Image Acquisitionand Proces

16 Image Acquisition Processing with LabVIEW
2.1.1 SCAN TYPES
The main types of cameras are broken into three distinct groups: (1) progressive
area scan, (2) interlaced area scan and (3) line scan.
2.1.1.1 Progressive Area Scan
If your object is moving quickly, you should consider using a progressive scan
camera. These cameras operate by transferring an entire captured frame from the
image sensor, and as long as the image is acquired quickly enough, the motion will
be frozen and the image will be a true representation of the object. Progressive-scan
camera designs are gaining in popularity for computer-based applications, because
they incorporate direct digital output and eliminate many time-consuming processing
steps associated with interlacing.
2.1.1.2 Interlaced Area Scan
With the advent of television, techniques needed to be developed to minimize the
amount of video data to be transmitted over the airwaves, while providing a satisfactory
resulting picture. The standard interlaced technique is to transmit the picture
in two pieces (or Þelds), called 2:1 Interlaced Scanning (Figure 2.2). So an image
of the letter “a” broken down into its 2:1 interlace scanning components would yield
the result shown in Figure 2.3.
One artifact of interlaced scanning is that each of the Þelds are acquired at a
slightly different time, but the human brain is able to easily combine the interlaced
Þeld images into a continuous motion sequence. Machine vision systems, on the
other hand, can become easily confused by the two images, especially if there is
excessive motion between them, as they will be sufÞciently different from each
other. If we consider the example in Figure 2.3, when the object has moved to the
right between the acquisition of Field A and Field B, a resulting interlaced image
would not be a true representation of the object (Figure 2.4).
Field B
Line x+1
Field B
Line x+2
Field B
Line x+3
Field A
Line 1
Field A
Line 2
Field A
Line 3
Field A
Line 4
Field B
Line 2x
Field A
Line x
FIGURE 2.2 Interlaced scanning schematic.