Changes between Version 169 and Version 170 of UvmatHelp


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Timestamp:
Jan 22, 2015, 5:24:55 PM (6 years ago)
Author:
vaillant1p
Comment:

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  • UvmatHelp

    v169 v170  
    832832''' - 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.
    833833
    834 ''' - {Manual fix:} ''' Interactive fixing with the mouse is also possible, see [section 4.2->#4.2].
    835 
    836 ''' - {Code for FixFlag:} '''
    837  * !FixFlag(i)=0 :default, everything is fine
    838  * !FixFlag(i)=1. : value 1 for the second digit: vector removed by criteria on vec_F or/and low correlation value
    839  * !FixFlag(i)=1..: value 1 for the third digit: vector selected by a criterium of difference with another field, for instance filter, or a field with differnt time interval.
    840  * !FixFlag(i)=...1, 2, 3  different criteria for elimination set by the program Fix of CIVx (not yet documented).
     834''' - Manual fix: ''' Interactive fixing with the mouse is also possible, see [section 4.2->#4.2].
     835
    841836
    842837=== 11.4 PATCH ===
    843 '''PATCH1: ''' interpolates the velocity values on a regular grid with a smoothing parameter rho. This also provides the spatial derivatives (vorticity, divergence) needed for the refined processing civ2. The numbers of grid points in x and y are set by nx and ny. The vectors which are too far from the smoothed field (erratic vectors) can be eliminated, using a threshold expressed in pixel displacement. The thin plate spline method must be done by subdomains for computational saving, use the default value 300.
     838'''PATCH1: ''' interpolates the velocity values on a regular grid with a smoothing parameter '''[num_!FieldSmooth]'''. This also provides the spatial derivatives (vorticity, divergence) needed for the refined processing civ2. 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.
    844839
    845840=== 11.5 CIV2 ===
    846 -'''CIV2:''' provides a refined calculation of the velocity field, using the civ1 result as previous estimate. The search range is determined by the civ1 field so there is no parameter. Use the option decimal shift to reduce peaklocking effects (advised). The option deformation is useful in the presence of strong shear or vorticity. Then image deformation and rotation is introduced before calculating the correlations. The other parameters (rho, ibx,iby, grid, mask) are used like for civ1 (take the same by default). The image pair for civ2 can be different than the one used in civ1 to get the first estimate. It is generally advised to use a moderate time interval for civ1, to avoid false vectors, and to take a larger intervel for civ2, in order to optimize precision. As civ2 already knows where to look and takes into account image strain and rotation (with the option deformation) it allows for higher time intervals.
     841-'''CIV2:''' provides a refined calculation of the velocity field, using the civ1 result as previous estimate. The civ1 field provides an estimated shift for each measurement point, so there is no edit box to enter shift parameter. The other parameters, in particular '''[!CorrBoxSize]''' and '''[!SearchBoxSize]''', have the same meaning as for Civ1. The image pair for civ2 ( set by the menu '''[!ListPairCiv2]''', can be different than the one used in civ1. It is generally advised to use a moderate time interval for civ1, to provide a first estimate avoiding false vectors, and to take a larger intervel for civ2, in order to optimize precision. As civ2 already knows where to look and takes into account image strain and rotation (with the option 'deformation') it allows for higher time intervals.
     842
     843Image deformation and rotation can be introduced before calculating the correlations, by selecting '''[!CheckDeformation]'''. This option  is useful in the presence of strong shear or vorticity.
    847844
    848845-'''FIX2:''' like FIX1, except for a new possible flag value vec_F=4 which indicates that the difference between the estimator and the result is more than 1 pixel. This should be selected for excellent data (otherwise it may be too strict).   
     
    853850Further 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.
    854851
    855 === 11.6 Stereoscopic PIV ===
    856 
    857 [sec11.6<-]
     852=== 11.6 Stereoscopic PIV === #sec11.6
    858853
    859854To obtain the three velocity components in a plane with stereoscopic PIV, use the following procedure:
     
    861856Install two cameras viewing a common field with angle about 45 ° on each side. A system of titled objective lenses (Sheimpflug) allows to optimize the focus in the whole image.
    862857
    863 Make a careful geometric calibration, by taking the images of a grid positioned in the plane of the laser sheet used for particle illumination. Introduce a Tsai calibration with the calibration tool associated with the  image acquisition software Acquix. Calibration can be adjusted with the UVMAT interface (see calibration).
    864 
     858Make a careful geometric calibration, by taking the images of a grid positioned in the plane of the laser sheet used for particle illumination, as described in section #GeometryCalib.
    865859This calibration model is valid in air or with an interface air-water perpendicular to the line of sight for each camera. Otherwise, the calibration problem is more complex.
    866860
    867 Perform usual PIV for each image series:
    868 
    869 The images from each camera (camA and camB) must be in the same directory and the PIV results in a single subdirectory. It is advised to do the successive operations including civ2 and fix2. This double series of PIV processing can be done in two steps (keeping the same CIV subdirectory) or simulatenously, using the compare ('-') option.  Then the check boxed test_stereo1 and test_stereo2 in the PATCH1 and PATCH2 frames must be unselected.
    870 
    871 For PIV near a staigth wall, it is advised to create a grid for each image series, corresponding to a common array of physical positions. This can be done by the pusch button grid in UVMAT (bottom right).
    872 
    873 Combine PIV fields:  this is done by selecting PATCH1 and  test_stereo1 (at level civ1), or PATCH2 and  test_stereo2 (at level civ2) in the stereo mode (check box compare selected). The resulting fields camA-camB are in physical coordinates. They are final results which cannot be processed for further Civ operations: one must go back to the two image series for that purpose. Note that this stereo combination is only  possible in the RUN mode for the moment (no BATCH)**.
     861Perform usual PIV for each image series.
     862
     863For PIV near a staigth wall, it is advised to create a grid for each image series, corresponding to a common array of physical positions. This can be done by the upper menu bar option '''!Tools/Make Grid''' in the GUI '''uvmat'''( see section [#a9.2Grids]).
     864
     865Combine PIV fields: TO UPDATE**.
    874866
    875867=== 11.7 Description of the velocity files: === #civdata