: minimize cTx
the sum of the costs times the

quantities shipped (cTx = c ·x)
flow balance is coupling: matrix x*k - xk* = -bk

xac
xad
xae
xba
xbc
xbe
xdb
.
.
.

= -

-1 -1 -1 1 0 0 0 0 1 0…
0 0 0 -1 -1 -1 1 …
...

ba
bb
bc
bd
.
.
.

minimize cTx
subject to
Ax = -b
x >= 0
a linear programming problem


Слайд 18
Specific Examples #3: Points in Polys
point in convex

Specific Examples #3: Points in Polyspoint in convex poly defined by

poly defined by planes n1 · x >= d1 n2 ·

x >= d2 n3 · x >= d3
farthest point in a direction in poly, c:


n1

n2

n3


x

Ax >= b,
linear inequality

min -cTx
s.t. Ax >= b
linear programming


Слайд 19
Specific Examples #3: Points in Polys (cont.)
closest point

Specific Examples #3: Points in Polys (cont.)closest point in two polys

in two polys min (x2-x1)2 s.t. A1x1 >= b1

A2x2 >= b2
stack ‘em in blocks, Ax >= b


n1

n2


x1

n3


x2

x1
x2

x =

A1 A2

A =

what about (x2-x1)2, how do we stack it?

b1
b2

b =


Слайд 20 Specific Examples #3: Points in Polys (cont.)
how do

Specific Examples #3: Points in Polys (cont.)how do we stack x1,x2

we stack x1,x2 into single x given (x2-x1)2 = x22-2x2•x1+x12


x1
x2

x1T x2T

1 -1
-1 1

= x22-2x2 • x1+x12 = xTQx

min xTQx
s.t. Ax >= b

a quadratic programming problem

x2 = xTx = x · x
1 = identity matrix



Слайд 21 Specific Examples #3: Points in Polys (cont.)
more points,

Specific Examples #3: Points in Polys (cont.)more points, more polys! min

more polys! min (x2-x1)2 + (x3-x2)2 + (x3-x1)2








x1
x2
x3

x1T x2T x3T

2 -1 -1
-1 2 -1
-1 -1 2

min xTQx
s.t. Ax >= b
another quadratic programming problem

same form for all these poly problems
never specified 2d, 3d, 4d, nd!

= xTQx


Слайд 22 Specific Examples #4: Contact
model like IK constraints a =

Specific Examples #4: Contactmodel like IK constraints a = Af +

Af + b a >= 0, no penetrating f >= 0,

no pulling aifi = 0, complementarity (can’t push if leaving)


f1

f2

a1

a2



f1

f2

a1

a2

linear complementarity problem

it’s a mixed LCP if some ai = 0, fi free, like for equality constraints


Слайд 23 Specific Examples #5: Joint Limits in CCD IK
how

Specific Examples #5: Joint Limits in CCD IKhow to do child-child

to do child-child constraints in CCD?
parent-child are easy, but

need a way to couple two children to limit them relative to each other
how to model this & handle all the cases?
define dn= gn - an
min (x1 - d1)2 + (x2 - d2)2
s.t. c1min <= a1+x1 - a2-x2 <= c1max
parent-child are easy in this framework: c2min <= a1+x1 <= c2max
another quadratic program: min xTQx s.t. Ax >= b









a1

g1

a2

g2

a1

g1


Слайд 24 What Unifies These Examples?
linear equations Ax = b
linear

What Unifies These Examples?linear equations Ax = b linear inequalities Ax

inequalities Ax >= b
linear programming min cTx s.t. Ax >= b, etc.
quadratic

programming min ½ xTQx + cTx s.t. Ax >= b Dx = e
linear complimentarity problem a = Af + b a >= 0, f >= 0 aifi = 0

Слайд 25 QP is a Superset of Most
quadratic programming min ½xTQx

QP is a Superset of Mostquadratic programming min ½xTQx + cTx

+ cTx s.t. Ax >= b Dx

= e

linear equations
Ax = b
Q, c, A, b = 0
linear inequalities
Ax >= b
Q, c, D, e = 0
linear programming
min cTx s.t. Ax >= b, etc.
Q, etc. = 0

but MLCP is a superset of convex QP!


