Delayed exposure to reduce reflections from other cameras

When to use delayed exposure

Exposure delay can be used in environments where the strobe light from one set of cameras disturbs marker capture in other cameras. It is good to first consider the following changes to the setup.

1. Test a different camera positioning. The reflections can be reduced if the cameras do not look straight at each other.

2. Can anything be done to the floor or any other reflective area of the room to make it less reflective.

3. It can also help to reduce the exposure time or increase the marker threshold, because the unwanted reflections are usually less intense than the markers.

Exposure delay will shift the time of exposure for different groups of cameras. For example, a 6-camera system has three cameras located on one side and three on the other side of a room, with a highly reflective floor, where the strobe from cameras on one side disturbs cameras on the other side. Assigning the first three cameras to one exposure group and the other three cameras to another exposure group will eliminate the problem of strobe interference.

Setting up delayed exposure

Follow these instructions to use the delayed exposure.

1. To use the delayed exposure, you must first find out which cameras are causing reflections in other cameras. It is usually best to use the marker intensity mode to see where the reflection comes from.

2. When you know the cause of the reflections you place the cameras in different groups on the Cameras page in the Project options dialog, see chapter Exposure delay.

   1. Activate the Exposure delay mode called Camera groups, which will calculate the delay for each group automatically by comparing it to the longest exposure time of the cameras in the previous group. Do not use the Advanced mode unless you are absolutely sure of what you are doing.

   2. Select the cameras that you want in the group from the list to the right.

   3. Then select the group on the Camera groups setting. Make sure that you always start with group 1 and continue with 2 and so on.

   4. Repeat steps b and c for each group that you want to create. Usually for a setup with cameras on two sides of the volume it is enough with two groups, one for each side. It is also possible to change the exposure group of a camera by right-clicking on the camera live feed in the 2D view window and selecting the exposure group via the context menu.

   5. When the exposure delay is activated Using delayed exposure is displayed in the status bar.

3. To check which groups the cameras are in, go to the 2D view and check the delayed exposure setting next to the camera number.

Guidelines for use of exposure groups for fast movements

By delaying the exposure between camera groups, the markers may have moved between the respective group exposures. The 3D tracking algorithm applies a compensation for this displacement of the 2D positions for the delayed cameras based on the measured movement of the markers. The compensation is effective for most capture applications of human movement, such as gait or running. For optimizing the compensation for fast movements, for example for baseball pitching or golf, take the following guidelines into account:

1. Minimize the number of exposure groups

2. Make sure that most cameras are assigned to the lowest possible exposure groups, so that most cameras are in exposure group 1, followed by group 2, etc.

3. Use short exposure times to minimize the delay

4. Use high capture frequencies

The compensation for the 2D positions is also used in the calibration. It is important to move the wand slowly for an optimal calibration. There is a warning in the Calibration results dialog if the wand speed is considered too high.