How to select one area from several in a logical array (mask)?

Elena Novoselova - 2023-02-06T13:42:13+00:00

Question: How to select one area from several in a logical array (mask)?

I have a 3D logical array where is several spatial areas marked as "true" or "1". 1st dimension is latitude, 2nd – longitude, 3d – depth. I need to choose only one area which contains a preassigned point in space. This point is always inside one of the areas. As a result, I should get the similar logical array, which has only one area with values "1".   I think the first problem is to find the border for each area. I tried to use "alphaShape" for it but I’m not sure that I actually need it. Besides, I had a problem with it because my areas stick together into one. I have added a data file in the hope that it will make my question clearer.     shp = alphaShape(lon3(mask), lat3(mask), z3(mask), 1); plot(shp) % lat3, lon3, z3 - 3d array of latitudes, longitudes and depths % mask - logical 3d array with several spatial areas % core_lat, core_lon, core_z - coordinates of preassigned point (red cross % in picture) (in data) An example of data is shown in the figure below. The figure above shows a vertical section along the latitude of 69.63 N, the figure below shows the distribution at the depth of 417 meters. Yellow is several areas, from which I need to select one. Red cross is preassigned point.    

Related Questions

Expert Answer

Neeta Dsouza answered . 2025-11-20

• mask is a 2D binary matrix (true|false values)
• target is the (x,y) coordinates of the target that lies within one of the objects in the image.

RGB = imread('pillsetc.webp'); % image processing toolbox
mask = imbinarize(rgb2gray(RGB));
target = [430,170];

1. The closest true pixel in the image to the target is identified using pdist2.
2. Each group of true values in the binary image is assigned a group number using bwlabel.
3. The group number that belongs to the group closest to the target is stored in targetGroup.
4. All other groups of true values in the binary matrix are removed.

 

[rownum, colnum] = find(mask); 
[~,I] = pdist2([colnum,rownum],target,'cityblock','smallest',1);

maskLbl = bwlabel(mask);
targetGroup = maskLbl(rownum(I),colnum(I));

maskClean = mask;
maskClean(maskLbl ~= targetGroup) = false;

figure()
tiledlayout(1,2)
nexttile
imagesc(mask)
colormap(parula(2))
hold on
plot(target(1), target(2), 'rx', 'DisplayName', 'target')
plot(colnum(I),rownum(I), 'ro', 'DisplayName', 'Nearest true value')
title('full binary image')
axis tight
axis equal

nexttile
imagesc(maskClean)
colormap(parula(2))
hold on
plot(target(1), target(2), 'rx', 'DisplayName', 'target')
plot(colnum(I),rownum(I), 'ro', 'DisplayName', 'Nearest true value')
title('trimmed binary image')
axis tight
axis equal
lg = legend();
lg.Layout.Tile = 'south';

 

Not satisfied with the answer ?? ASK NOW