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Image Quality
Understanding what constitutes quality images and how to optimize the quality of images used in 3VR systems is critical to developing viable solutions. Meeting specific requirements for 3VR facial surveillance is much more challenging than in any traditional CCTV deployment. Accordingly, many partners and users are unaccustomed to being concerned about these issues and can easily overlook them.
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Better qualify what opportunities are strong fits for 3VR
Set the appropriate level of expectations with partners and users
Design a system that accommodates real world conditions
Deliver a solution that is optimally effective
Imaging Background
Overview
Two aspects of imaging are most important in understanding and optimizing 3VR facial surveillance:
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Without understanding the impact of these two aspects, it will not be possible to master use of 3VR for either importing images or capturing video. As such, prior to analyzing facial surveillance or its application to 3VR, these aspects shall be explained and relevant introductory material shall be presented.
Key Elements in Optimizing Images for Facial Surveillance
This report will examine and explain the many elements that are critical to using 3VR for facial surveillance. While fundamental principles apply to images from photographic cameras as well as CCTV cameras, the application will differ. The following summarizes the key elements so that the reader may have a quick reference for future use.
Any image, whether video or photo, requires sufficient detail. Detail is determined by
the level of resolution in the image
the size of the person’s face relative to the size of the image
The person’s face must look directly towards the camera. This affects both the class of imported photos that are acceptable and how video cameras must be positioned to capture faces.
Imported images have to meet the requirements listed above. With the exception of mugshots and passport photos, most photos do not meet these requirements.
Video cameras must be positioned specifically to capture faces. The cost may be inexpensive but the skill is not trivial. Care must be taken to precisely position cameras to capture faces consistently.
Resolution
Resolution, as applied to images used in the 3VR system, is defined as the level of visual detail in the image.
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Field of View
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Camera Placement for Facial Analysis - 4 Factors
Field of View Determines Size and Resolution of Face
Facial analysis requires a certain minimum resolution level to be effective. This resolution level is measured in pixels.
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Excellent FOV | |
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Feet 3.5; Face: 1/6 of the image |  |
Horizontal Angle | |||
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Facial analysis requires a clear image of the full face, directly facing the camera - with minimal turning of the head to the left or right (horizontal angle) relative to the camera.
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Vertical Angle
Facial analysis requires a clear image of the full face, directly facing the camera - with minimal tilting of the head up or down (vertical angle) relative to the camera.
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Cameras need to be mounted low enough or far away enough so that the vertical angle or slope does not exceed 20% above the eye level when subjects are in focus in the foreground. Given an average eye height of 5 feet, a camera 10 feet away can not be mounted higher than 20% of 10 feet (2 feet) above the eye height of 5 feet - so not higher than 5 + 2 = 7 feet. A camera 20 feet away can be mounted as high as 9 feet (20% of 20 = 4 feet above 5 feet, 4 + 5 = 9). See “Determining Camera Mounting Height” later in this section for more details.
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Lighting Level
Facial analysis requires even levels of lighting that clearly shows the detail in a face. Facial analysis requires lighting conditions that do not produce shadows and/or dark areas in the face (underexposure) and lighting conditions that do not produce glare and/or washed-out areas in the face (overexposure). A face with lots of detail visible and a wide range of dark and light pixels (referred to as “dynamic range”) is required for facial analysis.
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Photo D – Underexposed | Image |
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There are substantial areas of shadow and/or not enough light. There is a narrower dynamic range - |  |
Additional Considerations for Megapixel Cameras
Field of View
With the addition of megapixel cameras to your security solution, you can now utilize the higher resolutions available to ultimately provide a wider field of view. In essence, this allows the use of less cameras and more coverage while still capturing face profiles. It is extremely important to understand that the same principles still apply for facial recognition; this includes pixels in between the eyes, horizontal and vertical angles, and lighting.
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Resolution | Megapixels | Width for Face (Feet) |
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1024 X 768 | 0.7 | 6.5 |
1280 X 1024 | 1.3 | 8.0 |
1600 X 1200 | 2 | 10.5 |
2048 X 1536 | 3 | 13.5 |
The appearance of the field of view is obviously different with a width of 8.5’.
Pixels Between the Eyes
The same principles that apply to lower resolution cameras apply to megapixel cameras; 3VR still requires 35 pixels between the eyes. However, analysis is conducted on the full megapixel frame of the camera’s output. For example, with a resolution of 1280 x 1024, 3VR conducts facial analysis at 1280 x 1024 versus an analog camera at 4CIF (704 x 576).
Image Use and Optimization for Facial Analysis
Overview
3VR can analyze faces from two primary sources:
Digital images such as photographs and mugshots can be imported
CCTV cameras can be connected to a 3VR and video can be continuously analyzed
Best practices for each source differ significantly. Both sources are described and in Imported Images section below.
