How do I find the minimum pixel value for every point along a line automatically ?

I have a point in an image with a small crack at the notch tip as shown in the figure 1. I want to find the minimum value of the pixel at every horizontal line as shown in fig 2. The ultimate motive is to find the crack length automatically without imtool.

Answer 1

[Bjorn Gustavsson]

Calculate the pixel intensities along a line-segment with interp2:

x_line = 123:432; % or something like that
y_line = linspace(321,312,numel(x_line)); %
Igray = rgb2gray(YourImg);
I_line = interp2(Igray,x_line,y_line);
% Then find minimum along I_line:
[I_min,idxMin] = min(I_line);

HTH

[Darshika Gumare]

Hey Bjorn,

Thanks for the help. But while running this code I encountered the folloeing error.

Error using griddedInterpolant

Sample values must be a single or double array.

Error in interp2>makegriddedinterp (line 228)

F = griddedInterpolant(varargin{:});

Error in interp2 (line 112)

F = makegriddedinterp({X,Y},V,method,extrap);

[Bjorn Gustavsson]

When you get an error like that do the following:

dbstop if error

Then rerun your code, matlab will interrupt at the offending line and give you a debug-prompt there so that you can inspect the variables in that function workspace/scope, and also step up in the function call-stack, to figure out what is happening.

In this case it might be that your gray-scale image is not in a format interp2 accepts, i.e. double or single float. To fix such problems I typically cast my images to double:

Igray = double(rgb2gray(YourImg));

HTH

[Darshika Gumare]

Sorry, but the code is not giving any output

[Bjorn Gustavsson]

Are the I_min and idxMin both empty? How have you modified the code-snippet?

[Darshika Gumare]

The code stops at after Igray output. I made the changes as suggested by you. Igray = double(rgb2gray(YourImage))

[Bjorn Gustavsson]

So you get an error from interp2?

[Darshika Gumare]

Yes.

[Bjorn Gustavsson]

Darishka, how do you think anyone can proceed with advicing you from that one-word-answer?

[Darshika Gumare]

Sir, actually I have already said in my answer previously the whole error which is appearing after running the code.

[Bjorn Gustavsson]

Interesting. I run this code-snippet without problems:

Igray = peaks(1234);
subplot(4,1,1:3)
imagesc(Igray)
hold on
x_line = 623:662;
y_line = linspace(421,412,numel(x_line)); %;
plot(x_line,y_line,'k')
I_line = interp2(qwe,x_line,y_line);
subplot(4,1,4)  
plot(x_line,I_line)
[I_min,idxMin] = min(I_line) % returns:
% I_min =
%      -1.8975
%idxMin =
%    40

Can you share your code-snippet?