File: transformEllipse.m

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## Copyright (C) 2024 David Legland
## All rights reserved.
## 
## Redistribution and use in source and binary forms, with or without
## modification, are permitted provided that the following conditions are met:
## 
##     1 Redistributions of source code must retain the above copyright notice,
##       this list of conditions and the following disclaimer.
##     2 Redistributions in binary form must reproduce the above copyright
##       notice, this list of conditions and the following disclaimer in the
##       documentation and/or other materials provided with the distribution.
## 
## THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS ''AS IS''
## AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
## IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
## ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR
## ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
## DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
## SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
## CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
## OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
## OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
## 
## The views and conclusions contained in the software and documentation are
## those of the authors and should not be interpreted as representing official
## policies, either expressed or implied, of the copyright holders.

function res = transformEllipse(elli, transfo)
%TRANSFORMELLIPSE Apply an affine transformation to an ellipse.
%
%   ELLI2 = transformEllipse(ELLI, TRANSFO)
%
%   Example
%     % apply an arbitrary transform to a simple ellipse
%     elli = [5 4 3 2 0];
%     rot = createRotation(pi/6);
%     sca = createScaling([2.5 1.5]);
%     tra = createTranslation([4 3]);
%     transfo = sca * rot * tra;
%     elli2 = transformEllipse(elli, transfo);
%     % display original and transformed ellipses
%     figure; hold on; axis equal; axis([0 20 0 20]);
%     drawEllipse(elli, 'k');
%     drawEllipse(elli2, 'b');
%     % Compare with transform on polygonal approximation
%     poly = ellipseToPolygon(elli, 100);
%     drawPolygon(transformPoint(poly, transfo), 'm');
%
%   Reference
%   https://math.stackexchange.com/questions/3076317/what-is-the-equation-for-an-ellipse-in-standard-form-after-an-arbitrary-matrix-t
%
%   See also 
%     ellipses2d, transforms2d, transformPoint

% ------
% Author: David Legland
% E-mail: david.legland@inrae.fr
% Created: 2022-09-05, using Matlab 9.12.0.1884302 (R2022a)
% Copyright 2022-2023 INRAE - BIA Research Unit - BIBS Platform (Nantes)

% first extract coefficients of cartesian representation
coeffs = ellipseCartesianCoefficients(elli);

% writing the matrix of the general conic equation x^t * Q * x = 0
Q = [...
     coeffs(1)   coeffs(2)/2  coeffs(4)/2; ...
    coeffs(2)/2   coeffs(3)   coeffs(5)/2; ...
    coeffs(4)/2  coeffs(5)/2   coeffs(6)];

% compute the matrix form of the transformed ellipse
Minv = inv(transfo);
Q2 = Minv' * Q * Minv;

coeffs2 = [Q2(1,1) 2*Q2(1,2) Q2(2,2) 2*Q2(1,3) 2*Q2(2,3) Q2(3,3)];
res = createEllipse('CartesianCoefficients', coeffs2);