Documentation

Determining the Adaptor Name

To determine the name of the adaptor, enter the imaqhwinfo function at the MATLAB prompt without any arguments.

imaqhwinfo
ans = 

    InstalledAdaptors: {'dcam'  'winvideo'}
        MATLABVersion: '7.4 (R2007a)'
          ToolboxName: 'Image Acquisition Toolbox'
       ToolboxVersion: '2.1 (R2007a)'

In the data returned by imaqhwinfo, the InstalledAdaptors field lists the adaptors that are available on your computer. In this example, imaqhwinfo found two adaptors available on the computer: 'dcam' and 'winvideo'. The listing on your computer might contain only one adaptor name. Select the adaptor name that provides access to your image acquisition device. For more information, see Determining the Device Adaptor Name.

Determining the Device ID

To find the device ID of a particular image acquisition device, enter the imaqhwinfo function at the MATLAB prompt, specifying the name of the adaptor as the only argument. (You found the adaptor name in the first call to imaqhwinfo, described in Determining the Adaptor Name.) In the data returned, the DeviceIDs field is a cell array containing the device IDs of all the devices accessible through the specified adaptor.

info = imaqhwinfo('dcam')
info = 

       AdaptorDllName: [1x77 char]
    AdaptorDllVersion: '2.1 (R2007a)'
          AdaptorName: 'dcam'
            DeviceIDs: {[1]}
           DeviceInfo: [1x1 struct]

Determining the Supported Video Formats

To determine which video formats an image acquisition device supports, look in the DeviceInfo field of the data returned by imaqhwinfo. The DeviceInfo field is a structure array where each structure provides information about a particular device. To view the device information for a particular device, you can use the device ID as a reference into the structure array. Alternatively, you can view the information for a particular device by calling the imaqhwinfo function, specifying the adaptor name and device ID as arguments.

To get the list of the video formats supported by a device, look at SupportedFormats field in the device information structure. The SupportedFormats field is a cell array of character vectors where each character vector is the name of a video format supported by the device. For more information, see Determining Supported Video Formats.

dev_info = imaqhwinfo('dcam',1)

dev_info = 

          DefaultFormat: 'F7_Y8_1024x768'
    DeviceFileSupported: 0
             DeviceName: 'XCD-X700  1.05'
               DeviceID: 1
  VideoInputConstructor: 'videoinput('dcam', 1)'
 VideoDeviceConstructor: 'imaq.VideoDevice('dcam', 1)'
       SupportedFormats: {'F7_Y8_1024x768'  'Y8_1024x768'}

Viewing the Video Input Object Summary

To view a summary of the video input object you just created, enter the variable name vid at the MATLAB command prompt. The summary information displayed shows many of the characteristics of the object, such as the number of frames that will be captured with each trigger, the trigger type, and the current state of the object. You can use video input object properties to control many of these characteristics. See Step 5: Configure Object Properties (Optional) for more information.

vid

Summary of Video Input Object Using 'XCD-X700  1.05'.

    Acquisition Source(s):  input1 is available. 

   Acquisition Parameters:  'input1' is the current selected source.
                            10 frames per trigger using the selected source.
                            'Y8_1024x768' 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. 
                     0 frames acquired since starting.
                     0 frames available for GETDATA.

Types of Image Acquisition Objects

The toolbox uses two types of objects to represent the connection with an image acquisition device:

A video input object represents the connection between MATLAB and a video acquisition device at a high level. The properties supported by the video input object are the same for every type of device. You created a video input object using the videoinput function in step 3.

When you create a video input object, the toolbox automatically creates one or more video source objects associated with the video input object. Each video source object represents a collection of one or more physical data sources that are treated as a single entity. The number of video source objects the toolbox creates depends on the device and the video format you specify. At any one time, only one of the video source objects, called the selected source, can be active. This is the source used for acquisition. For more information about these image acquisition objects, see Creating Image Acquisition Objects.

Viewing Object Properties

To view a complete list of all the properties supported by a video input object or a video source object, use the get function. To list the properties of the video input object created in step 3, enter this code at the MATLAB prompt.

get(vid)

The get function lists all the properties of the object with their current values.

General Settings:
    DeviceID = 1
    DiskLogger = []
    DiskLoggerFrameCount = 0
    EventLog = [1x0 struct]
    FrameGrabInterval = 1
    FramesAcquired = 0
    FramesAvailable = 0
    FramesPerTrigger = 10
    Logging = off
    LoggingMode = memory
    Name = Y8_1024x768-dcam-1
    NumberOfBands = 1
    Previewing = on
    ReturnedColorSpace = grayscale
    ROIPosition = [0 0 1024 768]
    Running = off
    Tag = 
    Timeout = 10
    Type = videoinput
    UserData = []
    VideoFormat = Y8_1024x768
    VideoResolution = [1024 768]
    .
    .
    .

