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Acquiring Image Data

After completing chapters 3 and 4, you can see your adaptor included in the list of adaptors returned by imaqhwinfo and you can create a video input object using the videoinput function. Now it's time to acquire data from your device. In this chapter, you flesh out the stub implementations of the adaptor class virtual functions that work together to acquire data.

User Scenario

The following example shows how a toolbox user initiates the acquisition of image frames. The example calls the videoinput function to create a video input object and then calls the start function to start the object. Note in the summary that ten image frames are acquired.

vid = videoinput('winvideo');


Summary of Video Input Object Using 'IBM PC Camera'.

   Acquisition Source(s):  input1 is available.

  Acquisition Parameters:  'input1' is the current selected source.
                           10 frames per trigger using the selected source.
                           'RGB555_128x96' video data to be logged upon START.
                           Grabbing first of every 1 frame(s).
                           Log data to 'memory' on trigger.

      Trigger Parameters:  1 'immediate' trigger(s) on START.

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


In the previous example, the start function opens the connection with the device but does not actually cause the acquisition of image data. The toolbox uses triggers to control image acquisition. By default, video input objects are configured with an immediate trigger so, in the example, when you start the object, an immediate trigger fires.

The toolbox also supports two other types of triggers: manual and hardware. With a manual trigger, after starting a video input object, you must call the trigger function to acquire data. With hardware triggers, you start the object and it waits until it receives a signal from an external device to start acquiring data.

The toolbox handles immediate and manual triggering automatically; you do not have to include any special processing in your adaptor. Supporting hardware triggers, requires some adaptor development work. For more information, see Supporting Hardware Triggers.

Overview of Virtual Functions Used to Acquire Data

The pure virtual functions in your adaptor class that you must implement work together to acquire data. However, the main steps are:

  1. Specify the format of the video data in the getMaxHeight(), getMaxWidth(), getNumberOfBands(), and getFrameType() functions — see Specifying the Format of the Image Data.

  2. Open a connection with your device in the openDevice() function — see Opening and Closing Connection with a Device.

  3. Start acquiring data in the startCapture() function — see Starting and Stopping Image Acquisition.

  4. Stop acquiring data in the stopCapture() function — see Starting and Stopping Image Acquisition.

  5. Close the connection with the device in the closeDevice() function — see Opening and Closing Connection with a Device.

The following diagram shows this flow of control in graphical form. This diagram picks up where the diagram in Chapter 3 ends, after the object has been created — see Acquiring Image Data.


The diagrams do not show the calls your adaptor makes to the image acquisition device's SDK because these calls vary with each device's SDK.

Flow of Control Among the Adaptor Acquisition Functions

Flowchart that shows the steps required for acquiring image data between MATLAB, the toolbox engine, and the adaptor.