Video feed will not start() in my axes object, but works with preview().

Hello,

I am trying to build a GUI that will allow a user to switch between a selection of cameras in a dropdown menu. The camera selected will show up in a display window on the GUI called cameraAxes. (The dropdown menu build is just fine.) Here is my code:

   function varargout = NWEMultiLEDGUI(varargin)
   % NWEMULTILEDGUI MATLAB code for NWEMultiLEDGUI.fig
   % Begin initialization code - DO NOT EDIT
   gui_Singleton = 1;
   gui_State = struct('gui_Name',       mfilename, ...
                      'gui_Singleton',  gui_Singleton, ...
                      'gui_OpeningFcn', @NWEMultiLEDGUI_OpeningFcn, ...
                      'gui_OutputFcn',  @NWEMultiLEDGUI_OutputFcn, ...
                      'gui_LayoutFcn',  [] , ...
                      'gui_Callback',   []);
   if nargin && ischar(varargin{1})
       gui_State.gui_Callback = str2func(varargin{1});
   end

   if nargout
       [varargout{1:nargout}] = gui_mainfcn(gui_State, varargin{:});
   else
       gui_mainfcn(gui_State, varargin{:});
   end
   % End initialization code - DO NOT EDIT

   % --- Executes just before NWEMultiLEDGUI is made visible.
   function NWEMultiLEDGUI_OpeningFcn(hObject, eventdata, handles, varargin)

   % Choose default command line output for NWEMultiLEDGUI
   handles.output = hObject;

   %Initialize camera selection dropdown menu display:
   imaqreset;
   result = imaqhwinfo;
   adapterName = result.InstalledAdaptors{1}; %this is hardcoded; change later
   axes(handles.cameraAxes); %makes current axes object the cameraAxes
   axis off;
   hold('off');
   %currently 'winvideo' since it's the only adaptor. 
   %Will have to write more code if more adaptor options are needed
   camNum = 1;
   assignin('base','adapterName', adapterName);
   assignin('base','camNum', camNum);
   info = imaqhwinfo(adapterName);
   for count = 1: length(info.DeviceIDs)
       info = imaqhwinfo(adapterName, count);
       name = info.DeviceName;
       posList{count} = sprintf('%d: %s', count, name);
   end
   set(handles.selectCameraMenu,'String',posList);
   disp('Camera selection menu built...');

   %Camera display settings:
   %handles.video = videoinput(adapterName, camNum); %create video input
   imshow(imread(strcat(pwd,'\testImg.jpg'))); %currently displaying a test image.
   assignin('base','newCam', 'true'); % a flag variable to let the start/stop code know when to redefine the video source
   %assignin('base','newCam', 'false');

   %Turn on start/stop camera once initialization is ready
   set(handles.startStopCameraButton,'Enable','on');
   set(handles.statusBar,'String','Select a camera');
   disp('End initialization sequence.');

   % Update handles structure
   guidata(hObject, handles);

   % --- Outputs from this function are returned to the command line.
   function varargout = NWEMultiLEDGUI_OutputFcn(hObject, eventdata, handles) 
   % Get default command line output from handles structure
   varargout{1} = handles.output;

   % --- Executes on selection change in selectCameraMenu.
   function selectCameraMenu_Callback(hObject, eventdata, handles)
   assignin('base','camNum', get(handles.selectCameraMenu,'Value'));
   assignin('base','newCam','true'); %a flag variable to let start/stop know if you       need to load a new camera.

