846 | | '''PATCH1: ''' interpolates the velocity values on a regular grid with a smoothing effect controlled by the parameter '''[num_!FieldSmooth]'''. The interpolation relies on the thin plate spline method, see [#tps: section 5.1]. This also provides the spatial derivatives (vorticity, divergence) needed for the refined processing civ2 (with option 'deformation'). The vectors which are too far from the smoothed field (erratic vectors) can be eliminated, using a threshold expressed in pixel displacement (set by '''[num_!MaxDiff]'''). The thin plate spline method must be done by subdomains for computational saving. The parameter [num_!SubDomain] gives the typical number of vectors used to determine the partition in subdomains. The default value 1000 fits the memory capacity of usual computers. |
847 | | |
848 | | The button '''[TEST]''' in the '''[Patch]''' frame allows the user to quantify the difference between the '''[Civ]''' result ('civ1') and the smoothed velocity fields 'filter1' given by '''[Patch]'''. It opens a graph in a new '''Figure1''' plotting the rms difference between these two quantities (over the whole field) as a function of the smoothing parameter varied around the value specified by '''[num_!FieldSmooth]''' parameter. This rms difference, expressed in pixel should be of the same order as the expected PIV error: the value 0.2 is indicated as a typical target for the choice of the corresponding smoothing parameter. |
| 846 | '''PATCH1: ''' interpolates the velocity values on a regular grid with a smoothing effect controlled by the parameter '''[num_!FieldSmooth]'''. The interpolation relies on the thin plate spline method, see [#tps: section 5.1]. This also provides the spatial derivatives (vorticity, divergence) needed for the refined processing civ2 (with option 'deformation'). |
| 847 | |
| 848 | The vectors which are too far from the smoothed field (erratic vectors) are eliminated (marked by an error flag), using a threshold set by '''[num_!MaxDiff]''' (expressed in pixel displacement). The interpolation is performed again by the program after this elimination. |
| 849 | |
| 850 | For fields involving many vectors, the thin plate spline method must be done by subdomains because of computer memory limitations. The parameter [num_!SubDomain] gives the typical number of vectors used to determine the partition in subdomains. The default value 1000 fits the memory capacity of usual computers. |
| 851 | |
| 852 | The button '''[TEST]''' in the '''[Patch]''' frame allows the user to quantify the difference between the '''[Civ]''' result ('civ1') and the smoothed velocity fields 'filter1' given by '''[Patch]'''. It opens a graph in a new '''Figure1''' plotting the rms difference between these two quantities (over the whole field) as a function of the smoothing parameter, varied around the value specified by '''[num_!FieldSmooth]''' parameter. This rms difference, expressed in pixel should be of the same order as the expected PIV error: the value 0.2 is indicated as an appropriate target for the choice of the corresponding smoothing parameter. A second curve indicates the proportion of vectors excluded by the criterion of excessive difference between the civ1 and filter1 fields. This criterion should apply only for erratic isolated vectors, so the corresponding proportion should not exceed a few percents. |
857 | | - '''PATCH2:''' like '''[PATCH1]'''. Using the '''[TEST]''' option, we can here accept a difference between '''[Civ2]''' and '''[Patch2]''' higher than 0.2. |
858 | | |
859 | | Further iterations: improvements can be obtained by further iterations of the civ2-fix2-patch2 process. Open again the interface, and consider the previous civ2 result as the prior guess civ1. It will be recopied and relabelled as civ1 in the new NetCDF file produced. |
| 863 | - '''PATCH2:''' like '''[PATCH1]'''. Using the '''[TEST]''' option, we can similarly quantify the influence of the smoothing parameter. The recommended value of the rms difference between '''[Civ2]''' and '''[Patch2]''' (field 'filter2') is now set to 0.1, to safely avoid systematic distortion by smoothing in the final result. |
| 864 | |
| 865 | Further iterations: improvements can be obtained by further iterations of the civ2-fix2-patch2 process. Open again the interface, and consider the previous civ2 result as the prior guess civ1, and select the option civ3 in the checkbox which now appears in the panel civ2. The result will be put in a new NetCDF file, but still labelled as Civ2 field. |