Beach racket is a quite popular hobby on Greek beaches. The Greek Association for Beach Racket plans to upgrade this hobby to a regular sport. The game is played by teams of two or four players. The required equipment is rackets made of wood and a tennis ball. In order to be able to objectively and accurately keep the score in a beach racket game, it commissioned Mobics Ltd to develop an IT system for supporting beach racket games. Mobics is a spin-off company of the National and Kapodistrian University of Athens, Greece.
The Challenge: Outdoor Imaging in a Tough Environment
The design of such a system was rather complex, since the application involved many challenging requirements. First of all, the system had to be able to operate on beaches with extreme weather conditions: temperatures over 50 degrees Celsius (direct exposure to sunlight), for more than 8 hours, without power-off periods, and a lot of dust. Moreover, the system should be able to detect very fast ball hits, with a maximum velocity near 125 km/hour (a very good hit can get near 118 km/h). Finally, the system should be accurate (independent of the ball direction), reliable and hack-proof. These requirements were the main reason for not choosing a radar-based solution, which has inherent cosine errors and can be jammed. After much research, a solution based on image processing with high-frame rate cameras from Allied Vision Technologies was chosen.
The solution: Ball-tracking at 130 fps
For its machine vision solution, Mobics selected the AVT Prosilica GE680, a fast CCD camera with VGA resolution (0.3 Megapixel) and a GigE Vision interface.
The camera stands opposite to a dark-colored background on which the position of the ball can be easily tracked. The camera’s ROI is configured in order to isolate the dark-colored background.
Before the beginning of a tournament, a short calibration phase takes place, where the size of the ball is aligned to its distance from the camera. After this training phase, the distance is inferred from the ball size and the velocity is calculated with trigonometric formulas. The exposure and gain settings are set to Auto in order to compensate for luminance conditions.
In order to track the rapidly moving balls, the camera needs to capture images at 130 frames per second. This is the minimum frame rate required in order to capture at least 2 frames of the ball at maximum velocity in the center of the court, and calculate the velocity vector. This is no issue for the AVT Prosilica GE680 that delivers up to 205 fps at full resolution. The OpenCV library was used for frame processing and implementation of the respective machine vision algorithms.
Official ball-tracking system at national tournament
The system was successfully used in 6 official competitions of the 1st Pan-Hellenic Beach Racket Tournament during the summer of 2011.
“We actually went far beyond AVT’s specifications for the camera, with operating temperature above 50°C and a camera exposed to dust without any protective enclosure”, says Vassileios Tsetsos, Chief Technology Officer of Mobics Ltd. “In spite of that, the AVT Prosilica GE680 proved to be an extremely reliable camera for demanding applications”.