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trouble using im2bw for image thresholding

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Hello, I need to threshold an image. This is what I've tried (see below) but I just get every pixel of the image with a value of 1. The original data is an ascii file. If it isn't possible to use im2bw in this situation then can anyone suggest an alternative way of thresholding?
imagedata2 = dlmread('C:\data\3d_cells\17_8um_exp0_002_bf_run1.asc');
clims=[500 4000];
image2=imagesc(imagedata2,clims);colormap gray;
level = graythresh(image2);
BW2 = im2bw(image2,level);
clims = [0,1.5];
imagebw=imagesc(BW2,clims); colormap gray


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Accepted Answer

Natasha on 20 May 2011
I did it this way;
imagedata2thresh=+(imagedata2>1420); %put threshold value
so I just put in the actual value that I want to use for the threshold instead of faffing around with levels.

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More Answers (1)

Sean de Wolski
Sean de Wolski on 17 May 2011
level = graythresh(imagedata2);
BWS = im2bw(imagedata2,level);
If this doesn't work what are class is imagedata2?
and what is the Otsu effectiveness metric?
[level, em] = graythresh(imagedata2);


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Natasha on 20 May 2011
the Otsu metric comes out as level= 0 , em=0 but even if I manually set the level to something that isn't zero I still get the same thing happening.
Natasha on 20 May 2011
Also it's not sparse so that's not the problem
Sean de Wolski
Sean de Wolski on 20 May 2011
Yes, the behavior your seeing is expected and I would say not properly documented. Graythresh requires that the data be uint8 so it silently converts your data to uint8. The conversion method bins all data close to each other together it doesn't pick a good range. That's why I asked what class your data is. If it's uint16, you should convert it to uint8 by selecting the range of values yourself and then thresholding. I think this behavior needs to have better documentation and I've considered writing an enhancement request.

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