Working with Image Data in MATLAB Workspace
Understanding Image Data
The illustrations in this documentation show the video stream and the contents of the memory buffer as a sequence of individual frames. In reality, each frame is a multidimensional array. For more information about using multidimensional arrays, see Multidimensional Arrays. The following figure illustrates the format of an individual frame.
Format of an Individual Frame
The following sections describes how the toolbox
This section also describes several ways to view acquired image data.
Determining the Dimensions of Image Data
The video format used by the image acquisition device is the
primary determinant of the width, height, and the number of bands
in each image frame. Image acquisition devices typically support multiple
video formats. You select the video format when you create the video
input object (described in Specifying the Video Format). The video input object stores
the video format in the VideoFormat
property.
The video input object stores the video resolution in the VideoResolution
property.
Each image frame is three dimensional; however, the video format
determines the number of bands in the third dimension. For color video
formats, such as RGB, each image frame has three bands: one each for
the red, green, and blue data. Other video formats, such as the grayscale
RS170 standard, have only a single band. The video input object stores
the size of the third dimension in the NumberOfBands
property.
Note
Because devices typically express video resolution as width-by-height,
the toolbox uses this convention for the VideoResolution
property.
However, when data is brought into the MATLAB® workspace,
the image frame dimensions are listed in reverse order, height-by-width,
because MATLAB expresses
matrix dimensions as row-by-column.
ROIs and Image Dimensions
When you specify a region-of-interest (ROI) in the image being
captured, the dimensions of the ROI determine the dimensions of the
image frames returned. The VideoResolution
property
specifies the dimensions of the image data being provided by the device;
the ROIPosition
property specifies the dimensions
of the image frames being logged. See the ROIPosition
property
reference page for more information.
Video Format and Image Dimensions
The following example illustrates how video format affects the size of the image frames returned.
Select a video format — Use the
imaqhwinfo
function to view the list of video formats supported by your image acquisition device. This example shows the video formats supported by a Matrox® Orion frame grabber. The formats are industry standard, such as RS170, NTSC, and PAL. These standards define the image resolution.info = imaqhwinfo('matrox'); info.DeviceInfo.SupportedFormats ans = Columns 1 through 4 'M_RS170' 'M_RS170_VIA_RGB' 'M_CCIR' 'M_CCIR_VIA_RGB' Columns 5 through 8 'M_NTSC' 'M_NTSC_RGB' 'M_NTSC_YC' 'M_PAL' Columns 9 through 10 'M_PAL_RGB' 'M_PAL_YC'
Create an image acquisition object — This example creates a video input object for a Matrox image acquisition device using the default video format, RS170. To run this example on your system, use the
imaqhwinfo
function to get the object constructor for your image acquisition device and substitute that syntax for the following code.vid = videoinput('matrox',1);
View the video format and video resolution properties — The toolbox creates the object with the default video format. This format defines the video resolution.
vid.VideoFormat ans = M_RS170 vid.VideoResolution ans = [640 480]
Bring a single frame into the workspace — Call the
getsnapshot
function to bring a frame into the workspace.frame = getsnapshot(vid);
The dimensions of the returned data reflect the image resolution and the value of the
NumberOfBands
property.vid.NumberOfBands ans = 1 size(frame) ans = 480 640
Start the image acquisition object — Call the
start
function to start the image acquisition object.start(vid)
The object executes an immediate trigger and begins acquiring frames of data.
Bring multiple frames into the workspace — Call the
getdata
function to bring multiple image frames into the MATLAB workspace.data = getdata(vid,10);
The
getdata
function brings 10 frames of data into the workspace. Note that the returned data is a four-dimensional array: each frame is three-dimensional and the nth frame is indicated by the fourth dimension.size(data) ans = 480 640 1 10
Clean up — Always remove image acquisition objects from memory, and the variables that reference them, when you no longer need them.
delete(vid) clear vid
Determining the Data Type of Image Frames
By default, the toolbox returns image frames in the data type
used by the image acquisition device. If there is no MATLAB data type that matches the object's
native data type, getdata
chooses a MATLAB data type that preserves numerical
accuracy. For example, in RGB 555 format, each color component is
expressed in 5-bits. getdata
returns each color
as a uint8
value.
You can specify the data type you want getdata
to
use for the returned data. For example, you can specify that getdata
return
image frames as an array of class double
. To see
a list of all the data types supported, see the getdata
reference
page.
The following example illustrates the data type of returned image data.
Create an image acquisition object — This example creates a video input object for a Matrox image acquisition device. To run this example on your system, use the
imaqhwinfo
function to get the object constructor for your image acquisition device and substitute that syntax for the following code.vid = videoinput('matrox',1);
Bring a single frame into the workspace — Call the
getsnapshot
function to bring a frame into the workspace.frame = getsnapshot(vid);
View the class of the returned data — Use the
class
function to determine the data type used for the returned image data.class(frame) ans = uint8
Clean up — Always remove image acquisition objects from memory, and the variables that reference them, when you no longer need them.
delete(vid) clear vid
Viewing Acquired Data
Once you bring the data into the MATLAB workspace, you can view it as you would any other image in MATLAB.
The Image Acquisition Toolbox™ software
includes a function, imaqmontage
, that you can
use to view all the frames of a multiframe image array in a single MATLAB image object. imaqmontage
arranges
the frames so that they roughly form a square. imaqmontage
can
be useful for visually comparing multiple frames.
MATLAB includes two
functions, image
and imagesc
,
that display images in a figure window. Both functions create a MATLAB image object to display the
frame. You can use image object properties to control aspects of the
display. The imagesc
function automatically scales
the input data.
The Image Processing Toolbox™ software includes an additional
display routine called imshow
. Like image
and imagesc
,
this function creates a MATLAB image
object. However, imshow
also automatically sets
various image object properties to optimize the display.