A few thoughts I have:
I would think taking an image of the field without anybody on it would work pretty well to represent the background. Subtracting the background from each image using absolute subtraction would leave the player and ball as pretty bright objects. If this is indoors the lighting will be more constant. Outdoors the brightness could vary significantly, especially on a partly cloudy day.
You might want to consider marking the player and the ball. For example, get a bright red ball and have the player wear red armbands and leg bands. Put a red filter on your camera and you will see the red items pretty clearly. You would still have to subtract the background, but it would probably be easier. Matching the colors of red could be difficult, though. If you are using a color camera (without color filter), you could look at the hue and color threshold the reds. You could also do two different colors for the player and the ball and search for them separately. This would eliminate the need for subtracting the background.
If you can, use a grayscale camera. Color cameras are a trade-off: You lose resolution and detail, but gain color information.
A wide angle lens will have a pretty big depth of field, so focusing on the ball shouldn't be too difficult unless the ball gets very close to the camera. If your budget is tight, you could probably prototype this with a webcam. They tend to have fairly wide angle lenses.
Bruce
Bruce Ammons
Ammons Engineering
