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Advanced Imaging Magazine

Updated: July 8th, 2008 05:26 PM CDT

How To... Handle FOV Changes in Fixed Bandwidth Environments

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Some vision applications demand that the field-of-view (FOV) for the image being captured be repeatedly adjusted from wide to narrow and back again. In other words, the camera/sensor being used must be able to capture a wide-angle view of a scene, "zoom in" on a specific object or section of the FOV, then "zoom out" again to see the big picture.

In most cases, this challenge can easily be handled by equipping the camera with a large, variable optical zoom lens. This enables a camera of moderate resolution (and cost) to capture enough detail in both wide-angle and close-up views, but adds the considerable expense and weight of the zoom lens to the vision system. In some cases, a less expensive solution might be to use a high resolution camera with fixed optics, provided there is sufficient pixel density in the image to identify or analyze small objects by digitally "zooming" into an area of interest.

This challenge, however, becomes much more demanding when the bandwidth of the data output must be considered. For example, modern unmanned aerial vehicles (UAVs) typically must transmit both wide angle and close-up video information via an RF link of limited bandwidth. Similarly, some machine vision or traffic applications call for multiple cameras sending both wide angle and close-up views over a single network.

In these applications, it may be inefficient or even impossible to use a megapixel camera with digital zoom, because the wide angle view from a 4 MP camera operating at 15 fps would generate over 60 Mbytes of data per second at 8-bit gray scale. This is enough to choke even a high bandwidth network and well beyond what a typical RF link can bear. The zoom lens option might satisfy the bandwidth requirement, but at an added cost and weight that greatly reduces the attractiveness of the system, especially for a UAV-type application.

Fortunately, a third alternative now has become available using a patent-pending approach called resolution proportional digital zoom (RPDZ). With this method, a high resolution camera with fixed optics and digital zoom is used, but the data rate being transmitted is the same regardless of whether a wide or narrow FOV is captured.

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