One of the constraints of extracting image detail from a live or recorded video image is that the user is limited by the fact that any detail must be present in the original image.
The image below depicts varying degrees of pixelization. A better method of defining image detail is by pixel density on a target (pixels per foot or pixels per meter).
Pixel density is obtained by dividing the camera horizontal resolution by field of view width. Pixel density is an established metric in the security industry.
Figure 1. Image detail quantified in pixels per meter or pixels per foot (PPM/PPF numbers based on mm per pixel specified by European Standard EN 62676-4: 2015)
The higher the pixel density, the higher the quality of the picture.
With higher pixel density, you can distinguish smaller details.
By default, the IP Video System Design Tool shows camera zones according to European Union standard EN 62676-4: 2015 (and older EN 50-132-7). Part 4 of that EU standard covers the different operator’s tasks, icluding identification, recognition, observation, detection, and monitoring.
DORI Zones
DORI stands for Detection, Observation, Recognition, Identification. You can use IP Video System Design Tool or online Pixel Density Calculator to see DORI zones.
Identification: The first type of camera zone is the identification zone. In the program, this is marked in red. In this area, you can positively identify a person beyond reasonable doubt. Here we have sufficient picture quality and detail to identify an individual. The EU standard defines the identification area as “more than 4mm at the target distance per pixel”. If we covert 4mm per pixels to pixel density (1000mm divide by 4 mm per pixel), the result is 250 pixels per meter (PPM) or about 76 pixels per foot (PPF).
Recognition: The second zone is recognition, marked in yellow. In this area, the security operator will be able to recognize a person known to them. Viewers can verify with a high degree of certainty whether or not an individual shown is the same as someone they have seen before. In this area, the pixel density is not less than 125 PPM (38 PPF) or 8mm per pixel. Also in this area, it is possible to recognize license plates manually, but the camera resolution may be not sufficient for Automatic number plate recognition (ANPR) systems.
Observation: Next we have the observation zone (green). In this area, some characteristic details of the individual, such as distinctive clothing, can be seen. The pixel density is 62 pixels per meter or about 19 PPF (16mm per pixel by EN 62676-4)
Detection: Next is the detection zone (light green). In this area, where the operator will be able to detect a human presence: 25 PPM or 8 PPF (40mm per pixel).
Monitoring: The monitoring zone (blue) is used to monitor or perform crowd control. 12PPM / 4 PPF/ 80mm per pixel.
There is also a special kind of zone called Inspection or Strong Identification: 1000 PPM or 303 PPF (or 1mm per pixel by EN 62676-4 or EN 50132-7), marked as purple in the program.
Figure 1 shown above depicts the difference in image detail at various levels from 4 pixels per foot (12 pixels per meter) to 76 pixels per foot (250 pixels per meter).
It can be a time-consuming task to find available camera resolution and lens combinations for the required pixel density if done manually.
This task is automated with the IP Video System Design Tool, which has an integrated pixel density calculator that calculates the pixels per foot or per meter and illustrates the detail level with color coded (red, yellow, green, blue) areas. Apart from IP Video System Design Tool you can also use our online PPF PPM Calcualtor here.
Part 2: Pixel density, camera zones, identification, recognition, detection and EN 62676-4: 2015
Figure 3. Camera Coverage illustration
There are other standards that are supported by the IP Video System Design Tool.
See also: Pixel Density Calculator
Department of Homeland Security 2013 (DHS 2013) Recommendations
Department of Homeland Security 2013 (DHS 2013) published its own recommendations for pixels per foot for different video surveillance functions in the Digital Video Quality Handbook (pages 27-28).
1. Observation: 20 PPF
2. Forensic review: 40 PPF
3. Recognition: 80 PPF
We can see that the DHS Observation: 20 pixel per foot zone is close to EU Observation: 19 pixel per foot zone (or 16mm per pixel by EN62676-4), but 80 PPF of the DHS Recognition zone is closer to the European EN Identification zone.
New French standard*
The newly-proposed French standard discusses different pixel density and PPM numbers for Detection/Observation/Recognition/Identification (DORI) tasks.
1. Detection: 30 PPM
2. Recognition: 100 PPM
3. License Plate Reading: 200 PPM
4. Identification: 400 PPM
*Please note that this is a draft. Please do not cite or distribute.
You can also specify your own PPF/PPM values in the Camera Visualization Window (Settings menu item) of the IP Video System Design Tool. The built-in PPM Calculator shows Pixel per meters for the specified distance in the status bar of IP Video System Design Tool near the face of the test man.
Site Plan, Floor Plan, and Obstacles
A site/floor plan can be loaded from a file or created in the software using objects such as walls, boxes, doors, windows, fences or stairs. The user can add test people, cars, trees, furniture, and other test objects.
By positioning all of these objects and viewing your site plan, you can decide where to locate and how to position your cameras.
On the next 3D View tab of the IP Video System Design Tool, you will see the Camera 3D View enlarged.
Using your mouse, you can adjust the camera direction and position.
Figure 4. Computer simulation of image size on screen
Conclusion
Pixel density is a popular metric in the video surveillance industry.
The IP Video System Design Tool software performs pixel density calculations and allows the use of customer-provided site plan drawings (PDF files, JPEG/PNG images, and AutoCAD DWG drawings) to find optimal camera locations, camera resolution,field of view and lens focal length.
We can also create a virtual working space with objects such as walls, people, cars and trees that make sense to our customers. Thus, we can show clients a schematic layout which is very similar to the real world as they evaluate whether the proposal is acceptable.
A 90-day trial version of IP Video System Design Tool can be downloaded from jvsg.com. Video tutorials are here and here. An youtube visual explanation of EN64676 (EN50132-7) operator tasks is here.
Source: https://www.jvsg.com/pixel-density/
