The DigImage particle tracking system is able to simultaneously determine additional information about the flow by accessing the larger scale aspects of the image intensity field. There are three main options: recording the background intensity in the neighbourhood of the particle, recording which of two possible regions the particle falls in, or masking off a particular region on the basis of the background intensity. A typical case where any or all of these options may be of interest is if the tracking is to be undertaken in a turbulent plume. Suppose we were interested in the entrainment flow, so the particles are initially in the quiescent fluid. During any subsequent analysis it is important to know whether a given particle lies inside or outside the plume, or, if the experiments are of adequate quality, what the concentration of some tracer is in the neighbourhood of the particle. This information may be encoded during the initial experiments by dying the plume (eg. with flourescene), and recording during the tracking process which particles are where.
Note that static background removal (as described by [qqB: Background removal]) is performed during the acquisition phase prior to determining the local dynamic background intensity.
This option determines the intensity of the background in the neighbourhood of the particle by setting up a window around the particle and looking for the modal value of an intensity histogram. If this option is chosen, then you must ensure the background intensity evaluation parameters in [qLB: Background intensity evaluation] are set up in an appropriate manner. Note that this option can not be used when tracking more than 511 particles. If it is set, then starting the tracking interactively with [;UH Start particle tracking - 4095 particles] will generate a warning message.
With this option, the region lying within the specified intensity contour is excluded from the particle tracking process.
With this option, the region lying outside the specified intensity conotour is excluded from the particle tracking process.
If this option is chosen, then there is no recording of the region in which a particle lies and the entire tracking region is utilised.
This option also includes the entire tracking window during particle tracking, but records whether a particle lies inside or outside the specified contour level. Note that this option can not be used when tracking more than 511 particles. If it is set, then starting the tracking interactively with [;UH Start particle tracking - 4095 particles] will generate a warning message.