| 775 | 775 | |
| 776 | 776 | * '''PIV''': makes image comparisons on sliding index pairs, as usual for measuring velocities by particle displacements. A second menu '''[!ListPairMode]''' in the panel '''[!PairIndices]''' then selects one among three possibilities (not always available depending on the file indexing): |
| 777 | 777 | * ''pair j1-j2:'' selects a given j index pair (sometimes denoted by letter index) for the whole time series. This is the most common case for PIV. Pair selection is performed in the menu '''[!ListPairCiv1]''' and '''[!ListPairCiv2]''' for the second iteration Civ2, see below. If timing from an XML file [#a3.5Imagedocumentationfiles.xml <!ImaDoc>] has been detected, this is indicated in the edit box '''[!ImaDoc]''' and the corresponding time intervals are indicated (in ms). For some applications, this time interval may evolve in time, so that reference indices ref_i and ref_j are chosen for the display. |
| 778 | 778 | * series (Di): selects a given index interval for the i index, around a set of reference i indices. The intervals are denoted Di=0|1, -1|1, -1|2, -2|2 ... , corresponding to the index pairs i|i+1, i-1|i+1, i-1|i+2, i-2|i+2 ...around each reference index i. Pair selection is then performed in the menu '''[!ListPairCiv1]''' and '''[!ListPairCiv2]''' like for 'pair j1-j2'. |
| 797 | | This series of operations is chosen by selecting the corresponding check boxes on the left of the GUI '''civ_series''', which give access to the corresponding parameter input panels. Note that the result of each of these operations is stored in the output NetCDF file, so the process can be split in several runs. When an existing NetCDF velocity file has been initially opened, the GUI '''civ.fig''' is automaticaly configured to perform the next operation (fix1, patch1, civ2...) needed in the process. |
| | 797 | This series of operations is chosen by selecting the corresponding check boxes on the left of the GUI '''civ_series''', which give access to the corresponding parameter input panels. Note that the result of each of these operations is stored in the output NetCDF file, so the process can be split in several runs. When an existing NetCDF velocity file has been initially opened, the GUI '''civ.fig''' is automaticaly configured to perform the next operation (fix1, patch1, civ2...) needed in the process. |
| 804 | | ('''[num_!CorrBoxSize_1,_2,_3], [iby])''' set the size (in pixels) of the 'correlation box', the sliding window used to get image correlations. '''[num_!SearchBoxSize_1,_2,_3]''' set the size of the 'search box' in which image correlation is calculated, see figure. This search box can be shifted with respect to the correlation box by parameters ([num_!SearchBoxShift_1,_2,_3]). This is useful in the presence of a known mean flow. The default value !SearchBoxSize=(25,25) is generally good, use a larger size for images with few particles, use an elongated box , e.g. (11,41), to optimise the resolution in one direction (for instance in a boundary layer). |
| 805 | | |
| 806 | | A parameter '''[num_CorrSmooth] ''' controls the smoothing of the image corelation curves used in civ, the default value 1 is generally used. |
| 807 | | |
| 808 | | The search parameters (!SearchBoxSize,!SearchBoxShift) can be estimated using the press button '''[search range]'''. First introduce the estimated minimum and maximum values of each velocity component u and v. The result depends on the time interval of the image pair. Change the selected image pair if the maximum displacement (ibsx-ibx)/2 is too small (lack of precision) or too large (bad image correlations and risks of false vectors). |
| | 804 | ('''[num_!CorrBoxSize_1,_2,_3])''' set the size (in pixels) of the 'correlation box', the sliding window used to get image correlations. '''[num_!SearchBoxSize_1,_2,_3]''' set the size of the 'search box' in which image correlation is calculated, see figure. This search box can be shifted with respect to the correlation box by parameters ([num_!SearchBoxShift_1,_2,_3]). This is useful in the presence of a known mean flow. The default value !SearchBoxSize=(25,25) is generally good, use a larger size for images with few particles, use an elongated box , e.g. (11,41), to optimise the resolution in one direction (for instance in a boundary layer). |
| | 805 | |
| | 806 | A parameter '''[num_CorrSmooth]''' controls the smoothing of the image corelation curves used in civ, the default value 1 is generally used. |
| | 807 | |
| | 808 | The search parameters (!SearchBoxSize,!SearchBoxShift) can be estimated using the press button '''[search range]'''. First introduce the estimated minimum and maximum values of each velocity component u and v (expressed in pixel displacement). The result depends on the time interval of the image pair. |
| 818 | | The grid determines the positions of measured velocity vectors: it sets the central positions of the correlation boxes (in pixels) for the first image. A default regular grid can be set by the meshes '''[num_Dx] ''' and '''[num_Dy]''' (in pixels). Alternatively a custom [grid->#sec3.6_grid] can be stored in a text file and selected by the check box '''[GET_GRID]'''. This is convenient to limitate the processing to a subregion or to fine tune the resolution. |
| 819 | | |
| 820 | | A subregion can be alternatively selected by a mask image, selecting the edit box '''[MASK]'''. If a mask image with an appropriate name is found in the image directory, it wil be detected, and the indication 'xxmask' appears in the edit box. (xx is the number of slices, equal to 1 for a single mask). Otherwise a browser appears to select a (single) mask file. |
