Matlab Guide: Developing a basic webcam viewer

Question

Code below is supposed to display the webcam preview in figure called axes1, when button preview is clicked. It must display the webcam images in real time.

% --- Executes on button press in Preview.
function Preview_Callback(hObject, eventdata, handles)
% hObject    handle to Preview (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)
    img = snapshot(cam);

    imshow(img,'Parent',handles.axes1);

    disp(clock);
    pause(0.5);

Error appearing is: Error while evaluating UIControl Callback

Undefined function or variable 'cam'.

Error in Main>Preview_Callback (line 161) img = snapshot(cam);

Error in gui_mainfcn (line 95) feval(varargin{:});

Error in Main (line 43) gui_mainfcn(gui_State, varargin{:});

Error in @(hObject,eventdata)Main('Preview_Callback',hObject,eventdata,guidata(hObject))

Error while evaluating UIControl Callback

If I place cam=webcam; before img = snapshot(cam); the error states Error using Main>Preview_Callback (line 161) An active connection to webcam, Integrated Webcam, already exists. You cannot create another connection to the same webcam.

Error in gui_mainfcn (line 95) feval(varargin{:});

Error in Main (line 43) gui_mainfcn(gui_State, varargin{:});

Error in @(hObject,eventdata)Main('Preview_Callback',hObject,eventdata,guidata(hObject))

Error while evaluating UIControl Callback

It is clear that the initialization is at the incorrect place. Where should one intialise/ state connection to the webcam?

Sade Sade · Accepted Answer · 2017-07-18T08:18:31

Intialise counter as follows:
handles.PressOne=0;


if handles.PressOne==0
     Connect to the camera (cam=webcam). 
     Update counter for handles.PressOne.
else
     Capture the image (img=snapshot(cam)) and disp the data. 
end

Matlab Guide: Developing a basic webcam viewer

1 Answers