Image Acquisition Toolbox 3.4

Creating Time-Lapse Video Using Timer Events

The Image Acquisition Toolbox™ makes it easy to produce time-lapse video. In this demo, we will use timer events to acquire frames to an AVI file. This technique of time decimation has the advantage that it allows you to make a decision about each frame before storing it. An application of this would be to only store frames that meet certain illumination levels, have motion relative to the previous frame, etc.

Watch a day long time-lapse sequence. (21 seconds)

Contents

Create a Video Input Object

Before acquiring images using the Image Acquisition Toolbox, create a video input object.

% When executing the following code, you may need to
% modify it to match your acquisition hardware.
vid = videoinput('winvideo',1,'RGB24_400x300');

Configure the Timer

To generate timer events, we specify two things: what happens when it occurs, and how often it should occur. The TimerFcn property specifies the m-file callback function to execute when a timer event occurs. A timer event occurs when the time period specified by the TimerPeriod property expires.

The configuration we will use is

set(vid,'TimerPeriod',1);
set(vid,'TimerFcn',@timelapse_timer);

Store the AVI File Object

Store the AVIFile object in the UserData property of the video input object so that it will be available inside the callback.

avi = avifile('timelapsevideo');
set(vid,'UserData',avi);

Configure the Video Input Object to Use Manual Triggering

Each time the timer event occurs

triggerconfig(vid, 'manual');
set(vid,'FramesPerTrigger',1);
set(vid,'TriggerRepeat',9);
vid

Summary of Video Input Object Using 'Creative WebCam (WDM)'.

   Acquisition Source(s):  input1 is available.

  Acquisition Parameters:  'input1' is the current selected source.
                           1 frames per trigger using the selected source.
                           'RGB24_400x300' video data to be logged upon STAR
T.
                           Grabbing first of every 1 frame(s).
                           Log data to 'memory' on trigger.

      Trigger Parameters:  10 'manual' trigger(s) upon TRIGGER.

                  Status:  Waiting for START.
                           0 frames acquired since starting.
                           0 frames available for GETDATA.

Perform the Time-Lapse Acquisition

Now, start the time lapse acquisition, and wait for up to 20 seconds for the acquisition to complete.

start(vid);
wait(vid,20);

Close the AVI File

Once the capture is completed, retrieve the AVI file object stored in the UserData property, and use the close function to release the resources associated with it.

avi = get(vid,'UserData');
avi = close(avi);

Play Back the Time-Lapse AVI Sequence

To play back the time-lapse AVI sequence, right-click on the filename in the MATLAB® Current Directory browser, and choose Open Outside MATLAB from the context menu.

Clean Up

When you are done with the video input object, you should use the delete function to free the hardware resources associated with it, and remove it from the workspace using the clear function.

delete(vid);
clear vid;

The Timer Callback Function

The following is a description of the callback function executed for each timer event

The AVI file object is stored in the UserData property of the object.

type timelapse_timer

function timelapse_timer(vid,event)

    % trigger the acquisition and get the frame
    trigger(vid);
    frame = getdata(vid,1);

    % Retrieve the total number of frames acquired
    numframes_acquired = get(vid,'FramesAcquired');

    % Drop every other frame:  If the frame is odd,
    % keep it.  If it is an even frame, do not keep it.
    keepframe = (mod(numframes_acquired,2) == 1);

    % Insert your processing code here

    if(~keepframe)
        return;
    end

    % Retrieve the AVI file object stored in the UserData
    % property.
    avi = get(vid,'UserData');

    % Add the frame to the AVI
    avi = addframe(avi,frame);

    % Update the AVI file object stored in the UserData
    % property.
    set(vid,'UserData',avi);
end