Boolean operations with polyshape objects

Hello everyone, I am trying to subtract a sector from a circle but I am getting some unwanted cut in the result as highlighted in red in the attached image. Please how may I remove this? Or is there a better way to achieve the same thing? Your help is very much appreciated

Below is the code.

r1 = 1.5; sec_ang=40; sec_ang=sec_ang*pi/180;

beta=60; beta= beta*pi/180; r2 = r1 + 2;

beta = beta*ones(1,2); % rotation angle

sec_ang = sec_ang*ones(1,2); % sectorial angle

r = [r1, r2] ; % radii

x0 = 0; y0 = 0;

theta = 0:pi/360:2*pi;

geom=[];

for i = 1:length(r)

circ = [r(i)*cos(theta)+x0 ; r(i)*sin(theta)+y0]; % generate points on the circle

a1 = beta(i); % start angle of arc

a2 = a1 + sec_ang(i); % end angle of arc

t = linspace(a1, a2);

xy = [x0 + (r(i)+1e-2)*cos(t); y0 + (r(i)+1e-2)*sin(t)]; % points on the arc of a sector

poly_sec= polyshape([x0,xy(1,:),x0], [y0, xy(2,:), y0]); %generate sector

p_circ = polyshape(circ(1,:), circ(2,:));

p_sub=subtract(p_circ,poly_sec); % subtract the sector from the circle

geom = [geom, p_sub.Vertices'];

end

p = polyshape(geom(1,:), geom(2,:));

plot(p,'FaceColor', 'k','LineWidth', 2)

axis equal

Answer 1

[John D'Errico]

Computers are such literal things. They do what you tell them to do, not what you want them to do. The fix lies in the mind reading toolbox, which is still under beta tasting. And sadly, it may never get out of beta testing. :)

Anyway, what you did was not overtly wrong. It just had that annoying horizontal line.

But how about this instead?

r = [1.5 3.5];
xy0 = [0 0];
theta = linspace(0,2*pi,1000)';
theta(end) = []; % avoids warning messages from polyshape

secang = 40*pi/180;
secrot = 60*pi/180;

Cin = polyshape(r(1)*[cos(theta),sin(theta)] + xy0);
Cout = polyshape(r(2)*[cos(theta),sin(theta)] + xy0);

ring = subtract(Cout,Cin); % This is an important part.
plot(ring)
axis equal

Note that there is no horizontal line at theta = 0 now. This is because I created the polyshape in a way that avoids it. There is a subtle difference in what I did if you think carefully about it.

Next, create a sector to remove. I'll go out to a distance of 100 from the origin. A triangular wedge of pie, essentially.

rotmatrix = @(rotangle) [cos(rotangle) sin(rotangle);-sin(rotangle) cos(rotangle)];
sector = polyshape([0 0;100 0;100*[cos(secang) sin(secang)]]*rotmatrix(secrot) + xy0);
partialring = subtract(ring,sector);
plot(partialring)
axis equal

Easy peasy. No steenkin' loops either. :)

[Oluwaseyi Ogun]

Great! thank you