Image Acquisitionand Proces

Image Acquisition 57
source LabVIEW Webcam library, released in July 2002, allows images to be captured
on machines running nearly any version of Windows at speeds of 7 to 10 frames per
second at 320 × 240 × 24 resolution. The library relies on a simple dynamic link library
(DLL) that communicates with the Video for Windows (VFW) programming interface
in order to initiate, configure, begin and end a capture session with an imaging device.
Labeling it a “Webcam library” is a slight misnomer, as VFW allows for communication
with practically any imaging device with a fully supported Windows driver.
3.4.2 FUNCTIONALITY
The Webcam library consists of two sets of VIs built around the low-level functionality
of the DLL. The first set assists in the capturing of images using the DLL
described previously. The second set provides a means of converting captured images
to the IMAQ format for processing, to the LabVIEW picture format for display on
the front panel, and to the bitmap format for file storage. When used together, the
capture and conversion VIs provide a straightforward means of acquiring and working
with image data from consumer-grade devices. A number of example uses of
the Webcam library are provided in the subsequent discussions.
3.4.2.1 Single and Continuous Capture with Display
Only two VIs are required to capture a single image from an imaging device and
display it on the front panel. The Webcam Snapshot VI can be coupled with the
Webcam Flat to Picture VI to accomplish this goal as shown in Figure 3.17.
FIGURE 3.17 Continuous Capture with Display.
A slightly more complex configuration of VIs is required to capture images
continuously. The Webcam Snapshot VI used in the single-shot acquisition shown
in Figure 3.17 initializes a session, grabs a single image, and then immediately closes
the session. A similar approach is taken when performing a continuous capture,
except that the grab, conversion and display operations are repeated inside a loop,
with the necessary initialization and discarding VIs outside, as shown in Figure 3.18.
3.4.2.2 Single Capture with Processing and Display
Any Webcam library image can easily be converted to the IMAQ format for image
processing and analysis using the LabVIEW Vision Toolkit. The Webcam Flat to
IMAQ VI is capable of creating a new buffer or using an existing IMAQ buffer to