In order to track particles in an experiment, we must sample their positions at regular intervals throughout the experiment. The first level of sampling is performed by the VTR which samples the experiment at the frame rate (25Hz for PAL, and 30Hz for NTSC) and a spatial resolution determined by a combination of the video camera and the VTR format. In fact, because of the interlacing technique used by standard video systems, the experiment is sampled at twice this rate, but with neighbouring samples having a small vertical offset.
During the tracking process, the information stored on the video tape itself needs to be sampled. Spatially it is digitized into a 512x512 (for PAL; 512x480 for NTSC) pixel array. The temporal information may be sampled at any multiple of the video frame rate by digitizing video frames at regular intervals. If we are prepared to accept a small vertical error, then we may utilise each video field separately and achieve a sampling rate of twice the frame rate.
The amount of storage required by DigImage during the tracking process is governed by a combination of the sample spacing, the total tracking time, and the average number of particles in each sample image. For each sample, 80+6*nPart bytes are required, where nPart is the average number of particles in the samples. Thus, for a given length of experiment, halving the sample spacing will double the amount of storage required.
DigImage may be made to track backwards in time by specifying a negative value for the sample spacing. Note that under these circumstances the direction of time, velocities etc. is effectively reversed.
If you are tracking from a movie file (specified by [V Video type]), then it is important that the sample spacing specified here matches that of the movie file acquired with [;TMR Record movie].