As many have stated, the lighting at the events is indeed different from what is in your classroom or shop. I'll give a general description for how we arrived at values, I'll attempt to answer some of the camera related questions, and I'll give a few hints on how to tune your camera up. I won't be giving absolute values as I fully expect that different events will have variations in lighting brightness, arrangement, color, and in backdrop. Because if this, knowing how to find the values is far more important than knowing the values of a single field. I highly encourage you to take advantage of the Th field time to tune your camera. Be prepared for a BIG post. If you like, you can then share the values with other teams.
First off, the brightness and other camera settings are persisted on the camera. The camera settings are modified via HTTP CGI protocol and can be set interactively using the camera web pages, the C based camera demo application, or via the LV wrapper VIs. The Find Two Color LV code included a brightness parameter on the panel specifically for tuning. Other camera settings such as exposure are set and behave similarly.
Lighting on the practice field seems to work reasonably well with default settings. It is perhaps a bit brighter than the average classroom, but the tests I ran in DC worked. The lighting on the competition field is quite different. There are two trusses of lights running along the long sides of the field approximately 12 ft outside the edges of the field and suspended ~30 ft in the air. Those trusses are packed with lights. They are adjusted for even coverage and to produce few shadows.
With the default settings, the test went reasonably well on three sides of the field, but glare was present and caused some white streaks on the targets. The fourth side of the field, the one with the black curtain will unfortunately not work with the default settings. The reason for this is that the camera is sampling the pixels in a frame to determine how to adjust future exposures to capture enough light with the camera sensor. This typically works quite well, but since the camera has no idea what portion of the scene you are most interested in, it uses a statistical sampling and when the background of the image is not returning any light, it causes the camera to lengthen the exposure resulting in an image with lots of glare. This same effect may be seen if the house lights in the stands are set very low. If the overall light from the background is too low, the brightly lit targets will be blown out by the bright lights.
The solution is to shorten the exposure. With the Axis 206 camera, there are two modes to control exposure, leave auto exposure on and decrease brightness, or determine a way to calibrate the auto exposure and set it to hold. Personally, I think it is more repeatable and more robust to use auto exposure. Of the fields seen thus far, most decided to set the brightness to 0, one field determined that 10 was a better value to use. The brightness works similar to an exposure compensation setting on most consumer cameras. It modifies the determined exposure to let the human adjust for back lighting or dark backgrounds. For glare lower it. For backlit targets, raise it.
The other aspect of the competition field that is difficult to deal with is the light position. In some locations, especially as the camera gets near the target, it will be aimed upwards. In some locations on the field, the camera will then be staring into the lights. This further complicates matters by partially blinding the camera. Finally, in the worst locations on the field, the angle from a low mounted camera, a target, and the light will be symmetric. When the angle from the target's plane to the light and the angle from the plane to the camera sensor are roughly equal, the glare from the lights will go way up, practically turning the target white. This doesn't occur that many places on the field, but it would be wise to allow for the occasional dropped frame in your tracking code, your lead estimator, etc.
Because the camera will sometimes be staring at the lights, you will find that the color saturation of the target drops compared to the shop. Because of this, you may find it useful to lower the minimum values of saturation you are thresholding against. How much? As you can see, different teams determined different values. If taken too low, gray or pastel colors will start to be included in the mask. Not low enough and some areas of the field will flicker or stripe. To determine the value to use, it would be good to interactively move the camera and target around on the field viewing an HSL display. To do this, the LV example was modified to display more information. This will be made available for FTAs and teams to use if they wish. An announcement and instructions will be released soon.
A sampling of test images and the threshold results can be found at
http://flickr.com/photos/35780791@N07/
From the stands, I saw some camera work very well, many others were not even turned on. As weeks go on, I expect more successes. Feel free to post questions or results.
Greg McKaskle