Pixel density is usually referred to as pixels per inch (PPI). The higher the PPI, the more detail you’ll find within the image. The Pixel Density application allows you to manipulate different parameters and learn how they affect pixel density.
These parameters include zoom, resolution and distance. The two images to the left show the result of the changes you make to the settings using the sliders to the right. The face in the very left image is an enhancement of the face of the man on the roof. The color of the icon to the left of the enhanced face indicates if there is enough pixel density for: