How to use slider for image processing?

Dear Matlab Community,

I'm working to create a tool that would measure diameter of our fibers. We've decided to create a GUI for ease of use and to change parameters when needed.

I'm creating GUI with the help of GUIDE. I want to have image updated on the axes when slider is moved, but it doesn't seem to work. Disp line is executed when slider is moved, but I'm not sure why picture isn't displayed. How do I know if bwopen line is executed? Why do variable disappear from matlab workspace? Lastly, Am I using right technique to import and handle images?

I'm totally new to this. I'm referring books to learn about GUI. I'm struggling with slider for about two weeks.

I'm referring to this text: "Learning to Program with MATLAB - Building GUI Tools"

Is there any other tutorial or text you'd recommend?

Sincerely, Kev

Here is the code for GUI:

          % --- Executes on button press in load_file.
          function load_file_Callback(hObject, eventdata, handles)
          % hObject    handle to load_file (see GCBO)
          % eventdata  reserved - to be defined in a future version of MATLAB
          % handles    structure with handles and user data (see GUIDATA)

          global read_image
          [filename, user_canceled] = imgetfile(); 
          %fmt = 'jpg';
          %[path,user_cance]=imgetfile();

          if user_canceled
              msgbox(sprintf('Error'),'Error','Error');
              return
          end
          read_image = imread(filename);
          axes(handles.axes1);
          imshow(read_image);

        % --- Executes on button press in binarize.
        function binarize_Callback(hObject, eventdata, handles)
        % hObject    handle to binarize (see GCBO)
        % eventdata  reserved - to be defined in a future version of MATLAB
        % handles    structure with handles and user data (see GUIDATA)

        global read_image
        colour_to_gray = rgb2gray(read_image);

        % Apply median filtering on grayscale image for smoothing 
        filtered_image = medfilt2(colour_to_gray); 
        %imtool(filtered_image)

        % Convert grayscale into Binary using Otsu Method 
        level = graythresh(filtered_image);
        binary = im2bw(filtered_image, level);
         axes(handles.axes1);
         imshow(binary);

        % --- Executes on slider movement.
        function xSlider_Callback(hObject, eventdata, handles)
        % hObject    handle to xSlider (see GCBO)
        % eventdata  reserved - to be defined in a future version of MATLAB
        % handles    structure with handles and user data (see GUIDATA)

        % Hints: get(hObject,'Value') returns position of slider
        %        get(hObject,'Min') and get(hObject,'Max') to determine range of slider

        global binary

        x=get(handles.xSlider, 'Value');

        xs=num2str(x);
        set(handles.xText, 'String', xs);
        disp( ' Go, go, go, world! ' ); 
        bw_open = imopen(binary, strel('disk', x)); 
        axes(handles.axes1);
        imshow(bw_open)

Answer 1

[Image Analyst]

Set a break point on the imopen line by clicking in the left margin - a red spot will appear. If it stops there then it will execute it when you type F10. You may or may not see an effect when you display bw_open depending on the image and the size of x. You can subtract to see if anything changed.

difference = abs(double(bw_open) - double(binary));
imshow(difference, []);

If it's all black then there is no difference.

[Kev]

Dear Image Analyst,

I applied breakpoint before imopen, but axes disappears when imshow() line is executed. I also added above lines there and it didn't show any effect. My slider values are only integers between 1 to 10 and there would be noticeable changes for imopen with given values.

Thank you!

[Image Analyst]

Put in these lines and tell what it says

whos binary
whos bw_open
fprintf('Min = %d, Max = %d\n', min(binary(:)), max(binary(:)));
fprintf('Min = %d, Max = %d\n', min(bw_open(:)), max(bw_open(:)));
fprintf('Min = %d, Max = %d\n', min(difference(:)), max(differences(:)));

[Kev]

I'm receiving following output:

 Name        Size            Bytes  Class     Attributes

binary      0x0                 0  double    global    

Name         Size            Bytes  Class     Attributes

bw_open      0x0                 0  double

Min = , Max = Min = , Max = Undefined function or variable "differences".

Error in fig1>xSlider_Callback (line 141) fprintf('Min = %d, Max = %d\n', min(differences(:)), max(differences(:)))

Error in gui_mainfcn (line 96) feval(varargin{:});

Error in fig1 (line 42) gui_mainfcn(gui_State, varargin{:});

Error in @(hObject,eventdata)fig1('xSlider_Callback',hObject,eventdata,guidata(hObject))

Error while evaluating uicontrol Callback

[Image Analyst]

Well there's your problem. Your binary variable is no good.

[Kev]

How may I fix it, sir?

[Image Analyst]

Put "global binary" in your binarize_callback so that it becomes a global variable instead of a local variable seen only within that callback.

[Kev]

Dear Image Analyst,

It worked. :)

Thanks very much.

Answer 2

[cr]

Code looks ok. imopen might be giving you an output that you dont expect. i.e. the "opened" image might the same as the one already on axes that you dont see any change, misleadingly looking as if not functioning. The workspace variables inside a function are deleted as soon as the control exits the function. If you want access to them, either make them global, or better, use breakpoints and debugging.

[Kev]

Thanks for reply, but you didn't provide solution.

Please help