what does this error mean? Error while evaluating UIControl Callback.

this is my code:

% --- Executes on button press in gray_scale.

function gray_scale_Callback(hObject, eventdata, handles)

% hObject handle to gray_scale (see GCBO)

% eventdata reserved - to be defined in a future version of MATLAB

% handles structure with handles and user data (see GUIDATA)

a= getappdata(0,'a');

a_gray= im2gray(a);

setappdata(0, 'filename', a_gray);

axes(handles.axes3);

imshow(a_gray)

Answer 1

[Image Analyst]

It means that when you click on the button called gray_scale, there was an error. You didn't give the entire error message (ALL the red text) but just a snippet of it so we don't know what really went wrong. But possibly there was no (badly named) a in the appdata workspace. Are you sure you loaded it in before then? Or it could be that a exists but it is not an RGB image.

Attach your .fig and .m file if you need more help.