Real World Haskell

Chapter 12. Barcode Recognition

In this chapter, we’ll make use of the image-parsing library we developed in Chapter 10 to build a barcode recognition application. Given a picture of the back of a book taken with a camera phone, we could use this to extract its ISBN number.

A Little Bit About Barcodes

The vast majority of packaged and mass-produced consumer goods sold have a barcode somewhere on them. Although there are dozens of barcode systems used across a variety of specialized domains, consumer products typically use either UPC-A or EAN-13. UPC-A was developed in the United States, while EAN-13 is European in origin.

EAN-13 was developed after UPC-A and is a superset of UPC-A. (In fact, UPC-A has been officially declared obsolete since 2005, though it’s still widely used within the United States.) Any software or hardware that can understand EAN-13 barcodes will automatically handle UPC-A barcodes. This neatly reduces our descriptive problem to one standard.

As the name suggests, EAN-13 describes a 13-digit sequence, which is broken into four groups:

Number system

The first two digits. This can either indicate the nationality of the manufacturer or describe one of a few other categories, such as ISBN (book identifier) numbers.

Manufacturer ID

The next five digits. These are assigned by a country’s numbering authority.

Product ID

The next five digits. These are assigned by the manufacturer. (Smaller manufacturers may have a longer manufacturer ID and shorter product ID, but they still ...