How to apply a different b/w threshold to each row of the image?

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Nut 2016-6-13

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评论： Image Analyst 2020-1-22

Hi,

I would convert an image from grayscale to black-and-white using the im2bw function, and I need to apply a different threshold to each row of the image. Which is the most efficient way to do this? Is there a way to avoid the "for" cycle?

Thank you very much.

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Roman Boldin 2020-1-22

Hello, do you still have the code of for and loop?

Image Analyst 2020-1-22

Roman, it would be something like

[rows, columns, numberOfColorChannels] = size(theImage);
binaryImage = false(rows, columns);
for row = 1 : rows
    % Get the threshold for this row - however you do it (I don't know).
    thisThreshold = whatever;
    % Now threshold/binarize the image for this row only.
    binaryImage(row, :) = theImage(row, :) > thisThreshold;
end

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Answer 1

Andrew Bliss 2016-6-13

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Depending on the input type of your image, you may be able to just do a simple thresholding operation (as below), otherwise you'll have to delve a little deeper into image processing.

asdf=imread('example.tif');
figure,imshow(asdf)
zxcv=rgb2gray(asdf);
figure,imshow(zxcv)
thresh=[50*ones(325,1);150*ones(325,1)]; %here you set the threshold for each row
threshMatrix=repmat(thresh,1,600);
BW=zxcv>threshMatrix;
figure,imshow(BW)

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Nut 2016-6-14

Thank you very much for your answer, it may be a good way. I'll compare this with the for loop.

Nut 2016-6-14

You're right. The execution time with your solution is about 1.6 seconds, instead using im2bw and the for loop it is about 5.1 seconds. Very good idea, thank you again.

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Answer 2

Image Analyst 2016-6-14

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编辑：Image Analyst 2016-6-14

No, you'll have to use a for loop. It's not a problem though. It will be very fast. No need to worry about for loops that are only a few thousand iterations.

Why do you need a different threshold for each row anyway?

You might be able to use a different function. There are new binarization functions. See Steve's blog:

http://blogs.mathworks.com/steve/2016/05/09/image-binarization-im2bw-and-graythresh/

http://blogs.mathworks.com/steve/2016/05/16/image-binarization-new-r2016a-functions/

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Image Analyst 2016-6-14

编辑：Image Analyst 2016-6-14

When you assumed that you needed a new threshold at each row and said that your current approach was to do it row by row, the most logical assumption (at least to me) was that the threshold depended on the image data in that row. Doesn't that seem reasonable? You mentioned using im2bw() but that determines a global threshold based on the whole image if you just pass it the whole image, and that was unacceptable to you. Indicating that using im2bw to do the automatic threshold was an acceptable threshold determination method but that you needed it to give different thresholds based on each row was a strong indication that you needed to pass each row to im2bw() and did not know the thresholds in advance.

Note that Andrew's answer assumes that somehow you know the threshold in advance for each row. If you do, then use his answer. If you need to determine the threshold on a row-by-row basis then you'll need to use my answer since you would need to get the row of image data to examine it.

Since you accepted Andrews answer you must know the thresholds in advance, so tell me how you got thresholds in advance that were different for each row? Do the thresholds not depend on the image data?

Image Analyst 2016-6-15

OK. I have a nice manual/interactive thresholding utility in my File Exchange if you're interested: http://www.mathworks.com/matlabcentral/fileexchange/29372-thresholding-an-image

Nut 2016-6-16

I'll try it, thank you!

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