Imported Images
Any image that is in a supported digital format may be imported into the 3VR system. Supported formats include: BMP, JPEG and GIF. Only faces in images that meet the requirements enumerated in the previous two sections (pixels between the eyes, angles, lighting) may be successfully analyzed by the 3VR system.
Passport Photos/Mugshots
Generally, passport photos and mugshots provide high quality images for facial analysis. The angles and the lighting on these photos are generally within guidelines. Users should check that the pixels between the eyes are sufficient. 3VR uses an import photo process that provides immediate feedback if the number of pixels between the eyes are insufficient.
Images from DVRs
Images from DVRs may not provide sufficient quality for facial analysis. Users should qualify and set appropriate expectations. Video from DVRs are often recorded at resolutions lower than 4CIF such as 2CIF or CIF. Because of the reduction of resolution, the field of view needed for sufficient detail increase significantly. For example, images at CIF resolution would need to have the face in focus in a field of view no wider than 2.25 feet.
Furthermore, the field of view of cameras on traditional DVRs is often 7’ to 12’ which is significantly greater than the 4.5 feet maximum field of view even if the video was recorded at 4CIF.
General Photos from Digital Cameras
Images from digital cameras may provide sufficient quality for facial analysis. To the extent that these photos exhibit the same properties as passport photos, suitability will be more likely.
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Digital cameras provide megapixel images. Given the very high resolution of these images (compared to analog photos), the width of the field of view may be very wide (as much as 20’ wide). Users should be careful to verify that the image has not been reduced in resolution. This is a common technique to reduce the file size of an image. However, if this is done it will significantly reduce proportionally the number of pixels between the eyes in faces in the image.
CCTV Video
Many existing camera positions are designed for alarm assessment and activity monitoring. Those cameras must be adapted for use in facial analysis. Existing infrastructure can and may be used for facial analysis but it must be optimized for use in meeting the imaging guidelines enumerated in this document.
The remainder of this section is a lengthy but important treatment of design and deployment options for CCTV video use.
Environment
First, examine the physical layout of the facility to identify natural “choke points” – areas where subjects will appear within a limited field of view and are most likely to look “straight-on” toward the camera(s). Ideal choke points include entrances and hallways. This area should be free from any obstructions that might come between the camera and subject, (including transparent barriers that can create glare/reflection problems). Also, these areas should be devoid of distractions that may entice subject to look away from the camera while passing through. Ideally, subjects will spend 3 seconds passing through a choke-point area.
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Scene lighting must be sufficient to produce a clear, sharp image. Excessive background lighting, blooming or shadowing conditions must be avoided.
Camera Positioning
Cameras should be centered to increase the opportunities to obtain straight-on (perpendicular) face images. In addition, the distance from the subject should be the greatest allowable (subject to lens specifications that provide a proper Field Of View - less than 4.5 feet, as explained in Section 3). The further the distance between camera and subject, the longer the Depth of Field – the area within which the subject will remain in focus – thus delivering more usable ‘face frames’.
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While cameras are often mounted higher than the optimal face-level height, adequate Face Capture can occur provided the maximum Vertical Angle of Incidence is less than a 20% slope, (see illustration below). The greater the distance between camera and subject, the higher the camera can be mounted, while maintaining this threshold.
Determining Camera Mounting Height - No more than 20% slope to eyes in the face
The first step is to determine an average “face-height”. This value is application-specific, (ie: an environment with young children will require a lower value). For this example, we will assume an average face height of 5 feet.
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Example: A potential entry point offers 2 appropriate mounting positions, one at 12 feet away, the other at 24 feet away. In this case, the furthest mounting location is most appropriate.
Camera/Lens Specifications
Cameras designated for face-based recording must comply with a wide range of specifications, primary among them is the use of high-resolution cameras, (480 TVL or greater). The camera and lens must be appropriate to the scene lighting conditions. For example, black & white low-lux or day/night camera for poorly lit areas, and super-dynamic cameras where severe back-lighting can occur.
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VariFocal Lenses are available with manual or automatic iris features. The automatic iris format is used in applications where lighting conditions may vary, for example, areas exposed to daytime sunlight.
Lens Selection
Typical VariFocal Lenses fall within the following approximate ranges; 3 to 8 millimeters (mm), 3 to 12mm, 5 to 50mm, and 20 to 100mm. The most common and least expensive are the 3 to 8mm versions, although most Face Capture applications will require the larger size VariFocal Lenses.
Guidelines for Determining Lens Focal Length
A 4’ wide FoV requires 1.2mm per foot of distance from subject; a 4½’ wide FoV requires 1.1mm per foot.
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With megapixel cameras, deployment no longer requires such a large telephoto range when comparing to analog and non-megapixel IP cameras. Evidently, this will depend on the particular application. Please refer to the table in the Field of View section for determining the maximum width for each application.
Camera Face Requirement Chart in 3VR
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Result
Quality Image
With better understanding on optimization of image quality you can now determine:
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