PIV Commands

FFT mode scale_factor

    mode = 0  return the magnitude scaled by scale_factor

    mode = 1   return the real part scaled by scale_factor

    mode = 2   return the imaginary part scaled by scale_factor

    mode = 3   return inverse - input is real part scaled by scale_factor

    mode = 4   return inverse - input is imaginary part scaled by scale_factor

    mode = 5  return the log of the magnitude squared scaled by scale_factor

    mode = 6  return the magnitude squared scaled by scale_factor

    mode = 7  return the autocorrelation scaled by scale_factor

    mode = 8  return inverse - input is a filter scaled by n


MAKPIV no_of_points

    Randomly generate a piv particle image. no_of_points is how many point in the image. Max (and default) is 20,000.


MASK width height

    Make a “mask” for inverse fft operations. The central region of the current image with the width and height specified will be set to 1. The area around the central region is set to 0. (See FFT command option 8.)


OF2PIV scale_factor average_flag x_vel_file y_vel_file

    Convert optical flow images into PIV vectors Reads in OMA image formats for x and y velocities Use PIVINC and PIVSIZE to determine how often to plot a vector Scale factor can be float. Average flag (integer) is 0=off 1=on.


PAIRLS

    List dx dy and magnitude for pairs.  Points must first be generated via the POINTS command and pairs generated via the PAIRS command.


PAIRPR

    Process pair statistics for an entire image. Assumes that matching PIV and POINT have already been calculated (e.g., via the PIV and POINTS or PNTMOSAIC commands). Prints out statistics corresponding to pairs for each velocity vector.


PAIRS dx dy radius

    Find pairs of particles separated by dx, dy (to within radius) assumes arrays exist that contain image points and that the points lie within the bounds of the current image. The rectangle is changed to correspond to the entire image.


PEAK #_to_print sum_area exclude_radius


PEAKIMAGE peakval sum_area exclude_radius

    Replace the image data by a peaks only image. Points greater that peakval are included.


PIV- dx dy

    Subtract a constant vector from the currently loaded vectors.


PIV sum_area exclude_radius


PIV* scale

    Multiply the currently loaded vectors by a scale factor.


PIV/ scale

    Divide the currently loaded vectors by a constant.


PIV+ dx dy

    Add a constant vector to the currently loaded vectors.


PIVABS

    Returns the absolute value of all components in a PIV Field. Note that this only affects the top (visible) layer of vectors and does NOT alter the underlying, alternate peaks.


PIVADD <filename>

    Add vector file to currently loaded vectors.


PIVALTER flag

    Alter a point calculated by the PIV command. If flag is not 0, turn on alter mode. Interactively shuffle through different peaks. To change a vector, point at the plotted arrow. Rotate a all others up one position.


PIVANG value

    Strip bad vectors out of a PIV image by checking that it's angle does not differ too much from those around it. The value is a deviation in degrees (15 is the default).


PIVAVG

    Calculates average vector over entire vector field.


PIVBAD mag angle replace_flag

    Remove vectors which deviate more than mag in magnitude and angle in angle from those in immediate neighborhood. replace_flag = 1 means put in average of surrounding for bad vectors


PIVBIG

    Kind of does what PIVMAX used to do. It returns the 3 components of the maximum visible (top) vector to the Command Window.


PIVC-

    See PIVC+


PIVC* factor

    Conditional PIV arithmetic. Multiplies all NON-ZERO vectors by the scale factor specified in the command line. Only affects the visible (top) vectors.


PIVC/

    See PIVC*


PIVC+ xcomp ycomp

    Conditional PIV arithmetic. Adds the X & Y components specified in the command line to each NON-ZERO vector in a buffer. Only affects the visible (top) vectors.


PIVCNV

    Convert Pixels/Delay to m/s using parameters from dialog.


PIVCRS

    Cross correlation PIV command. The second image is stored in the background image (use the “SBACK filename” command).


PIVDIRECTION

    Alter PIV vectors to orient vectors in preferred direction (as set by PIVSET command).


PIVDIVIDE

    Divide the currently loaded vectors by the scalar image in the current buffer. This could be used to do conditional averages -- the number of points in the scalar field would be the same as the number of vectors.


PIVDOT

    Dot product of PIV buffer with PIV in file.


PIVEXE

    Execute throughput PIV processing algorithm. (see EXECUTE command).


PIVFILL

    Interpolate zero vectors in field from neighbors.


PIVFLH

    Flip the PIV Field in the OMA PIV buffer horizontally. This transformation is applied to all the peaks in the image, including the visible vectors.


PIVFLV

    Flip the PIV Field in the OMA PIV buffer vertically. This transformation is applied to all the peaks in the image, including the visible vectors.


