Frame grabbers ease processing burden

Frame grabbers with FPGAs can offload processing tasks from a vision system's CPU.

Ann R. Thryft, Contributing Technical Editor -- Test & Measurement World, 6/1/2011 12:00:00 AM

Read more from the June 2011 Machine-Vision & Inspection Test Report

As more inspection systems run faster, use multiple cameras, and transmit huge amounts of data to CPUs for processing, many designers of vision systems are turning to image-acquisition cards, also know as frame grabbers, to manage the data flow. Frame-grabber cards help offload the host CPU by performing preprocessing and other functions. As image sensors get faster, frame grabbers may become more important, aided by bigger, smarter, onboard FPGAs.

FPGAs have been present in high-end boards before, but they have become so inexpensive that processing functions are migrating down to lower-end frame grabbers, said Mike Miethig, technical manager for Teledyne Dalsa. “At the same time, camera bandwidth is going through the roof,” he said. A third intersecting trend is the availability of faster and cheaper CMOS image sensors. Many observers, therefore, expect frame grabbers to become even more important as the speed and performance of FPGAs and CMOS image sensors improve while their prices fall, and as the migration to lower-cost, higher-bandwidth data-transmission technology continues.

With their ability to perform image-processing and preprocessing tasks, today’s frame grabbers can offload some of the computational burden from the host CPU, said Pierantonio Boriero, product line manager for Matrox Imaging. “That makes sense when you’re looking at the high data rates you can achieve with these new [camera interface] standards,” he said. “The whole goal is to perform repetitive tasks by offloading to the frame grabber the pure number-crunching algorithms that don’t depend much on image content.” That frees up the host CPU to do more of the sophisticated algorithms with heuristics that are image-content-dependent, not the brute force or repetitive algorithms.

In addition, camera-plus-frame-grabber systems handle much higher bandwidths than do direct-to-PC cameras. Systems that employ frame grabbers also use less CPU bandwidth to transfer data into host memory, said Miethig. “The frame grabber can do preprocessing functions like unpacking data and Bayer interpolation,” he said. “In multicamera systems, where there’s a huge amount of data to process, the frame grabber [offers] an advantage even when the cameras are lower-bandwidth models.”

Jim Blasius, solutions architect for ADLink Technologies, acknowledged that a frame grabber with an FPGA can offload some of the processing work of the CPU, but he said that the configuration you need depends on your application. “If you have smart cameras that can do the job, you don’t need frame grabbers,” he said. “If you can’t do what you need with smart cameras, then shifting to cameras with frame grabbers that do preprocessing makes sense. If you’re taking large frames at a low frame rate, you’ve used smart cameras in the past, and you can re-use the smart-camera processing software in a low-power, low-cost, multicore industrial computer, you can save some money.”

Although frame grabbers are not required in vision systems that use digital direct-connect buses such as GigE Vision or IEEE 1394 (FireWire), they provide advantages in some factory-floor applications, said Kamalina Srikant, National Instruments’ product marketing manager for vision hardware and software. Srikant explained that “many plug-in boards offer better synchronization and I/O connectivity than built-in NICs [network-interface cards], which is especially useful for online systems on the factory floor.”

Too many interfaces?

A major concern for manufacturers of frame grabbers is the growing number of interface standards, including Camera Link, GigE Vision, IEEE 1394b, USB, CoaXPress, and Camera Link HS. “The biggest headache for everyone in industrial vision is the fact that the supply chain is too complex, and that there are too many standards,” said Jacky Lin, ADLink’s product marketing manager for measurement and automation. “We think that the number-one trend will be ease of use, including configurations. So, we think Power over Ethernet will be especially important.”

Having three main vision interface standards—GigE Vision, CoaXPress, and Camera Link HS—will complicate R&D for manufacturers of frame grabbers, said Matrox’s Boriero. “A camera vendor can pick only one, but frame-grabber vendors must ensure maximum compatibility. So, Matrox is actively participating on all three fronts. The market will have to decide which one will win out or if there will be equal use of the three.”

The new CoaXPress standard features the trigger capabilities, low jitter, and deterministic I/O of Camera Link, while improving on that standard’s bandwidth, ease of use, and cable length, said Michael Chee, Matrox Imaging’s product manager. “Camera Link HS, aimed at similar goals of increasing bandwidth and increasing cable length, is still under development. Work is still being done to define data communication and triggering methods.” The Camera Link HS specification is targeted for release during 2011, according to the Website of the AIA (American Imaging Association).

Although each standard has its assets and advantages, users don’t always know which way to go, especially when trying to decide between CoaXPress and Camera Link HS, said Marc Damhaut, Euresys’ senior VP of product management. “When both standards are out, we may start to make a decision,” he said. “I would prefer not to have to develop products for both at the same time.”

Over the last two years, Euresys has seen an increase in sales of medium- and full-configuration Camera Link frame grabbers, mainly for applications where there’s no real high-speed alternative, said Damhaut. This has prompted the company to redesign its Grablink cards to more fully exploit the Camera Link standard while keeping the cards cost-effective. Euresys achieved this by packing as many functions as possible, including Camera Link deserializers, into a single FPGA.

In the past, Euresys integrated some preprocessing functions for linescan cameras into its frame grabbers, such as pixel compensation and shading correction, but those have now been fully integrated into the camera, said Damhaut. “Currently, we integrate some standard processing that users expect, such as Bayer-pattern decoding, a type of preprocessing, and look-up table,” he said. “Look-up table processing can also be done on a PC. Bayer-pattern processing is a little more difficult to program on the PC side, so it makes sense to include this function in the frame grabber.”

Protocols designed for machine vision, such as Camera Link and Camera Link HS, tend to be simpler than general-purpose protocols, such as USB and FireWire, said Teledyne Dalsa’s Miethig. More importantly, they directly support features required by machine vision, such as low latency triggers or GPIO within the cable, and asymmetric data bandwidth for transferring high-bandwidth video data from camera to frame grabber and for transferring lower-bandwidth command control data from frame grabber to camera.

The FireWire standard is still around and still very strong in Asia, but it’s no longer growing rapidly, said National Instruments’ Srikant. Meanwhile, GigE Vision’s popularity is increasing. “It uses the Ethernet infrastructure that
everyone already has, so adopting it is an easy decision,” she said.

“For the past 10 years, most of our customers required lower-speed base [configuration] Camera Link frame grabbers,” said Damhaut. “That market segment has now been largely taken over by GigE Vision.” GigE Vision cameras have taken off in Asia, the US, and Europe, so the technology has become mainstream, he said, but not everyone wants a networking approach. Bandwidth is currently limited to 1 Gbps on a single cable, and GigE Vision cameras are more difficult to set up.

“Engineers have to play with additional tasks, such as choosing power supplies and setting IP addresses,” Damhaut said. “On the other hand, issues such as jitter, triggering, and CPU load have been ironed out and drivers have been optimized. Most of the applications I see now for GigE Vision use NICs, so the need for an interface card is basically the same, whether or not we call them frame grabbers.”

Some frame-grabber makers also consider USB, especially USB 3.0, which is a major contender in camera interfaces, said ADLink’s Blasius. “It [USB 3.0] has overcome a lot of USB’s overall disadvantages,” he said. “For example, USB 3.0 is a lot faster and more deterministic.”

The main issue in choosing an interface standard will be software development and the problem of legacy applications. “That’s why FireWire is still around, because of the software investment,” explained Blasius. “It’s just not that easy to convert everything to another standard: It’s a major investment.”  T&MW