MATLAB: Webcam preview in GUI not showing up

guiMATLABwebcam

Using MatlabR2019a. Trying to get a webcam preview to show up in a GUI, but just getting a black box in the resulting axes – see image below
gui.JPG
Here is the code:
function align_GUI_OpeningFcn(hObject, eventdata, handles, varargin)
% This function has no output args, see OutputFcn.
% hObject    handle to figure
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)
% varargin   command line arguments to align_GUI (see VARARGIN)
% Choose default command line output for align_GUI
handles.output = hObject;
cam=webcam(2)
imWidth=640;
imHeight=480;
axes(handles.axes1);
hImage=image(zeros(imHeight,imWidth,3),'Parent',handles.axes1);
preview(cam,hImage)
Webcam preview when just using the command window works fine using:
cam=webcam(2);
preview(cam)

Best Answer

The issue seems to be because the cam object is a local variable within the OpeningFcn function. When the function completes, the cam object is destroyed. The fix to add it to the handles structure so that it is available for the lifetime of the GUI. 
function align_GUI_OpeningFcn(hObject, eventdata, handles, varargin)
% This function has no output args, see OutputFcn.
% hObject    handle to figure
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)
% varargin   command line arguments to align_GUI (see VARARGIN)
% Choose default command line output for align_GUI
handles.output = hObject;
handles.cam=webcam(2)
imWidth=640;
imHeight=480;
axes(handles.axes1);
handles.hImage=image(zeros(imHeight,imWidth,3),'Parent',handles.axes1);
preview(handles.cam, handles.hImage)
guidata(hObject, handles)
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