To view the properties of the currently selected video source object associated with this video input object, use the getselectedsource function in conjunction with the get function. The getselectedsource function returns the currently active video source. To list the properties of the currently selected video source object associated with the video input object created in step 3, enter this code at the MATLAB prompt.

get(getselectedsource(vid))

The get function lists all the properties of the object with their current values.

General Settings:
    Parent = [1x1 videoinput]
    Selected = on
    SourceName = input1
    Tag = 
    Type = videosource

  Device Specific Properties:
    FrameRate = 15
    Gain = 2048
    Shutter = 2715

Setting Object Properties

To set the value of a video input object property or a video source object property, you reference the object property as you would a field in a structure, using dot notation.

Some properties are read only; you cannot set their values. These properties typically provide information about the state of the object. Other properties become read only when the object is running. To view a list of all the properties you can set, use the set function, specifying the object as the only argument.

To implement continuous image acquisition, the example sets the TriggerRepeat property to Inf. To set this property, enter this code at the MATLAB prompt.

vid.TriggerRepeat = Inf;

To help the application keep up with the incoming video stream while processing data, the example sets the FrameGrabInterval property to 5. This specifies that the object acquire every fifth frame in the video stream. (You might need to experiment with the value of the FrameGrabInterval property to find a value that provides the best response with your image acquisition setup.) This example shows how you can set the value of an object property by referencing the property as you would reference a field in a MATLAB structure.

vid.FrameGrabInterval = 5; 

To set the value of a video source object property, you must first use the getselectedsource function to retrieve the object. (You can also get the selected source by searching the video input object Source property for the video source object that has the Selected property set to 'on'.)

To illustrate, the example assigns a value to the Tag property.

vid_src = getselectedsource(vid);

vid_src.Tag = 'motion detection setup';

Running the Example

To run the example, enter the following code at the MATLAB prompt. The example loops until a specified number of frames have been acquired. In each loop iteration, the example calls getdata to bring the two most recent frames into the MATLAB workspace. To detect motion, the example subtracts one frame from the other, creating a difference image, and then displays it. Pixels that have changed values in the acquired frames will have nonzero values in the difference image.

The getdata function removes frames from the memory buffer when it brings them into the MATLAB workspace. It is important to move frames from the memory buffer into the MATLAB workspace in a timely manner. If you do not move the acquired frames from memory, you can quickly exhaust all the memory available on your system.

% Create video input object. 
vid = videoinput('dcam',1,'Y8_1024x768')

% Set video input object properties for this application.
vid.TriggerRepeat = 100;
vid.FrameGrabInterval = 5;

% Set value of a video source object property.
vid_src = getselectedsource(vid);
vid_src.Tag = 'motion detection setup';

% Create a figure window.
figure; 

% Start acquiring frames.
start(vid)

% Calculate difference image and display it.
while(vid.FramesAvailable >= 2)
    data = getdata(vid,2); 
    diff_im = imabsdiff(data(:,:,:,1),data(:,:,:,2));
    imshow(diff_im);
    drawnow     % update figure window
end

stop(vid)

Note that a drawnow is used after the call to imshow in order to ensure that the figure window is updated. This is good practice when updating a GUI or figure inside a loop.

The following figure shows how the example displays detected motion. In the figure, areas representing movement are displayed.

Image Data in the MATLAB Workspace

In the example, the getdata function returns the image frames in the variable data as a 480-by-640-by-1-by-10 array of 8-bit data (uint8).

whos 
  Name           Size                   Bytes  Class

  data           4-D                  3072000  uint8 array
  dev_info       1x1                     1601  struct array
  info           1x1                     2467  struct array
  vid            1x1                     1138  videoinput object
  vid_src        1x1                      726  videosource object

The height and width of the array are primarily determined by the video resolution of the video format. However, you can use the ROIPosition property to specify values that supersede the video resolution. Devices typically express video resolution as column-by-row; MATLAB expresses matrix dimensions as row-by-column.

The third dimension represents the number of color bands in the image. Because the example data is a grayscale image, the third dimension is 1. For RGB formats, image frames have three bands: red is the first, green is the second, and blue is the third. The fourth dimension represents the number of frames that have been acquired from the video stream.