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Logging Data at Constant Intervals

This example shows how to log data at intervals instead of logging the entire acquisition.

In certain applications, it may not be necessary to log every frame provided by an image acquisition device. In fact, it may be more practical and resourceful to log frames at certain intervals.

To log frames at a constant interval, configure the video input object's FrameGrabInterval property. Configuring the property to an integer value N specifies that every Nth frame should be logged, starting with the first frame.

Note, specifying a FrameGrabInterval value does not modify the rate at which a device is providing frames (device frame rate). It only specifies the interval at which frames are logged.

Step 1: Access and Configure a Device.

Create a video input object and configure the desired logging interval. The logging interval is determined by the value of the FrameGrabInterval property.

% Access an image acquisition device.
vidobj = videoinput('winvideo', 1);

% Configure the number of frames to log.
framesToLog = 9;
vidobj.FramesPerTrigger = framesToLog;

% Configure the logging interval. This specifies that
% every 10th frame provided by the device is to be logged.
grabInterval = 10;
vidobj.FrameGrabInterval = grabInterval;

% Access the device's video source and configure the device's frame rate.
% FrameRate is a device specific property, therefore, it may not be supported by
% some devices.
frameRate = 30;
src = getselectedsource(vidobj);
src.FrameRate = num2str(frameRate);

Step 2: Log and Retrieve Data.

Initiate the acquisition of images and retrieve the logged frames and their timestamps.

% Start the acquisition.

% Wait for the acquisition to end.
wait(vidobj, 10)

% Retrieve the data.
[frames, timeStamp] = getdata(vidobj);

Step 3: Calculate the Time Difference Between Frames.

Knowing the device's actual frame rate and the grab interval at which frames were logged, the number of frames logged per second can be calculated.

% Number of frames logged per second.
loggedPerSec = frameRate/grabInterval
loggedPerSec =


Knowing the number of frames logged per second, the expected time interval between each logged frame can be calculated and compared.

% Expected number of seconds between each logged frame.
loggingRate = 1/loggedPerSec
loggingRate =

% Actual time difference between each logged frame.
% Note that frames were logged at a constant interval.
ans =

    0.3332    0.3338    0.3331    0.3332    0.3330    0.3332    0.3331    0.3330
% Determine the average time difference between frames.
avgDiff = mean(diff(timeStamp'))
avgDiff =

percentError = ( abs(loggingRate-avgDiff) ) * 100
percentError =

% Once the video input object is no longer needed, delete
% it and clear it from the workspace.
clear vidobj
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