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

3 views (last 30 days)
Nut on 13 Jun 2016
Commented: Image Analyst on 22 Jan 2020
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.
##### 2 CommentsShowHide 1 older comment
Image Analyst on 22 Jan 2020
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

Andrew Bliss on 13 Jun 2016
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.
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)
##### 2 CommentsShowHide 1 older comment
Nut on 14 Jun 2016
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.

Image Analyst on 14 Jun 2016
Edited: Image Analyst on 14 Jun 2016
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:
Nut on 16 Jun 2016
I'll try it, thank you!

### Categories

Find more on Image Processing and Computer Vision in Help Center and File Exchange

### Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!