I'm trying to use a toggle button to start and stop video acquisition but i get the following error. Could any1 help???

Trying to use a simple toggle button to start and stop my video processing i get an error.

my code under that of the toggle button being

% Hint: get(hObject,'Value') returns toggle state of startstop
clc; 
source= videoinput('winvideo');
set(source, 'ReturnedColorSpace', 'RGB');
set(source, 'FramesPerTrigger', 1);
set(source, 'TriggerRepeat', inf);
triggerconfig(source, 'manual');
start(source);
pause(2);
trigger(source);

if get(handles.startstop,'Value')
      set(handles.startstop,'String','Stop');
      set(handles.startstop,'Value',1);
      drawnow();
  else
      set(handles.startstop,'String','Start');
      set(handles.startstop,'Value',0);
      clear(source);
      stop(source);
end

i get the following error

??? Error using ==> imaqdevice.start at 91
Multiple VIDEOINPUT objects cannot access the same device simultaneously.

Answer 1

[Walter Roberson]

When you push the toggle button to start the video, you create the videoinput(). Then when you push the button to stop the video, you again create the videoinput.

When you have a togglebutton, the code does not somehow wait around for the togglebutton to change state and then continue. Instead each time you push the togglebutton, the entire callback is executed.

You need to find a way to save the value of "source" somewhere so it can be available for when you are turning off the video.

http://matlab.wikia.com/wiki/FAQ#How_can_I_share_data_between_callback_functions_in_my_GUI.3F

Note: clear() must be applied to a variable name, and after you clear the variable, it will no longer be in the workspace to be available to stop(). clear() does not stop a video object; see http://www.mathworks.com/help/imaq/clear.html, and it also does not delete the video object; see http://www.mathworks.com/help/imaq/delete.html. You should stop the object, and delete it. If it is a local variable then you do not need to clear it as the local reference will go out of scope when the function returns.

[Sanjeev]

could you pls comment on what is it that shld do? how shld i change my code?

[Walter Roberson]

Check whether the button is "on" first. If it is, then create your source, and store a copy of it outside the function (or using a persistent variable), and trigger the action. If the button is "off" then retrieve the copy of source and stop() it and delete() the video object.

[Sanjeev]

do i give a copy of the variable source inside the else loop and then stop and delete it?? like

source=value

stop(value)

delete(value)?

[Walter Roberson]

There probably no need to take a copy of it like that. When I say "retrieve the copy", I mean from where-ever you stored it outside the function. Refer to the FAQ item I linked to above for methods to store data outside of callback functions.

[Sanjeev]

Walter will u help me with the two lines of codes 2 be used??? im having no luck at all???

[Walter Roberson]

if get(handles.startstop,'Value')
  source= videoinput('winvideo');
  set(source, 'ReturnedColorSpace', 'RGB');
  set(source, 'FramesPerTrigger', 1);
  set(source, 'TriggerRepeat', inf);
  triggerconfig(source, 'manual');
  start(source);
  pause(2);
  trigger(source);
  set(handles.startstop,'String','Stop');
  set(handles.startstop,'Value',1);
  handles.source = source;
  guidata(gcbo, handles);
  drawnow();
else
  set(handles.startstop,'String','Start');
  set(handles.startstop,'Value',0);
  source = handles.source;
  stop(source);
  delete(source)
  handles.source = [];
  guidata(gcbo, handles)
end

[Sanjeev]

thanks Walter. ur code is working perfectly. if u dont mind could you explain what the codes u added does exactly??

handles.source = source;

guidata(gcbo, handles);

drawnow()

handles.source = [];

guidata(gcbo, handles

[Walter Roberson]

http://matlab.wikia.com/wiki/FAQ#How_can_I_share_data_between_callback_functions_in_my_GUI.3F