When 250ppi Binary Fingerprint Images Were Acceptable

(Part of the biometric product marketing expert series)

I remember the first computer I ever owned: a Macintosh Plus with a hard disk with a whopping 20 megabytes of storage space. And that hard disk held ALL my files, with room to spare.

For sake of comparison, the video at the end of this blog post would fill up three-quarters of that old hard drive. Not that the Mac would have any way to play that video.

That Mac is now literally a museum piece.

And its 20 megabyte hard disk illustrates the limitations of those days. File storage was a precious commodity in the 1980s and 1990s, and we therefore accepted images that we wouldn’t even think about accepting today.

This affected the ways in which entities exchanged biometric information.

The 1993 ANSI/NIST standard

The ANSI/NIST standard for biometric data interchange has gone through several iterations over the years, beginning in 1986 when NIST didn’t even exist (it was called the National Bureau of Standards in those days).

Fingerprints only

When I began working for Printrak in 1994, the image interchange standard in effect was ANSI/NIST-CSL 1-1993, the “Data Format for the Interchange of Fingerprint Information.”

Yes, FINGERPRINT information. No faces. No scars/marks/tattoos. signatures, voice recordings, dental/oral data, irises, DNA, or even palm prints. Oh, and no XML-formatted interchange either. Just fingerprints.

No logical record type 99, or even type 10

Back in 1993, there were only 9 logical record types.

For purposes of this post I’m going to focus on logical record types 3 through 6 and explain what they mean.

• Type 3, Fingerprint image data (low-resolution grayscale).
• Type 4, Fingerprint image data (high-resolution grayscale).
• Type 5, Fingerprint image data (low-resolution binary).
• Type 6, Fingerprint image data (high-resolution binary).

Image resolution in the 1993 standard

In the 1993 version of the ANSI/NIST standard:

• “Low-resolution” was defined in standard section 5.2 as “9.84 p/mm +/- 0.10 p/mm (250 p/in +/- 2.5 p/in),” or 250 pixels per inch (250ppi).
• The “high-resolution” definition in sections 5.1 and 5.2 was twice that, or “19.69 p/mm +/- 20 p/mm (500 p/in +/- 5 p/in.”
• While you could transmit at these resolutions, the standard still mandated that you actually scan the fingerprints at the “high-resolution” 500 pixels per inch (500ppi) value.

Incidentally, this brings up an important point. The series of ANSI/NIST standards are not focused on STORAGE of data. They are focused on INTERCHANGE of data. They only provided a method for Printrak system users to exchange data with automated fingerprint identification systems (AFIS) from NEC, Morpho, Cogent, and other fingerprint system providers. Just interchange. Nothing more.

Binary and grayscale data in the 1993 standard

Now let’s get back to Types 3 through 6 and note that you were able to exchange binary fingerprint images.

Yup, straight black and white images.

Why the heck would fingerprint experts tolerate a system that transmitted binary images that latent fingerprint examiners considered practically useless?

Because they had to.

Storage and transmission constraints in 1993

Two technological constraints adversely affected the interchange of fingerprint data in 1993:

• Storage space. As mentioned above, storage space was limited and expensive in the 1980s and the 1990s. Not everyone could afford to store detailed grayscale images with (standard section 4.2) “eight bits (256 gray levels)” of data. Can you imagine storing TEN ENTIRE FINGERS with that detail, at an astronomical 500 pixels per inch?
• Transmission speed. There was another limitation enforced by the modems of the data. Did I mention that the ANSI/NIST standard was an INTERCHANGE standard? Well, you couldn’t always interchange your data via the huge 1.44 megabyte floppy disks of the day. Sometimes you had to pull your your trusty 14.4k or 28.8k modem and send the images over the telephone. Did you want to spend the time sending those huge grayscale images over the phone line?

So as a workaround, the ANSI/NIST standard allowed users to interchange binary (black and white) images to save disk space and modem transmission time.

And we were all delighted with the capabilities of the 1993 ANSI/NIST standard.

Until we weren’t.

The 2015 ANSI/NIST standard

The current standard, ANSI/NIST-ITL 1-2011 Update 2015, supports a myriad of biometric types. For fingerprints (and palm prints), the focus is on grayscale images: binary image Type 5 and Type 6 are deprecated in the current standard, and low-resolution Type 3 grayscale images are also deprecated. Even Type 4 is shunned by most people in favor of new friction ridge image types in which the former “high resolution” is now the lowest resolution that anyone supports:

• Type 13, Variable-resolution latent friction ridge image.
• Type 14, Variable-resolution fingerprint image.
• Type 15, Variable-resolution palm print image.

We’ve come a long way.

Now that you’ve read this whole thing, I’ll share my video which covers everything in 25 seconds.

By the time I upload this video to Instagram, I’ll probably use Instagram’s music facilities to add this song as background music.

• And note that the band name is spelled Visage with one I, not Viisage with two I’s. (Now part of IDEMIA, along with Printrak.)
• But the spelling inaccuracy is not surpring. The band can’t spell “gray” either.