   % --- Executes during object creation, after setting all properties.
   function selectCameraMenu_CreateFcn(hObject, eventdata, handles)
   if ispc && isequal(get(hObject,'BackgroundColor'),
   get(0,'defaultUicontrolBackgroundColor'))
       set(hObject,'BackgroundColor','white');
   end

   % --- Executes on button press in startStopCameraButton.
   function startStopCameraButton_Callback(hObject, eventdata, handles)
   % Start/Stop Camera
   set(handles.acqImgButton,'Enable','on'); 
   axes(handles.cameraAxes);
   vid = evalin('base','vidFeed');
   %get(handles.video);
   %runs if camera is currently stopped/has never been started
   if strcmp(get(handles.startStopCameraButton,'String'),'Start Camera')
       % Camera is off. Change button string and start camera.
       %read in vars from base workspace
       if(strcmp(evalin('base','newCam'),'true'))
           disp('Establishing camera parameters...');
           set(handles.statusBar,'String','Establishing camera parameters...');
           handles.video = videoinput(evalin('base','adapterName'),
   evalin('base','camNum'));
           axes(handles.cameraAxes);
           hold(handles.cameraAxes, 'off')
           set(handles.video,'TimerPeriod', 0.05, ...
           'TimerFcn',['if(~isempty(gco)),'...
           'handles=guidata(gcf);'... % Update handles
           'image(getsnapshot(handles.video));'... % Get picture using GETSNAPSHOT    and put it into axes using IMAGE
           'set(handles.cameraAxes,''ytick'',[],''xtick'',[]),'... % Remove    tickmarks and labels that are inserted when using IMAGE
           'else '...
           'end']);
           triggerconfig(handles.video,'manual');
           handles.video.FramesPerTrigger = Inf; % Capture frames until we manually    stop it
           set(handles.statusBar,'String','Loading camera settings...');
           assignin('base','newCam','false');
           assignin('base','vidFeed','vid');
       end
       hold('off');
       disp('Starting camera...');
       axes(handles.cameraAxes);
   %    preview(handles.video);  %This line works when uncommented, so I think the    handle is still alive here.
       start(handles.video);
       set(handles.statusBar,'String','Camera is live.');
       set(handles.contImgButton,'Enable','on');
       set(handles.startStopCameraButton,'String','Stop Camera');
       imaqfind %SEE BELOW FOR OUTPUT FROM THIS LINE
   else
       % Camera is on. Stop camera and change button string.
       disp('Stopping camera...');
       stop(handles.video);
   %    preview(handles.video);    %This line works when uncommented.
       set(handles.statusBar,'String','Camera stopped.');
       set(handles.contImgButton,'Enable','off');
      set(handles.startStopCameraButton,'String','Start Camera');
   end
   guidata(hObject, handles); % Update handles structure

The output when I run imaqfind inside startStopCameraButton is:

   Video Input Object Array:

   Index:   Type:          Name:  
   1        videoinput     YUY2_1280x960-winvideo-1
   2        videoinput     YUY2_1280x960-winvideo-1

There is no error output when I try to start or stop the camera.

The commented out "preview(handles.video)" line works in both the 'Start section' and 'Stop section' of the startStopCameraButton code.

The test image I load upon opening the GUI shows up throughout the testing sequence and appears unchanged. If I comment out the test image line, the region where the cameraAxes object exists on my GUI remains blank.

Hopefully someone has some insight. Thank you! -Lisa

Answer 1

[Rohit Kudva]

Hi Lisa,

I understand that you are unable to view any image on your GUI. I looked into your code and one of the possible reason for this issue can be that the condition for the 'if' statement of the 'TimerFcn' callback function is never true. In order to figure this out, write a separate function in MATLAB and set the 'TimerFcn' property of the videoinput object to the handle of that function

function mytimercallback(src,events)

% input parameter 'src' will be the handle to the 'videoinput' object
% The code goes here

end

And in the main function

set(handles.video,'TimerPeriod', 0.05, 'TimerFcn', @mytimercallback);

You can now add breakpoints to the 'mytimercallback' function to see if the 'if' statement condition is ever true.

Another possible reason can be that the reference to 'handles' in the callback function may be empty.

handles=guidata(gcf); % handles may be []

This again can be checked by debugging into the callback function.

Regards,

Rohit