Perfect pixel count

Pixel density

The growth of IP surveillance makes us rethink how we define our operational requirements.

Instead of using vertical height as a percentage, we can instead focus on pixel density, which refers to the number of pixels representing our object of interest – usually a person’s face.

The human face has distinctive identifying features, and the variations in face width are less than those of body height or width. The average human face is 16 cm across (6.3 inches). After recommendations from the Swedish National Laboratory of Forensic Science, and our own test results at Axis, we have elected to use 80 pixels as the requirement for facial identification in challenging conditions¹. (see Table 3).

Some sources² recommend 40 pixels for a face, but although 80 px/face may sound like a lot, Axis recommends this number due to the fact that there are few other usable indicators for identification. For recognition, previous knowledge adds factors such as how a person moves – easy to observe and recognize, but difficult to identify and describe accurately. To ensure sufficient video quality even when the object is not facing the camera directly, or in sub-optimal lighting, 80 pixels gives a good safety margin.

Operational requirement Horizontal pixels / Face Pixels / cm Pixels / inch
Identification (Challenging conditions) 80 px/face 5 px/cm 12,5 px/in
Identification (Good conditions³) 40 px/face 2,5 px/cm 6,3 px/in
Recognition 20 px/face 1,25 px/cm 3,2 px/in
Detection 4 px/face 0,25 px/cm 0,6 px/in


Table 3: The Axis definition of operational requirements.


¹ Challenging conditions: Situations with very varying or weak lighting. People, objects and vehicles are seen from an angle where details are in shadow, or facing away from the camera, and in situations where people, objects and vehicles are moving at high speeds. Occurs more often in outdoor situations without additional lighting, or indoors in very dark conditions.
² See IEC International Standard IEC 62676-4.
³ Good conditions: Situations with decent lighting. People, objects and vehicles are moving at a reasonable speed, and are seen from an angle from which sufficient details are visible. Occurs more often in indoor situations with even lighting, or outdoors with additional lighting.

A simple model