doubt regarding bwconncomp example.

2015 8 21
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更新时间：2015 8 29
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This is a small doubt.....I'm stuck in the second example given for bwconncomp by Mathworks. This example removes the group of pixels having largest pixels connected (in an 8 connected neighbourhood):
BW = imread('text.png');
imshow(BW);
CC = bwconncomp(BW);
numPixels = cellfun(@numel,CC.PixelIdxList);
[biggest,idx] = max(numPixels);
BW(CC.PixelIdxList{idx}) = 0;
figure, imshow(BW);
I want to change the code to erase groups of pixels which have between 20 to 70 connected pixels (in their 8 connected neighbourhood). How can I do this?
If I do:
idx = numPixels < 70;
I get an error saying : " Maximum variable size allowed by the program is exceeded."

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Meghana Dinesh
Meghana Dinesh 2015-8-22
编辑：Meghana Dinesh 2015-8-24
I also tried using bwlabeln. But how can I remove (i.e. make = 0) those pixels which repeat between 20 to 70 times?
Walter Roberson
Walter Roberson 2015-8-24
What is size(numPixels) ?
Meghana Dinesh
Meghana Dinesh 2015-8-26
编辑：Meghana Dinesh 2015-8-26
As an example:
c is a 4X6X3 logical matrix.
c(:,:,1) =
0 0 0 0 0 1
1 0 0 1 0 0
0 1 1 1 0 0
0 0 0 0 0 0
c(:,:,2) =
0 0 0 0 0 1
1 0 0 1 0 0
0 1 1 1 0 0
0 0 0 0 0 0
c(:,:,3) =
0 0 0 0 0 1
1 0 0 1 0 0
0 1 1 1 0 0
0 0 0 0 0 0
CC = bwconncomp(c);
numPixels = cellfun(@numel,CC.PixelIdxList);
numPixels has a value [15,3]
and
size(numPixels) = 1 2 ;
Ultimately, my size of c will be around 3000 X 3000 X 3. In this, I want to remove the points (i.e., make their values = NaN) which have less than say 100 number of connected points.
Walter Roberson
Walter Roberson 2015-8-26
With those sizes there is no reason why you should get the reported error message for idx = numPixels < 70 . If you had instead received an error message about being out of memory then that would be more understandable. "Maximum variable size allowed by the program is exceeded" is an error message that you would get on 32 bit MATLAB systems if you attempted to create an array that was larger than 2 gigabytes, and is not the error message given if you fill up your available memory.
If you do not need the actual number of pixels, just to know that the number is in the right range, then recall that you can use
inrange = @(x, a, b) a <= x & x <= b;
numPixelsInRange = cellfun( @(C) inrange(numel(C), 20, 70), CC.PixelIdxList);
this should save a small amount of memory at the expense of time.
Meghana Dinesh
Meghana Dinesh 2015-8-27
This is what I am doing:
I am first converting my point cloud (in this example, I have created a small 4 X 6 point cloud) into a logical matrix. (all NaNs = 0 while all other finite values = 1).
Here is a screenshot.
I don't seem to face any memory issues as of now.
Image Analyst
Image Analyst 2015-8-27
numPixels<10 produces a binary variable, true or false. It does not produce two numbers. There are not two numbers to try to stick into biggest, and idx like you're trying to do.
Walter Roberson
Walter Roberson 2015-8-28
It looks like you are trying to find a maximum, but it is not clear what you are trying to find a maximum of.
idx = 20 <= numPixels & numPixels <= 70;
valididx = find(idx);
validsizes = numPixels(idx);
if isempty(valididx)
lastvalid = [];
else
lastvalid = valididx(end);
end
[sizelargest, idxlargest] = max(validsizes);
ccidxlargest = valididx(idxlargest);
lastvalid would be for the case where you want to find the index (into CC) of the last cluster that is in the size range -- the maximum being taken over the indices
ccidxlargest would be for the case whee you want to find the index (into CC) of the largest cluster that is left after you isolate down to that range of sizes -- the maximum being taken over the remaining sizes.
Meghana Dinesh
Meghana Dinesh 2015-8-28
Maybe my approach is wrong.
This is my goal: I have a pointCloud (a 3D matrix including Inf, NaN and finite values). I want to remove the groups of all those points ("remove" i.e., make them = NaN) which have lesser than N (say, 100) points connected to it (in a 26-connectivity neighbourhood).
In order to do this, I am performing the steps mentioned above (converting all finite values to logical and applying bwconncomp).
Are my steps right?

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 采纳的回答

Image Analyst
Image Analyst 2015-8-22

0 个投票

Use bwareafilt instead.

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Meghana Dinesh
Meghana Dinesh 2015-8-24
编辑：Meghana Dinesh 2015-8-24
My data is in 3D.
Image Analyst
Image Analyst 2015-8-29
If you want to " want to remove the groups of all those points ("remove" i.e., make them = NaN) which have lesser than N (say, 100) points connected to it (in a 26-connectivity neighbourhood)." then the function you want to use is definitely 100% bwareaopen(). It's meant for exactly that. No need to get PixelIdxList at all.

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更多回答（1 个）

Walter Roberson
Walter Roberson 2015-8-28

1 个投票

N = 100;
a = imread('text.png');
c = a;
c(~isfinite(c)) = 0;
BW = logical(c);
CC = bwconncomp(BW);
numPixels = cellfun(@numel,CC.PixelIdxList);
idx = numPixels < N;
unwantedCC = CC(idx);
unwantedpixels = vertcat(unwantedCC.PixelIdxList);
b = a;
b(unwantedpixels) = NaN;

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Meghana Dinesh
Meghana Dinesh 2015-8-28
编辑：Meghana Dinesh 2015-8-28
I am still getting the same error.
RimPointCloud_filt_lst_sq is a 3D matrix.
Walter Roberson
Walter Roberson 2015-8-28
unwantedpixels = vertcat(unwantedCC.PixelIdxList(idx));
Meghana Dinesh
Meghana Dinesh 2015-8-28
编辑：Meghana Dinesh 2015-8-28
The line
unwantedCC = CC(idx);
gives me the error mentioned above.
So when I use:
unwantedpixels = vertcat(unwantedCC.PixelIdxList(idx));
I get unwantedCC as undefined.
Walter Roberson
Walter Roberson 2015-8-28
编辑：Walter Roberson 2015-8-28
Sorry, it is the early hours of the morning here.
Leave out the unwantedCC definition. Use
unwantedpixels = vertcat(CC.PixelIdxList{idx});
Meghana Dinesh
Meghana Dinesh 2015-8-29
:D Nevermind! Anyway, thanks for your help so far.
I am still not able to execute since the variable unwantedpixels is a cell. So the line:
b(unwantedpixels) = NaN;
gives an error saying: Function 'subsindex' is not defined for values of class 'cell'.
Walter Roberson
Walter Roberson 2015-8-29
unwantedpixels = cell2mat(CC.PixelIdxList(idx));

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