EIS (Electronic Image Stabilization) is an abbreviation of Electronic Image Stabilization - an image enhancement technique using electronic processing.
EIS minimizes blurring and compensates for camera shake often caused by the external factors, for example, wind, mount vibrations due to heavy traffic or incorrect camera installation, e.g. too high and too flexible mast.
EIS cannot prevent blur from subject movement or extreme camera shaking, but it is engineered to minimize blur from normal handheld lens shaking. Certain cameras and lenses are built with more advanced active modes.
Fig. 1. Camera image with EIS disabled and enabled
The image is stabilized when the image captured by the image sensor is being processed. The image processor traces the data from the image sensor in real-time. The data is analysed using different algorithms detecting and measuring any changes in the recorded image. If the algorithm identifies a shift as a shake, a transformation (usually image shift) is initiated to compensate the shake. For the EIS system to effectively compensate the shake and shift the image within specific limits, additional regions must be created at its edges using one of two methods.
The first involves digitally zooming in the central section of the image. A scene recorded in a single frame is part of a whole image, and its position on the image can be changed as required. If the content of the specific part of the image shifts in relation to the next image frame, the section borders also shift. As a result, the pixel that shifted in the image sensor is stored without shifting in the new coordinate system. The system can result in a slight decrease in the camera field of view, which can be observed after the system is enabled. The differences in the image are analysed between the frames, and the image is usually divided into zones. If a shake is observed in a certain zone, it will be interpreted as a movement of the recorded subject. If the movement covers a significant part of the image, it is identified as a background movement. As a result, the algorithm interprets the movement as a shake and compensates it as necessary. The system can easily misinterpret large moving subjects in the frame as a shake. In this case, more time is required for the algorithm to recognize if the subject moves or if the camera shakes.
The second method is similar, however, it is hardware-based. The image sensor has a region at its edges that is not used for recording images in normal operation. After detecting a shake, the coordinates of the centre of the region capturing the image move correspondingly.
Fig. 2. EIS function flow chart
1 - Camera movement up 2 - Camera movement down 3 - Camera movement up and right 4 - Image plan 5 - Camera movement
The efficiency of the EIS system depends on the efficiency of the detection methods. Introduction of high-resolution cameras has brought significant benefits in the video surveillance system design. Higher resolutions provide more image details and allow analysis of selected regions of interest. This approach is often based on telephoto lenses. As a result, the image quality may be reduced and the details are lost due to motion and shakes.
Fig. 3. Image recorded by Hikvision camera with EIS disabled and enabled
EIS may require some compromises but is cheap to implement. Electronic image stabilization decreases the resolution of the obtained image which is somewhat a secondary product - a result of an algorithm - in which contrast, sharpness, and field of view are often affected. A degree to which the image quality deteriorates depends on the amplitude of shakes compensated by the EIS system. With relatively large focal lengths and in typical conditions, it should not be noticeable to an average user. An advantage of the electronic system is high speed and no mechanical parts in its most basic version. The system does not affect the weight and dimensions of the device.