trouble using im2bw for image thresholding

2011 5 17
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更新时间：2019 1 22
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0 个投票

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?
Thanks
imagedata2 = dlmread('C:\data\3d_cells\17_8um_exp0_002_bf_run1.asc');
clims=[500 4000];
figure('Name','');
image2=imagesc(imagedata2,clims);colormap gray;
level = graythresh(image2);
BW2 = im2bw(image2,level);
clims = [0,1.5];
figure;
imagebw=imagesc(BW2,clims); colormap gray

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Natasha
Natasha 2011-5-20

0 个投票

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.

更多回答（1 个）

Sean de Wolski
Sean de Wolski 2011-5-17

1 个投票

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

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Natasha
Natasha 2011-5-20
I got the same thing happening (every pixel goes to 1). I don't need to use the graythresh part, I can just set the level myself, which is fine, I just put that in to be sure I wasn't choosing a stupid value, but actually I've tried setting a range of values for level (between 0 and 1) and I still always get the same thing. Imagedata2 is class double which should be ok according to the matlab help file;
"The input image can be of class uint8, uint16, single, int16, or double, and must be nonsparse"
I don't know what nonsparse means so maybe that's the problem...?
Natasha
Natasha 2011-5-20
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
Natasha 2011-5-20
Also it's not sparse so that's not the problem
Sean de Wolski
Sean de Wolski 2011-5-20
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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