PIVFTR

    Filter PIV based on local neighborhood mean.


PIVGET <filename>

    Load PIV data from file.


PIVINC

    Specify the increment (in pixels) between successive samples in the PIV command.


PIVLST

    List the current PIV data.


PIVM*

    Each vector in the PIV buffer is multiplied by a scalar variable equal to the value of the corresponding pixel in the OMA Image buffer. Only affects the visible (top) vectors.


PIVM/

    Each vector in the PIV buffer is divided by a scalar variable equal to the value of the corresponding pixel in the OMA Image buffer. Only affects the visible (top) vectors.


PIVMAG accuracy

    Kill any vector in image whose magnitude differs from the neighborhood average.  accuracy is the threshold of accuracy for vector comparison; 0=different; 1.0= identical; default is accuracy = 0.65.


PIVMAX

    Specifies the maximum allowed length of vectors in a PIV Field. Vectors longer than this limit are set to zero. Only affects the visible (top) vectors.


PIVMIN

    Specifies the minimum allowed length of vectors in a PIV Field. Vectors shorter than this limit are set to zero. Only affects the visible (top)  vectors.


PIVMSG

    Massage PIV data. Auto correct a PIV image This is done using an iterative application similar to that using PIVALT  mode 3.


PIVMULT

    Multiply the currently loaded vectors by the scalar image in the current buffer. This could be used to apply a mask (scalar image with 1's and 0's) to the vector field.


PIVPLT scale

    Plot the current PIV data. Scale vectors by scale (float).


PIVREFORMAT cp np

    Reformat piv data to have cp as center pixel, np peaks


PIVRMV [deviation_magnitude deviation_angle set_avg_flag]

    Try to remove bad vectors -- those with deviation > given. Set bad vectors = 0.0 or replace by local avg if requested.


PIVRND dx dy pix_value num_pairs

    Generate a simulated double-exposure PIV image.


PIVROT

    Rotate the PIV Field by 90 degrees clockwise. The Vectors are also rotated 90 degrees, swapping X & Y components. Four PIVROT will leave the original image unchanged.


PIVSAVE <filename>

    Save current PIV data to file.


PIVSET

    Set values of constants used in the PIV command.


PIVSHT [exrad]

    Remove vectors shorter than exrad.


PIVSIZE

    Specify the size of fft done in the piv command.


PIVSQRT

    Get square root of each component of the currently loaded vectors.


PIVSUB <filename>

    Subtract vector file to currently loaded vectors.


PIVSUC <filename>

    Subtract vector file: only do subtraction from vectors in currently loaded vector field.


PIVUODETECT

     PIV command that implements the Universal Outlier detection algorithm of J. Westerweel and F. Scarano "Universal Outlier Detection for PIV data" Experiments in Fluids [39]:1096-1100 (2005).


PIVX

    Extract the x-component.


PIVX2Y

    Writes the X-component of the visible vectors into the Y-component so that both are identical.


PIVY

    Extract the y-component.


PIVY2X

    Writes the Y-component of the visible vectors into the X-component so that both are identical.


PNTIMG

    Replace the image data by a points only image. Points must first be generated via the POINTS command.


PNTMOSAIC scale

    Find the points (particle images usually) in a large image. The large image is treated as a mosaic of smaller images (size specified by PIVSIZE). As in POINTS, the centroids of connected areas are found, along with integrated intensities and areas. This information is stored in memory. The clipvalue is determined as fraction of the range in each subimage. For example, a scale of 0.5 would use 50% of the largest peak height within the subimage as the clip value. Use the PIVSET command to enable the histogram clipping feature to exclude erroneous minimum and values.


POINTS clipval

    For those points greater than clipval, the centroids of connected areas are found, along with integrated intensities and areas. This information is stored in memory.


PRIMG type

    Replace the image data by a point-pair image. Points must first be generated via the POINTS command and pairs generated via the PAIRS command. Type = 0 assigns inten/area. Type = 1 assigns total inten. Type = 2 assigns ratio of intensities.


PRINFO

    Provide info on point pairs. Points must first be generated via the POINTS command and pairs generated via the PAIRS command.


VECIMG n

    Record the vector magnitudes (scaled by n) in the image buffer.


VECIMX n

    Record the x-component of velocity (scaled by n) in the image buffer.


VECIMY n

    Record the y-component of velocity (scaled by n) in the image buffer.


VORT2 scale

    calculate the vorticity another way -- simply differences


VORTICITY scale

    Calculate vorticity; calculation method from Reuss et al. -- circulation around a closed contour surrounding the point


XCORRELATE <filename>

    Another cross correlation PIV command. The second image is in the specified file. (P. Kalt algorithm)