determine within cell the coordinates (rows/columns) having equal 3 numbers

Question

Alberto Acri 2023-8-16

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https://ww2.mathworks.cn/matlabcentral/answers/2009302-determine-within-cell-the-coordinates-rows-columns-having-equal-3-numbers

评论： Dyuman Joshi 2023-8-18

采纳的回答： dpb

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How can you determine within cell 'out' the coordinates (rows/columns) having equal 3 numbers?

I would need to get a matrix that shows me the coordinates with 3 equal values.

rgb = randi(255,3,3,3,'uint8');
out = mat2cell(rgb,ones(1,size(rgb,1)),ones(1,size(rgb,2)),3);

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Answer 1

dpb 2023-8-16

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[r,c]=find(cellfun(@(c)all(c==c(1)),out));

If there are none, the above will return [] empty results; taking off the find() will always return a logical array the size of out. What's more convenient will depend mostly on the use.

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Answer 2

Dyuman Joshi 2023-8-16

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You don't need to convert the data to cell array to find that -

rgb = randi(4,3,3,3,'uint8')
rgb = 3×3×3 uint8 array
rgb(:,:,1) =

   4   4   1
   3   1   2
   3   3   4


rgb(:,:,2) =

   4   4   1
   3   3   3
   1   2   4


rgb(:,:,3) =

   2   1   1
   4   1   2
   1   3   2
%Method 1
[r,c]=find(rgb(:,:,2)==rgb(:,:,1) & rgb(:,:,3)==rgb(:,:,1))
r = 1
c = 3
%Method 2
[r,c]=find(all(rgb(:,:,2:3)==rgb(:,:,1),3))
r = 1
c = 3

Since the data is an unsinged integer, diff() will not be useful here, otherwise we can use this -

[r,c]=find(all(diff(rgb,1,3)==0,3));

The reason being - Subtracting a bigger number from a smaller number will result in a 0 for any unsigned integer, which will give incorrect result.

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DGM 2023-8-17

编辑：DGM 2023-8-17

I agree with @Dyuman Joshi here. This seems to all be part of a long composite thread about either selecting scattered pixels from a color image and getting a list of their color tuples, and it's morphed into a convoluted mess of seemingly unnecessary cell arrays.

Maybe it's all unrelated, and maybe it's hard to be certain from the outside perspective, but it sure smells like we're dealing with consequences of questionable decisions which need to be re-evaluated before we wind up digging a hole.

Dyuman Joshi 2023-8-18

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@Alberto Acri, Here's an approach -

rgb(:,:,1) =[4   4   1
   3   1   2
   1   2   4];
    rgb(:,:,2) =[4   4   1
       3   3   2
       1   2   4];
rgb(:,:,3) =[4   1   1
   4   1   2
   1   2   2];
rgb = uint8(rgb);
%find the linear indices
linx = find(rgb(:,:,2)==rgb(:,:,1) & rgb(:,:,3)==rgb(:,:,1));
arr = rgb(:,:,1);
%values
val = arr(linx)
val = 5×1
   4
   1
   2
   1
   2
%Get unique values and their indices
[vec, ~, ia] = unique(val, 'stable');  % Stable keeps it in the same order
%frequency of the unique values
bin = accumarray(ia, 1);
%Get indices of non-repeating values
[~,uniqueidx] = intersect(val,vec(bin<=1));
%Get the indices of repeating values by getting the difference
repeatidx = setdiff(1:numel(val),uniqueidx);
%Convert the corresponding repeating linear indices to subscripts
[r,c] = ind2sub(size(arr), linx(repeatidx))
r = 4×1
     3
     3
     1
     2
c = 4×1
     1
     2
     3
     3

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Answer 3

C B 2023-8-16

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I have taken dummy data instead of random inorder to get right co-ordinates, please let me know if you wanted to get same co-ordinates or different.

rgb = randi(255,3,3,3,'uint8')
rgb = 3×3×3 uint8 array
rgb(:,:,1) =

   235   202   208
   136     3    50
    20    50   245


rgb(:,:,2) =

   215    53   227
   214   246   143
   183    49   101


rgb(:,:,3) =

   216    11   108
   119   131   219
   199   236   125
out = mat2cell(rgb,ones(1,size(rgb,1)),ones(1,size(rgb,2)),3);
rgb = uint8([...
    100, 100, 100; 150, 150, 200; 255, 255, 255; 
    100, 150, 200; 200, 200, 200; 50, 50, 50;
    255, 0, 255; 0, 255, 0; 150, 150, 150]);
rgb = reshape(rgb, [3, 3, 3])
rgb = 3×3×3 uint8 array
rgb(:,:,1) =

   100   100   255
   150   200     0
   255    50   150


rgb(:,:,2) =

   100   150     0
   150   200   255
   255    50   150


rgb(:,:,3) =

   100   200   255
   200   200     0
   255    50   150
out = mat2cell(rgb, ones(1, size(rgb, 1)), ones(1, size(rgb, 2)), 3);
matForm = cellfun(@(x) numel(unique(x)), out);
[rows, cols] = find(matForm == 1);
equal_coords = [rows, cols];
disp(equal_coords)
     1     1
     3     1
     2     2
     3     2
     3     3