Advantages and disadvantages of vision chips

 

When compared to a vision processing system consisting of a camera and a digital processor, a vision chip provides many system level advantages. These include

• Speed: The processing speed achievable using vision chips exceeds that of the camera-processor combination. A main reason is the information transfer bottleneck between the imager and the processor. In vision chips information between various levels of processing is processed and transferred in parallel.
• Large dynamic range: Many vision chips use photodetectors and photocircuits which have a large dynamic range over at least 7 decades of light intensity. Many also have local and global adaptation capabilities which further enhances their dynamic range. Conventional cameras are at best able to perform global automatic gain control.
• Size: Using single chip implementation of vision processing algorithms, very compact systems can be realized. The only parts of the system that may not be scalable are the mechanical parts (like the optical interface).
• Power dissipation: Vision chips often use analog circuits which operate in subthreshold region. There is also no energy spent for transferring information from one level of processing to another level.
• System integration: Vision chips may comprise most modules, such as image acquisition, and low level and high level analog/digital image processing, necessary for designing a vision system. From a system design perspective this is a great advantage over camera-processor option.

Although designing single-chip vision systems is an attractive idea, it faces several limitations:

• Reliability of processing: Vision chips are designed based on the concept that analog VLSI systems with low precision are sufficient for implementing many low level vision algorithms. The precision in analog VLSI systems is affected by many factors, which are not usually controllable. As a result, if the algorithm does not account for these inaccuracies, the processing reliability may be severely affected. Vision chips also use unconventional analog circuits which may not be well characterized and understood.
• Resolution: In vision chips each pixel includes a photocircuit which occupies a large proportion of the pixel area. Therefore, vision chips have a low fill-factor and a low resolution. The largest vision chip reported has only 210 230 pixels, for a photocircuit consisting of six transistors only [Andreou and Boahen 94a].
• Difficulty of the design: Vision chips implement a specific algorithm in a limited silicon area. Therefore, often off-the-shelf circuits cannot be used in the implementation. This involves designing many new analog circuits. Vision chips are always full custom designed, and full custom design is known to be time consuming and error-prone.
• Programming: None of the vision chips are general purpose. In other words, many vision chips are not programmable to perform different vision tasks. This inflexibility is particularly undesirable during the development of a vision system.