Identification and recognition

Identification and recognition

Camera positioning

Camera positioning is critical for successful identification. This is not only to avoid difficult lighting situations, but also to ensure that persons or objects are captured at a favorable angle. A birds-eye perspective from a camera placed high above the ground will cause some degree of distortion, making it difficult to identify persons or objects.

Figure 4: An image with good lighting, both in intensity and direction. The camera is placed at the same level as the people, and the lens provides both focus and depth of field.

The camera should be firmly fixed in order to minimize blur caused by camera movement. This is of particular importance for PTZ cameras, where maneuvering the camera may cause vibrations that affect image quality. Stability can also be a challenge when the camera is mounted on a tall pole, and uses a zoom lens with a long focal length. In this situation, even small vibrations will translate to large movements in the resulting image.

Your system design must also consider the effects of motion in the scene. For identification purposes, a minimum frame rate of 5-8 frames per second is often recommended. Your surveillance objectives may require higher frame rates, for example to get a clearer picture of a series of events. If the scene being monitored includes persons or objects moving at high speeds, or close to the camera, you will probably want to increase the frame rate to ensure that the camera does not miss any of the action.

Furthermore, in order to capture sharp footage of fast-moving persons or objects, you will need to use fast shutter speeds. Using cameras that support progressive scan eliminates the blur that affects moving objects when using interlaced video.

Compression can greatly affect the usability of recorded materials for identification and recognition. High compression ratios will introduce blur or pixelation, which makes identification difficult. If the compression algorithm uses a bit rate limit, the compression might increase when there is motion, making otherwise clear footage unusable. When using variable bit rates on the other hand, the compression remains unchanged, but bandwidth usage will increase when there is motion.