# Matlab code for generating some shapes using signed distance

24 views (last 30 days)
SAMUEL AYINDE on 13 Oct 2018
Answered: SAMUEL AYINDE on 23 Apr 2019
In order to get a good understanding of my question, please, you may first run the matlab code below. This code can generate the shape for rectangle, ellipse and circle using signed distance if you uncomment the portion corresponding to each shape.
Please, I need a code that can give the shapes in the attached picture (Picture_1.jpg) using signed distance. Thank you so much.
%%%Signed distance code for generating shapes
clear all;
close all;
clc;
DomainWidth=2;
DomainHight=1;
ENPC=40;
ENPR=80;
EW = DomainWidth / ENPR; % The width of each finite element.
EH = DomainHight / ENPC; % The hight of each finite element.
M = [ ENPC + 1 , ENPR + 1 ];
[ x,y ] = meshgrid( EW * [ -0.5 : ENPR + 0.5 ] , EH * [ -0.5 : ENPC + 0.5 ]);
[ FENd.x, FENd.y, FirstNdPCol] = MakeNodes(ENPR,ENPC,EW,EH);
LSgrid.x = x(:); LSgrid.y = y(:); % The coordinates of Level Set grid 1
cx = DomainWidth/2;
cy= DomainHight/2;
a = cx;
b = 0.8*cy;
%%Generate circle
tmpPhi= sqrt ( ( LSgrid . x - cx ) .^2 + ( LSgrid . y - cy ) .^2 ) - DomainHight/2;
LSgrid.Phi = -((tmpPhi.')).';
%%Generate ellipse
% tmpPhi= ( (LSgrid . x - cx)/a ) .^2 + (( LSgrid . y - cy)/(0.8*cy) ) .^2 - 1;
% LSgrid.Phi = -((tmpPhi.')).';
%%Generate rectangle
% lower = [cx - 0.5 * DomainWidth, cy - 0.25 * DomainHight];
% upper = [cx + 0.5 * DomainWidth, cy + 0.25 * DomainHight];
% Phi11 =max(LSgrid.x - upper(1), lower(1) - LSgrid . x );
% Phi11 =max(Phi11,LSgrid.y - upper(2));
% Phi11 =max(Phi11,lower(2) - LSgrid . y );
% LSgrid.Phi = -Phi11;
FENd.Phi = griddata( LSgrid.x, LSgrid.y, LSgrid.Phi, FENd.x, FENd.y, 'cubic');
figure(10)
% Figure of the full contour plot of the signed distance of the shape
contourf( reshape( FENd.x, M), reshape(FENd.y , M), reshape(FENd.Phi, M));
axis equal; grid on;drawnow; colorbar;
figure(11)
% Figure of the scaled contour plot showing only the shape
contourf( reshape( FENd.x, M), reshape(FENd.y , M), reshape(FENd.Phi, M), [0 0] );
axis equal; grid on;drawnow; colorbar;
figure(12)
%Figure of the signed distance function
h3=surface(x, y, reshape(-LSgrid.Phi , M + 1)); view([37.5 30]); axis equal; grid on;
set(h3,'FaceLighting','phong','FaceColor','interp', 'AmbientStrength',0.6); light('Position',[0 0 1],'Style','infinite'); colorbar;
%%Code for creating nodes
function [NodesX, NodesY, FirstNdPCol] = MakeNodes(EleNumPerRow,EleNumPerCol,EleWidth,EleHight)
[ x , y ]= meshgrid( EleWidth * [ 0 : EleNumPerRow ], EleHight * [0 : EleNumPerCol]);
FirstNdPCol = find( y(:) == max(y(:)));
NodesX = x(:); NodesY = y(:);
end
jonas on 13 Oct 2018
Not sure if I can, but I will give it a try. Anyway, if the question is clear then usually someone will answer eventually :)
SAMUEL AYINDE on 13 Oct 2018
These are other pictures

jonas on 14 Oct 2018
Edited: jonas on 14 Oct 2018
I would attempt the following more 'general' algorithm.
• describe the shape with a set of x-y-coordinates
• create a mesh
• find the closest distance to each point in the mesh to the set of x-y-coordinates
If I understand correctly, that is what the "signed distance field" describe, i.e. the closest distance to a shape from any point in the domain.
This is the code for a single horizontal line from [0,0.5] to [1,0.5].
% The "shape"
x=0:0.01:1;
y=ones(size(x)).*.5;
% The "domain"
[X,Y] = meshgrid(0:0.01:1,0:0.01:1);
% Closest point between domain and shape
[X,Y] = meshgrid(0:0.01:1,0:0.01:1);
[id,Z] = dsearchn([x',y'],[X(:),Y(:)]);
scatter3(X(:),Y(:),Z,[],Z)
The only thing you have to change is the sign of those points that are inside of the "shape". This is however fairly simple, using the inpolygon function. Let's try one for the sinusoidal shape:
% Build polygons
x=0:0.01:1;
Y{1} = 0.2.*sin(x/0.05)+0.3
Y{2} = 0.2.*sin(x/0.05)+0.7
py = [Y{1},fliplr(Y{2})]
px = [x,fliplr(x)];
% Define domain
[X,Y] = meshgrid(0:0.01:1,0:0.01:1);
% Calculate distance
[~,Z] = dsearchn([px',py'],[X(:),Y(:)]);
[in]=inpolygon(X(:),Y(:),px,py)
Z(in) = -Z(in)
Z = -Z;
% Plot
surf(X,Y,reshape(Z,size(X)),'edgecolor','none');hold on
p = polyshape(px,py)
plot(p)
axis([0 1 0 1])
.
There seems to be some edge-effect, in particular to the left. Other than that, it looks quite OK.
jonas on 15 Oct 2018
Edited: jonas on 15 Oct 2018
My pleasure! I will update my previous answer with an image of the results if the run finishes :)
SAMUEL AYINDE on 16 Oct 2018
Thank you so much, @jonas!

SAMUEL AYINDE on 23 Apr 2019
Please, can someone help me with the code to generate the signed distance for this geometry? Thank you so much.