Diplay a dialog box when using a wrong callback using images in gui

I'm still learning how tu use GUI in matlab. i Have a gui that reads an image and display it. Then i have 3 buttons which converts the image into a grayscale image (R, G or B) then, using a slider, i set the threshold for a binary image. Well, when i use the slider without pressing any buttons (r, g or b) i get an error " Reference to non-existent fiel". And that's fine, i know i have to get an error there, but what i want to do is to display an error using waitfor(msgbox('')); when that error occurs. i'm still traying to do that, but i need a little help please.

Well, i want to use this gui when running a certain part from a code in a .mat file. For example. I run the code, read an image, then call the gui, binarize the same image, save the binarized image, and then use that saved image in the original code and continue the code. Hope it makes sense. How can i do this?

Here is the gui

function prueba_gui_OpeningFcn(hObject, eventdata, handles, varargin)
handles.output = hObject;

handles.imagen = imread('G:\Memoria\test\Imagen125.jpg');
axes(handles.axes1);
imshow(handles.imagen);

guidata(hObject, handles);

function varargout = prueba_gui_OutputFcn(hObject, eventdata, handles) 

varargout{1} = handles.output;

function binar_Callback(hObject, eventdata, handles)

handles.output = hObject;
axes(handles.axes1);
tempImg = handles.im;
thresholdValue = get(handles.binar, 'value');
binaryImage = tempImg> thresholdValue;
se = strel('square',4);
w = imdilate(binaryImage,se);
imshow(w);
guidata(hObject,handles);

function binar_CreateFcn(hObject, eventdata, handles)
if isequal(get(hObject,'BackgroundColor'), get(0,'defaultUicontrolBackgroundColor'))
    set(hObject,'BackgroundColor',[.9 .9 .9]);
end

function RED_Callback(hObject, eventdata, handles)

handles.output = hObject;
axes(handles.axes1);
handles.im = handles.imagen(:,:,1);
imshow(handles.im);
guidata(hObject,handles);

function GREEN_Callback(hObject, eventdata, handles)

handles.output = hObject;
axes(handles.axes1);
handles.im = handles.imagen(:,:,2);
imshow(handles.im);
guidata(hObject,handles);

function BLUE_Callback(hObject, eventdata, handles)

handles.output = hObject;
axes(handles.axes1);
handles.im = handles.imagen(:,:,3);
imshow(handles.im);
guidata(hObject,handles);

% --- Executes on button press in save.
function save_Callback(hObject, eventdata, handles)

handles.output = hObject;
%nothing here yet

Answer 1

[Image Analyst]

See the FAQ for how to share variables between different functions in your m-file. http://matlab.wikia.com/wiki/FAQ#How_can_I_share_data_between_callback_functions_in_my_GUI.28s.29.3F

[Alejandro]

sorry, can you be more especific please? i understand when the variable is a number or an array, but in this case my variable is an image.

[Image Analyst]

It doesn't matter - they're all variables. Why would you get an error like you said? Can you post the exact error message - all the red text?

[Alejandro]

this error:

Reference to non-existent field 'im'.
Error in prueba_gui>binar_Callback (line 74)
tempImg = handles.im;
Error in gui_mainfcn (line 96)
        feval(varargin{:});
Error in prueba_gui (line 42)
    gui_mainfcn(gui_State, varargin{:});
Error in @(hObject,eventdata)prueba_gui('binar_Callback',hObject,eventdata,guidata(hObject))

Error while evaluating uicontrol Callback

i actually know why that error occurs. The normal image (RGB image) is not defined in binar_callback. All i want to do is to tell the "user", that before using the slider to set the thresholdValue, to press the RED, GREEN or BLUE buttons and then use the slider. And if the "user" use the slider before pressing any of those buttons, display an error in a msgbox saying that a button needs to be press first (or the image has to be grayscale). I can't figure it out yet.

[Image Analyst]

% See if we can pull out the im field.
hasField = isfield(handles, 'im');
if ~hasField
  % Has no im field yet.
  uiwait(warndlg('You need to click the binar button first!'));
  return;
end

[Alejandro]

i added this:

hasField = isfield(handles, 'im');
if ~hasField
  % Has no im field yet.
  uiwait(warndlg('You need to click the binar button first!'));
  return;
else hasField = handles.im; 
end

and it works!! Thanks!! could you help me with the second question i asked please?

[Image Analyst]

I did. That is what the link was for. Just attach the variable to the handles structure or use setappdata() and get appdata(), or declare it global.

Please mark the answer as Accepted if we're done here. Thanks.

[Alejandro]

a ok, no problem. But i was asking for using the saved binarized image in a *.mat file that is the general code for my program. The gui is just for binarize an image, and i want to use this image in further processes in the code. that´s all i need for now.

[Image Analyst]

You can save it in a mat file. That's one way listed in the FAQ. Though it's slower because you have to write to disk and read from disk. It's mostly used for passing data between two separate GUIs rather than within functions of the same GUI.

[Alejandro]

thanks!