null or NaN entries in image arrays?

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Tim
Tim 2024-11-13，4:25
评论： Walter Roberson 2024-11-14，4:37
Is it possible to set some elements in animage array to null or in some way that they are ignored?
I have an automatically detected circular ROI outside of which is to be ignored (those values will skew the results) and need to step through the array using a for loop. Obviously a for loop can only step in rectangles so is there any way I can set an element in that image array to null or NaN that is acceptable?
Regards
Tim
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Manikanta Aditya
Manikanta Aditya 2024-11-13，5:01
Yes, you can programmatically set elements in an image array to NaN to ignore them in your calculations.
DGM
DGM 2024-11-13，5:31
What is the actual applied operation? Does it actually require a loop, or can it be done with basic logical indexing?

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Epsilon
Epsilon 2024-11-13，4:40
编辑：Epsilon 2024-11-13，4:42
Ran in:
Hi Tim,
It is possible to set the elements to NAN in order to be ignored. Attaching an example:
% Sample image array
img = rand(100);
% Define the circular ROI
[rows, cols] = size(img);
center = [50, 50];
radius = 20;
% Create a mask for the circular ROI
[x, y] = meshgrid(1:cols, 1:rows);
mask = sqrt((x - center(1)).^2 + (y - center(2)).^2) <= radius;
% Set elements outside the circle to NaN
imgModified = img;
imgModified(~mask) = NaN;
% Display original and modified images
figure;
subplot(2, 1, 1); imagesc(img); title('Original'); colorbar;
subplot(2, 1, 2); imagesc(imgModified); title('Modified'); colorbar;
Hope it helps.
  2 个评论
Tim
Tim 2024-11-14，3:35
That worked well. Thanks Epsilon
Walter Roberson
Walter Roberson 2024-11-14，4:37
This code relies upon img being type double or single . This code would not work if img is uint8() for example. You really should use im2double() like I showed in my Answer.

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更多回答（3 个）

Walter Roberson
Walter Roberson 2024-11-13，5:06
编辑：Walter Roberson 2024-11-13，5:12
The trick is that if your image array is not datatype single() or double(), then there is no way to set locations to be ignored as part of the same array
You can, however,
Imd = im2double(YourImageArray);
Imd(AppropriateLocations) = nan;
This will not in itself cause locations to be skipped. For example if you have
for ROW = 1 : size(YourImageArray,1)
for COL = 1 : size(YourImageArray,2)
do something with Imd(ROW,COL)
end
end
then this code in itself will not skip the locations that are NaN. You would need
for ROW = 1 : size(YourImageArray,1)
for COL = 1 : size(YourImageArray,2)
if ~isnan(Imd(ROW,COL))
do something with Imd(ROW,COL)
end
end
end
but if you are going to do that then you might as well use
for ROW = 1 : size(YourImageArray,1)
for COL = 1 : size(YourImageArray,2)
if YourImageMask(ROW,COL)
do something with YourImageArray(ROW,COL)
end
end
end
Steven Lord
Steven Lord 2024-11-13，5:10
Ran in:
If your images are stored in an integer data type, assigning a NaN value into that array will store a 0 in that location. There is no equivalent of a NaN value in the integer types; all the possible bit patterns that can be stored in that type represent finite values.
A = int16(magic(3))
A = 3×3
8 1 6 3 5 7 4 9 2
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<mw-icon class=""></mw-icon>
A(3, 3) = NaN % 2 replaced by 0 not NaN
A = 3×3
8 1 6 3 5 7 4 9 0
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B = uint32(magic(4))
B = 4×4
16 2 3 13 5 11 10 8 9 7 6 12 4 14 15 1
<mw-icon class=""></mw-icon>
<mw-icon class=""></mw-icon>
B(2, 4) = NaN % 8 replaced by 0 not NaN
B = 4×4
16 2 3 13 5 11 10 0 9 7 6 12 4 14 15 1
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Tim
Tim 2024-11-14，3:35
Thanks for your help guys.

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