Слайд 26 Karush-Kuhn-Tucker Optimality Conditions get us to MLCP
for QP
form

Karush-Kuhn-Tucker Optimality Conditions get us to MLCPfor QPform “Lagrangian” L(x,u,v) =

“Lagrangian” L(x,u,v) = ½ xTQx + cTx - uT(Ax -

b) - vT(Dx - e)
for optimality (if convex): ∂L/ ∂x = 0 Ax - b >= 0 Dx - e = 0 u >= 0 ui(Ax-b)i = 0
this is related to basic calculus min/max f’(x) = 0 solve

min ½ xTQx + cTx s.t. Ax - b >= 0 Dx - e = 0


Слайд 27 Karush-Kuhn-Tucker Optimality Conditions (cont.)
L(x,u,v) = ½ xTQx +

Karush-Kuhn-Tucker Optimality Conditions (cont.)L(x,u,v) = ½ xTQx + cTx - uT(Ax

cTx - uT(Ax - b) - vT(Dx - e)

y = ∂L/ ∂x = Qx + c - ATu - DTv = 0, x free
w = Ax - b >= 0, u >= 0, wiui = 0
s = Dx - e = 0, v free

x
v
u

Q -DT -AT
D 0 0
A 0 0

=

y
s
w

+

c
-e
-b

y, s = 0
x, v free
w, u >= 0
wiui = 0


Слайд 28 This is an MLCP
x
v
u

Q -DT -AT
D

This is an MLCPxvuQ -DT -ATD  0  0A

0 0
A 0

0

=

y
s
w

+

c
-e
-b

y, s = 0
x, v free
w, u >= 0
wiui = 0

a

=

A

f

b

+

aifi = 0

some a >= 0, some = 0
some f >= 0, some free
(but they correspond so complementarity holds)


Слайд 29 Modeling Summary
a lot of interesting problems can be

Modeling Summarya lot of interesting problems can be formulated as MLCPsmodel

formulated as MLCPs
model the problem mathematically
transform it to an

MLCP
on paper or in code with wrappers
but what about solving MLCPs?

Слайд 30 Solving MLCPs (where I hope I made you hungry

Solving MLCPs (where I hope I made you hungry enough for

enough for homework)
Lemke’s Algorithm is only about 2x as

complicated as Gaussian Elimination
Lemke will solve LCPs, which some of these problems transform into
then, doing an “advanced start” to handle the free variables gives you an MLCP solver, which is just a bit more code over plain Lemke’s Algorithm

Слайд 31 Playing Around With MLCPs
PATH, a MCP solver (superset

Playing Around With MLCPsPATH, a MCP solver (superset of MLCP!)really stoked

of MLCP!)
really stoked professional solver
free version for “small” problems
matlab

or C
OMatrix (Matlab clone) free trial (omatrix.com)
only LCPs, but Lemke source is in trial
not a great version, but it’s really small (two pages of code) and quite useful for learning, with debug output
good place to test out “advanced starts”
my Lemke’s + advanced start code
not great, but I’m happy to share it
it’s in Objective Caml :)

Слайд 32 References for Lemke, etc.
free pdf book by Katta

References for Lemke, etc.free pdf book by Katta Murty on LCPs,

Murty on LCPs, etc.
free pdf book by Vanderbei on

LPs
The LCP, Cottle, Pang, Stone
Practical Optimization, Fletcher
web has tons of material, papers, complete books, etc.
email to authors
relatively new math means authors are still alive, bonus!

Слайд 34 Specific Examples #5: Constraints for IK
compute “forces” to

Specific Examples #5: Constraints for IKcompute “forces” to keep bones together

keep bones together a1 = A11 f1 + b1 a1 :

relative acceleration at constraint f1 : force at constraint b1 : external forces converted to accelerations at constraints A11 : force/acceleration relation matrix





f1

fe


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