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Webcam preview in GUI not showing up

Asked by Jesse McAlister on 12 Jun 2019
Latest activity Answered by Geoff Hayes
on 13 Jun 2019
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)

  2 Comments

Jesse - try saving the cam objec to the handles structure. From your code, it looks like cam will be a local variable so when the OpeningFcn completes, the cam object may be destroyed. (I say may since I am only guessing...)
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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R2019a

1 Answer

Answer by Geoff Hayes
on 13 Jun 2019
 Accepted 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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