Refined CIV:

To improve the results, come back to the CIV menu, and follow these successive steps, corresponding to a sequence of operations.

1. Select the 'CIV1' operation, so the corresponding parameters show up. Improve the spatial resolution by selecting smaller correlation boxes in the civ1 menu, for instance 'ibx,iby' to 19 and 13 (size in image pixels). This is possible because of the good image quality and high particle density. The use of a smaller box in y allows to optimize the resolution in this direction. It is also possible to adjust the search range, using knowledge on extremal velocities, see histograms displayed by 'uvmat' (on the right side) . We introduce estimated bounds on each velocity components, 'min' to -2 and 'max' to 6 for 'u' and (-4, 4) for 'v', and press 'calcul search range' to get values for the search range 'isx, isy' and a systematic shift. Then we can refined the grid mesh for the velocity vectors, taking 'dx'=5, 'dy'=5.
2. Select the 'FIX1' operation, which will eliminate some false vectors. Introduce the mask by pressing 'GET_MASK' to suppress vectors outside the fluid region. It detects the mask image file with appropriate name 'UV_DEMO/EX_1/frame1_mask_1.png'. If this file is not present, an interface for making the mask appears. Keep the default values for the other parameters.
3. Select the 'PATCH1' operation, to interpolate the vectors on a regular grid. Select a high resolution 'nx'=200, 'ny'=60, representing the number of grid points in which the velocity is interpolated. These values correspond to dx=dy=5 pixels for an image 1000x300. Choose the default value 10 for the smoothing parameter 'rho' . You can later try different values, the smoothing effect increasing with rho. Keep the default values for the other parameters.
4. Select the 'CIV2' operation to improve the correlation results, using the information on local image deformation, provided by the previous knowledge on velocity spatial derivatives (calculated in 'patch1'). Use the same grid dx= dy=5 than for civ1. The spatial resolution can be slightly improved by decreasing the correlation box, using for instance ibx=iby=15. The search range is determined automatically using the prior knowledge on velocity, obtained with the civ1 and patch1 operations. Note that 'civ2' corresponds to a new measurement from the images, the previous civ1 and patch operations being used only as an initial guess for the search of optimal correlations.
5. Select 'FIX2' and 'PATCH2' with the same parameters as 'FIX1' and 'PATCH1'.
6. Press 'RUN' to run the calculation. The results are stored in a new subdirectory, A.0, so the previous results are not erased (you can also set the name of the subdirectory in the 'civ' interface, by the edit box 'SUBDIR_CIV1'.

The corresponding interface state is shown in Fig. 2. You can directly get this interface already filled by opening the file 'UV_DEMO/EX_1/frame_civ2.fig', for instance with the browser of 'uvmat'. Then just modify the input name and press 'RUN'. The resulting velocity fields are already stored in 'UV_DEMO/EX_1/VEL.0/frame_1-2.nc', so you can compare with your results. You can process in several steps, running for instance 'CIV1' and 'FIX1' alone, then coming back to the civ interface, selecting only 'PATCH1', 'CIV2'.

The result can be improved again by performing a third civ iteration, civ3. For that purpose, select only the 'civ2', 'fix2' and 'patch2' operations with the same parameters as previously, see 'UV_DEMO/EX_1/frame_civ3.fig'. The previous result is now considered as 'civ1', so set A.0 as the subdirectory in the edit window 'SUBDIR_CIV1'. Select a new subdirectory name, for instance 'A.civ3' in the edit window 'SUBDIR_CIV2', the new result will be obtained there. Further iterations can be similarly performed, but the improvement becomes negligible.

Figure 2: The 'civ' interface