Pixels and Pixies
I sometimes get asked to show someone how big a graphic is “at full size” and this request always confuses me because fundamentally, images don’t really have a size, per se. An image is an array of pixels, which don’t have sizes. Let’s look at the Beehive Logo, which is 899 pixels wide by 153 pixels high.
It looks like a complete picture, but it is actually composed of individual pixels, which are the smallest possible element of the picture. Here is a zoom view of some pixels around the ‘b’ with a single pixel colored black.
As an aside, if you notice the weird patterns of different shades of gold, that is anti-aliasing. It is designed to fool your eyes into seeing a jagged thing like this as a smooth curve. Anyhow, I know exactly how many pixels our logo has, but I can’t tell you how big it will be on your screen, or if you print it out. I am currently on a laptop that is very dense – 1920×1280 pixels, but it is a small (13”) screen. The logo is small on my screen. On yours, it is likely to be different. That’s because it doesn’t have an actual size, it just have pixels.
However, all is not lost. There is a property of an image called dpi – dots per inch. You can think of this as a “suggestion” of how the image should be sized. This is available as a file property in Windows 7, but they dropped it in Windows 8. You can see it in an image editor, though. Here it is in the “Image Adjustment” dialog in Photoshop.
This shows the image is 72 dpi, which is typical for the vast majority of images. 72 dpi is a typical screen resolution. That means on a typical screen (not every screen), this image will be 899 pixels / 72 dpi = 12.5 inches, as shown here. However, when I look at the logo in 1:1 zoom on my computer, it isn’t that big. That’s because my monitor isn’t 72 dpi, it is 108 dpi So, this logo is closer to 8 inches. As you can see, there is no absolute size to this image. There are pixels, and dpi, which provide an idea on size, but not an absolute size.
So what does this have to do with biometrics? A lot, actually. When we compare fingerprints, we are looking at the X and Y locations of minutiae points – ridge endings, splits, etc. In order to compare them, we need to know absolutely where they are, none of this “dpi suggestion” stuff like in the previous example. We need to know exactly where each point is. Luckily, the FBI solved this problem a long time ago, because they have a selfish interest in trying to make it easy to collect and compare fingerprints. They (along with NIST), established a standard that says all fingerprints are collected at 500 dpi. That is a different kind of measurement that in our logo example, because it is an indication of something collected from a scanner. So, we know for sure that 500 pixels is 1 inch. So, if a particular point is at the location (250,250), we know that point is at the location 1/2”, 1/2” on the finger. So, we can compare all fingers along the same scale.
You can see this in the following illustration of matched points. We still have to deal with rotation, but that is not too hard.
Face matching and iris matching are totally different, though. They do not rely on X,Y positions of points, so dpi is not important, just pixels. Additionally, faces and irises have easily locatable marker points that provide a basis for positional information. For faces, it is the eyes, and for iris it is the pupil. This allows faces and irises to be compared on the same scale without requiring precise DPI for collection.
In short, pixels have no absolute size, nor do images. The only thing that establishes a relationship between an image and anything approaching a real size is to know the dpi of the sensor that acquired a particular image.