how to create an array of ones with some zeros delimited by an ellipse?

I need to create an array with ones inside an ellipse and zeros outside it. The matrix has dimensions of NxM = 91x361. This matrix will be plotted polarly and the ellipse must be completely contained within the matrix.

[Walter Roberson]

One approach is to use an Ellipse ROI object https://www.mathworks.com/help/images/ref/images.roi.ellipse.html and createMask() from it.

This has the advantage of being fairly customizable as to center and axes lengths.

It is, though, overly complicated if you just happen to have the simple case of the ellipse being aligned with the x y planes.

Do you happen to be looking for the largest x y aligned ellipse that will fit completely inside your N x M ?

Answer 1

[Rik]

I'm going to ignore the part where you want to plot a cartesian object with polar coordinates.

You can either use ndgrid or meshgrid to generate a coordinate grid. Then you need to google the equation for an ellipse, after which it should be obvious how to adjust it to return all points inside the ellipse, instead of only the points exactly on the array.

To satisfy your requirement of each pixel being entirely inside the ellipse you can use the corners of each pixel and mark only the pixels if all four corners are inside.

Feel free to post a follow-up question in a comment if your have trouble implementing a specific step.

[Ricardo Souza]

I need to create a matrix of 1 inside the ellipse and zeros outside. I need to plot this, for example, centered at the origin and have it appear in the four quadrants, and then create a polar contour map.

theta = 360;
phi = 90;
[r t] = meshgrid(0:theta,0:phi);
cTheta = 40;
cPhi = 90;
rTheta = 40;
rPhi = 50;
elipse = (t - cTheta).^2 ./ rTheta^2 + (r - cPhi).^2 ./ rPhi^2 >= 1;
elipse = elipse.*20;
elipse = elipse-40;
elipse(cTheta+1,cPhi+1) = 0;

The problem with this code is when cTheta or cPhi is equal to 0. An ellipse will appear that appears in all four quadrants. Also, when tracing in polar it is like a drop and not an ellipse.

Below is the code for the polar contour map.

function polarplotindb(AF)
mindb = -40;
maxdb = 0;
resuloction = abs(mindb)*10;
[r,t] = meshgrid(deg2rad(0:90),deg2rad(0:360));
x = r.*cos(t);  
y = r.*sin(t);
z = AF';
   
z(z<mindb) = mindb;
     
levels = linspace(mindb, maxdb, resuloction);
    
contourf(x,y,z, levels, 'edgecolor', 'none'),
text(1.6,0,'0^\circ','FontSize',14),
text(1.61,0.1,'\rightarrow','FontSize',15,'rotation',90),
colorbar('Location', 'westoutside'),
colormap('jet'),
set(gca,'XTick',[],'YTick',[]),
xline(0,'LineStyle','-','Color','w'),
yline(0,'LineStyle','-','Color','w'),
axis square, xlabel '\phi \rightarrow ',
axis off
axis image
grid minor
end

[Rik]

cTheta=0;
imshow(elipse,[]),xlabel('\Theta'),ylabel('\Phi')

I don't really see the issue. This is an ellipse in your strange coordinate system. I have no clue what would happen if you would try to plot the edge of this object in polar coordinates. You also seem to be using completely separate approaches in your script and your function.

It is unclear to me what your goal is, and what exactly it is you need help with.

[Ricardo Souza]

You got the same result as mine. What I want is to plot an ellipse in polar coordinates, which will appear in the four quadrants and will be a matrix where the 0<theta<90 and 0<phi<360. I need to plot in polar form. When you plot your figure in polar form something other than an ellipse will appear. Finally, I want a matrix (0<theta<90 and 0<phi<360), which plots in polar an ellipse centered anywhere and appearing anywhere within the matrix boundary.

[Ricardo Souza]

the first image is my matrix (0<theta<90 and 0<phi<360) and the second is its respective polar plot. I need the description of the first matrix in its limits so that I can plot the second one.