Hardware Reference
In-Depth Information
When you run this, notice how the .jpg or .png
images scan much more reliably than the
camera images. The distortion from the analog-
to-digital conversion through the camera causes many
errors. This error is made worse by poor optics or low-end
camera imaging chips in mobile phones and webcams.
Even with a good lens, if the code to be scanned isn't
centered, the distortion at the edge of an image can throw
off the pattern-recognition routine. You can improve the
reliability of the scan by guiding the user to center the
tag before taking an image. Even simple graphic hints like
putting crop marks around the tag, as shown in Figure 9-5,
can help. When you do this, users framing the image tend
to frame to the crop marks, which ensures more space
around the code and a better scan. Such methods help
with any optical pattern recognition through a camera,
whether it's one- or two-dimensional barcodes, or another
type of pattern altogether.
Optical recognition forces one additional limitation
besides those mentioned earlier: you have to be able to
see the barcode. By now, most of the world is familiar with
barcodes, because they decorate everything we buy or
ship. This limitation is not only aesthetic. If you've ever
turned a box over and over trying to get the barcode to
scan, you know that it's also a functional limitation. A
system that allowed for machine recognition of physical
objects—but didn't rely on a line of sight to the identifying
tag—would be an improvement. This is one of the main
Figure 9-5
A two-dimensional barcode (a QR code, to be specific) with crop
marks around it. The image parsers won't read the crop marks,
but they help users center the tag for image capture.
reasons that RFID is beginning to supersede bar codes in
inventory control and other ID applications.
Radio Frequency
Identification (RFID)
Like barcode recognition, RFID relies on tagging objects
in order to identify them. Unlike barcodes, however, RFID
tags don't need to be visible to be read. An RFID reader
sends out a short-range radio signal, which is picked up by
an RFID tag. The tag then transmits back a short string
of data. Depending on the size and sensitivity of the
reader's antenna and the strength of the transmission,
the tag can be several feet away from the reader, enclosed
in a topic, box, or item of clothing, and still be read. In fact,
some clothing manufacturers are now sewing RFID tags
into their merchandise, which customers remove after
purchasing.
There are two types of RFID system: passive and active,
just like distance-ranging systems. Passive RFID tags
contain an integrated circuit that has a basic radio
transceiver and a small amount of nonvolatile memory.
They are powered by the current that the reader's signal
induces in their antennas. The received energy is just
enough to power the tag to transmit its data once, and the
signal is relatively weak. Most passive readers can only
read tags a few inches to a few feet away.
In an active RFID system, the tag has its own power supply
and radio transceiver, and it transmits a signal in response
to a received message from a reader. Active systems can
transmit for a much longer range than passive systems,
and they are less error-prone. They are also much more
expensive. If you're a regular automobile commuter, and
you pass through a toll gate during your commute, you're
probably an active RFID user. Systems like E-ZPass use
active RFID tags so that the reader can be placed several
meters away from the tag.
