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settings-panel: PIV - Interrogation Window.
Multi pass
Specify the number of passes. The result of ervery pass is used to preshift the interrogation windows during the next pass. Two final passes with the same interrogation window sizes are always useful. In this way, the interrogation windows overlap optimally in the last pass, the correlation result will be enhanced if compared to the first pass. Use successively smaller windows to cope with large displacements.
Interrogation window width
The horizontal size of the interrogation windows in pixel for each pass, respectively.
Interrogation window height
The vertical interrogation window size.
Search domain width/heigh
Only correlation peaks inside this area are accepted.
Horizontal/Vertical vector spacing
The distance between the vectors.
Region of interst (ROI)
Select this box to constrict the PIV evaluation to a portion of the image. Specify the upper left and the lower right corners of the region of interest in the text fields below.
Horizontal/Vertical pre-shift (first pass)
A constant pre-shift of the interrogation windows can be specified for the first pass. This is useful, if there is a uniform flow direction. Individual pre-shifts for following passes are taken from the previous passes, respectively.
Between passes:
Select some vector post processing routines to eliminate outliers in the pre-shift files. Normalized median test marks vectors as invalid using the normalized median test proposed by Jerry Westerweel and Fulvio Scarano (Experiments in Fluids (2005) 39: 1096-1100). The velocity data remains unchanged, but the value in the fifth column (generally the peak height) is set to -1. Parameters are set on the JPIV - Vector Processing - Normalized Median Test panel. All vector processing functions expect selected files in the "Files" frame. Replace invalid vectors by median replaces invalid vectors by the median of their eight surrounding vectors. A vector is considered to be invalid if its fifth column is negative or null. 3x3 median filter replaces every vector by the median of its surrounding vectors inclusive itself. 3x3 smoothing replaces every vector by the average of its surrounding vectors inclusive itself.
Deform (shear) interrogation windows
Simple shearing of interrogation windows using affine transformation. Only enhances evaluation accuracy of heavily sheared velocity fields.