% Section 8.4.1, Boyd & Vandenberghe "Convex Optimization"
% Original version by Lieven Vandenberghe
% Updated for CVX by Almir Mutapcic - Jan 2006
% (a figure is generated)
%
% We find the ellipsoid E of maximum volume that lies inside of
% a polyhedra C described by a set of linear inequalities.
%
% C = { x | a_i^T x <= b_i, i = 1,...,m } (polyhedra)
% E = { Bu + d | || u || <= 1 } (ellipsoid)
%
% This problem can be formulated as a log det maximization
% which can then be computed using the det_rootn function, ie,
%     maximize     log det B
%     subject to   || B a_i || + a_i^T d <= b,  for i = 1,...,m

% problem data
n = 2;
px = [0 .5 2 3 1];
py = [0 1 1.5 .5 -.5];
m = size(px,2);
pxint = sum(px)/m; pyint = sum(py)/m;
px = [px px(1)];
py = [py py(1)];

% generate A,b
A = zeros(m,n); b = zeros(m,1);
for i=1:m
  A(i,:) = null([px(i+1)-px(i) py(i+1)-py(i)])';
  b(i) = A(i,:)*.5*[px(i+1)+px(i); py(i+1)+py(i)];
  if A(i,:)*[pxint; pyint]-b(i)>0
    A(i,:) = -A(i,:);
    b(i) = -b(i);
  end
end

% formulate and solve the problem
cvx_begin
    variable B(n,n) symmetric
    variable d(n)
    maximize( det_rootn( B ) )
    subject to
       for i = 1:m
           norm( B*A(i,:)', 2 ) + A(i,:)*d <= b(i);
       end
cvx_end

% make the plots
noangles = 200;
angles   = linspace( 0, 2 * pi, noangles );
ellipse_inner  = B * [ cos(angles) ; sin(angles) ] + d * ones( 1, noangles );
ellipse_outer  = 2*B * [ cos(angles) ; sin(angles) ] + d * ones( 1, noangles );

clf
plot(px,py)
hold on
plot( ellipse_inner(1,:), ellipse_inner(2,:), 'r--' );
plot( ellipse_outer(1,:), ellipse_outer(2,:), 'r--' );
axis square
axis off
hold off

 
Calling Mosek 9.1.9: 34 variables, 15 equality constraints
   For improved efficiency, Mosek is solving the dual problem.
------------------------------------------------------------

MOSEK Version 9.1.9 (Build date: 2019-11-21 11:32:15)
Copyright (c) MOSEK ApS, Denmark. WWW: mosek.com
Platform: MACOSX/64-X86

Problem
  Name                   :                 
  Objective sense        : min             
  Type                   : CONIC (conic optimization problem)
  Constraints            : 15              
  Cones                  : 6               
  Scalar variables       : 24              
  Matrix variables       : 1               
  Integer variables      : 0               

Optimizer started.
Presolve started.
Linear dependency checker started.
Linear dependency checker terminated.
Eliminator started.
Freed constraints in eliminator : 0
Eliminator terminated.
Eliminator - tries                  : 1                 time                   : 0.00            
Lin. dep.  - tries                  : 1                 time                   : 0.00            
Lin. dep.  - number                 : 0               
Presolve terminated. Time: 0.00    
Problem
  Name                   :                 
  Objective sense        : min             
  Type                   : CONIC (conic optimization problem)
  Constraints            : 15              
  Cones                  : 6               
  Scalar variables       : 24              
  Matrix variables       : 1               
  Integer variables      : 0               

Optimizer  - threads                : 8               
Optimizer  - solved problem         : the primal      
Optimizer  - Constraints            : 8
Optimizer  - Cones                  : 6
Optimizer  - Scalar variables       : 18                conic                  : 18              
Optimizer  - Semi-definite variables: 1                 scalarized             : 10              
Factor     - setup time             : 0.00              dense det. time        : 0.00            
Factor     - ML order time          : 0.00              GP order time          : 0.00            
Factor     - nonzeros before factor : 30                after factor           : 30              
Factor     - dense dim.             : 0                 flops                  : 6.23e+02        
ITE PFEAS    DFEAS    GFEAS    PRSTATUS   POBJ              DOBJ              MU       TIME  
0   1.0e+00  1.5e+00  5.2e+00  0.00e+00   4.159870340e+00   0.000000000e+00   1.0e+00  0.00  
1   1.8e-01  2.6e-01  3.6e-01  9.61e-01   1.076588449e+00   3.304561450e-01   1.8e-01  0.01  
2   3.7e-02  5.4e-02  4.6e-02  8.56e-01   8.459002550e-01   6.798381990e-01   3.7e-02  0.01  
3   4.8e-03  7.0e-03  2.4e-03  7.20e-01   9.434346967e-01   9.199445707e-01   4.8e-03  0.01  
4   1.3e-04  2.0e-04  1.1e-05  9.71e-01   9.519921837e-01   9.513316720e-01   1.3e-04  0.01  
5   4.1e-06  6.1e-06  6.3e-08  9.99e-01   9.523018434e-01   9.522813332e-01   4.1e-06  0.01  
6   2.3e-07  3.4e-07  8.4e-10  1.00e+00   9.523076494e-01   9.523064894e-01   2.3e-07  0.01  
7   2.9e-08  4.2e-08  3.6e-11  1.00e+00   9.523075616e-01   9.523074184e-01   2.9e-08  0.01  
8   1.8e-09  2.6e-09  5.5e-13  1.00e+00   9.523075144e-01   9.523075056e-01   1.8e-09  0.01  
Optimizer terminated. Time: 0.02    


Interior-point solution summary
  Problem status  : PRIMAL_AND_DUAL_FEASIBLE
  Solution status : OPTIMAL
  Primal.  obj: 9.5230751439e-01    nrm: 1e+00    Viol.  con: 5e-09    var: 0e+00    barvar: 0e+00    cones: 5e-10  
  Dual.    obj: 9.5230750558e-01    nrm: 2e+00    Viol.  con: 0e+00    var: 4e-09    barvar: 2e-09    cones: 0e+00  
Optimizer summary
  Optimizer                 -                        time: 0.02    
    Interior-point          - iterations : 8         time: 0.01    
      Basis identification  -                        time: 0.00    
        Primal              - iterations : 0         time: 0.00    
        Dual                - iterations : 0         time: 0.00    
        Clean primal        - iterations : 0         time: 0.00    
        Clean dual          - iterations : 0         time: 0.00    
    Simplex                 -                        time: 0.00    
      Primal simplex        - iterations : 0         time: 0.00    
      Dual simplex          - iterations : 0         time: 0.00    
    Mixed integer           - relaxations: 0         time: 0.00    

------------------------------------------------------------
Status: Solved
Optimal value (cvx_optval): +0.952308