| 821 | | |
| 822 | | Finally thresholds on image intensity can be introduced to suppress underexposed or overexposed parts of the image: select the check box '''[THRESH],''' and edit the boxes '''[!MinIma] ''' and '''[!MaxIma] ''' which then appear. |
| 823 | | |
| 824 | | The velocity results are stored in the subdirectory set by the edit box '''[subdir_civ1].''' Use the browser''' [list_subdir_civ1]''' to check the existing subdirectories. |
| 825 | | |
| 826 | | -''' RUN and BATCH: ''' |
| 827 | | |
| 828 | | The PIV calculation is started by the press button [RUN], or [BATCH] if this option has been installed. BATCH sends the same computations as background computing tasks in a network. |
| 829 | | |
| 830 | | In both cases, the status of the computations can be checked by opening {.cmx } and {.log} files, using the UVMAT browser or any text editor. These files are writtent in the same subdirectory as the NetCDF result files. Each {.cmx} file contains the set of parameters used for a civ computation. It is written by the civ interface just before ordering the computations. By contrast , the {.log} file is produced after completion of the computations, and it contains some information on the process, including possible error messages. |
| | 817 | The grid determines the positions of measured velocity vectors: it sets the central positions of the correlation boxes (in pixels) for the first image. A default regular grid can be set by the meshes '''[num_Dx] ''' and '''[num_Dy]''' (in pixels). Alternatively a custom [grid->#MaskGrid] can be stored in a text file and selected by the check box '''[GET_GRID]'''. This is convenient to limitate the processing to a subregion or to fine tune the resolution. |
| | 818 | |
| | 819 | A subregion can be alternatively selected by a mask image, selecting the edit box '''[Mask]'''. If a mask image with an appropriate name is found in the image directory, it wil be detected, and the indication 'xxmask' appears in the edit box. (xx is the number of slices, equal to 1 for a single mask). Otherwise a browser appears to select a (single) mask file. |
| | 820 | |
| | 821 | Finally thresholds on image intensity can be introduced to suppress underexposed or overexposed parts of the image: select the check box '''[!CheckThreshold],''' and edit the boxes '''[num_!MinIma] ''' and '''[num_!MaxIma] ''' which then appear. |
| 833 | | The FIX operation is used after civ to remove false vectors, using different criteria: |
| 834 | | |
| 835 | | ''' - {warning flags:} ''' these are flags (vec_F) indicating problems with the image correlation process: |
| 836 | | * vec_F(i)=0 or 1 : default , the processing is fine |
| 837 | | * vec_F(i)=-2: the maximum of the correlation function is close to the border of the search box (at a distance of two pixels or less), so that its maximum cannot be reliably obtained: the search domain is too small |
| 838 | | * vec_F(i)=2: (only in civ1) we keep the less accurate Hart result (resulting from combining correlation functions from neighborhing points) and vec_C(i)=-1 is chosen by convention. Reference: Hart D. P. (2000) 'PIV error correction',{ Exp. Fluids} '''29''', 13-22. |
| 839 | | * vec_F(i)=3: the optimisation of the correlation function is unstable or local Intensity rms of the image =0 |
| 840 | | * vec_F(i)=4:only in civ2: the difference between the estimator and the result is more than 1 pixel. The two criteria, vec_F(i)=-2, and 3, should be selected (default options). The criterium vec_F=2 (Hart) should not be selected, while vec_F=4 (for civ2) could be selected only for excellent data (otherwise it may be too strict). |
| 841 | | |
| 842 | | ''' - {Threshold on the image correlation:} ''' (vec_C) can be introduced by the edit box [thresh_vecC] (value between 0 and 1). It removes vectors with poor correlation. |
| 843 | | |
| 844 | | ''' - {Threshold on the velocity modulus:} ''' (expressed in pixels): it can remove either too small values (menu '''[inf_sup1]''' set to '<'), or excessive values (menu '''[inf_sup1]''' set to '>'). Erratic zero velocity vectors, produced by a fixed image background, can be eliminated by this criterium. These criteria can be as well applied to the difference with a reference field, defined by a file series (edit box '''[ref_fix1]''') and a field inside the file (menu '''[field_ref1]'''). |
| 845 | | |
| 846 | | ''' - {Mask fix:} ''' It has the same effect as the one used in civ1, but the removed vectors are kept in memory, labelled as false. |
| | 824 | The FIX operation is used after civ to mark false vectors, using different criteria: |
| | 825 | |
| | 826 | ''' - warning flags: ''' these are flags (vec_F) indicating problems with the image correlation process. |
| | 827 | * vec_F=-2: select to eliminate vectors for which the maximum of the correlation function is close to the border of the search box (at a distance of two pixels or less), so that its maximum cannot be reliably obtained. |
| | 828 | * vec_F=3: the optimisation of the correlation function is unstable or local Intensity rms of the image =0. Must be selected. |
| | 829 | |
| | 830 | ''' - Threshold on the image correlation: ''' (vec_C) can be introduced by the edit box '''[num_!MinCorr]''' (value between 0 and 1). It removes vectors with poor correlation. |
| | 831 | |
| | 832 | ''' - Threshold on the velocity modulus: ''' (expressed in pixels). It can remove either excessive values (threshold set by '''[num_MaxVel]''') or too small values (threshold set by '''[num_MinVel]'''). Erratic zero velocity vectors, produced by a fixed image background, can be eliminated by the latter criterium. |
