is this the right way to find local minima of histogram

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Muhammad Ali Qadar
Muhammad Ali Qadar 2013-11-8
评论： Image Analyst 2016-8-26
hi, I am trying to find the local minima value of image's histogram, I am filtering image from 1*3 filter and then finding minima, waiting for your suggestions
I = imread('eight.tif');I=I(:)';I=double(I);
x=[1 1 1]/3;
f=filter2(x,I,'same');
y = sin(5*f);
idx = [false, y(3:end)>y(2:end-1) & y(2:end-1)<y(1:end-2), false];
xmin = f(idx)
plot(xmin,'o-')
thanks in Advance.

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Walter Roberson
Walter Roberson 2013-11-8
Not so bad, but remember
  1. you might match multiple local minima
  2. you are going to plot all the local minima adjacent to each other, not their original distance apart
  3. you are going to have trouble if the pattern is \__/ instead of \_/ i.e., if the local min is more than one point wide
  4. you might want to think about using diff()
  5. local min could occur at the endpoints -- with your current code, a continually-decreasing or -increasing set of data would not register any local min.
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Image Analyst
Image Analyst 2016-8-26
"count <= 0 will give you the logical array" and that is what he calls L.

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Image Analyst
Image Analyst 2013-11-8
Way too complicated for me. Actually I didn't even see you take the histogram, which you do with imhist(). I'm not sure what you're doing with the sine wave. Why not just use imhist() and imregionalmin()? It's much more straightforward and only 2 lines of code.
[counts, grayLevels] = imhist(I); % Get histogram into "counts"
minIndexes = imregionalmin(counts); % Find indexes of local mins in the histogram.
To get the gray levels instead of the indexes, you need to subtract 1 because index 1 is actually gray level 0.
grayLevelsAtLocalMins = minIndexes - 1;
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Muhammad Ali Qadar
Muhammad Ali Qadar 2013-11-11
@Image Analyst, I am sorry but I did not understand, that plot(grayLevelsAtLocalMins,'-o'), give what information, because its shows a strange number of lines can you please elaborate more that how I could get regionalmin points.

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