Changes between Version 193 and Version 194 of UvmatHelp
- Timestamp:
- Feb 20, 2015, 10:42:39 PM (9 years ago)
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UvmatHelp
v193 v194 284 284 If the two files are both images or scalar, their difference is introduced as the input field. If one field is an image (or scalar), while the other one is a vector field, the image will appear as a background in the vector field. This is convenient for instance to relate the CIV result to the quality of the images, or to relate vorticity to the vector field. 285 285 286 If two vector fields are compared, their difference is taken as the input field, and is then displayed and analysed. If the two fields are not at the same points, the velocity of the second field is linearly interpolated at the positions of the first one (using the Matlab function ''griddata.m''). The color and flags are then taken from the first field.286 If two vector fields defined at the same points are compared, their difference is taken as the input field, and is then displayed and analysed. If the vectors are not at the same points, the program proposes to create an interpolation grid for vector subtraction (a projection plane as described in [#ProjMode: section 6]). An alternative possibility is to introduce the [#FieldTransform: transform function] ''diff_vel.m'' which linearly interpolates the vectors of the second field to the positions of the first one before subtraction. The color and flags are then taken from the first field. 287 287 288 288 The two file series will be scanned simultaneously by '''[runplus]''' ( '->') and '''[runmin]''' ('<-') , according to their own nomenclature. It is also possible to manually edit the second file indices '''[!FieldIndex_1] ''' to compare two fields with different indices. If available, the time of the second field is indicated in the edit box '''[abs_time_1] ''' at the very right, below the time of the main field. … … 819 819 [[Image(11-2 CV1.png)]] 820 820 821 The button '''[TEST]''' allows the user to witness the correlation as a live plot. It first opens the source image in a new figure '''view_field'''. By moving the mouse in the figure, the local correlation box and the corresponding search box are drawn in the image, and the 2D correlation result then appears in a new figure 'Figure1 Image Correlation' which automatically pops up. It is possible to freeze the current correlation plot by left mouse selection. The figure belows shows the correlation process and the '''[!SearchBox]''' and '''[!CorrBox]''' explained before.821 The button '''[TEST]''' allows the user to witness the correlation as a live plot. It first opens the source image in a new figure '''view_field'''. By moving the mouse in the figure, the local correlation box and the corresponding search box are drawn in the image, and the 2D correlation result then appears in a new figure 'Figure1 Image Correlation' which automatically pops up. It is possible to freeze the current correlation plot, and get the values in the Matlab work space, by left mouse selection. The figure belows shows the correlation process and the '''[!SearchBox]''' and '''[!CorrBox]''' explained before. 822 822 823 823 [[Image(civ1_test.jpg)]] [[Image(Correlation for PIV.png)]] … … 837 837 * vec_F=3: the optimisation of the correlation function is unstable or local Intensity rms of the image =0. Must be selected. 838 838 839 839 '''- 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. 840 840 841 841 '''- 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. … … 859 859 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. 860 860 861 === 11.6 Stereoscopic PIV === #sec11.6861 === 11.6 Stereoscopic 3C PIV === #sec11.6 862 862 To obtain the three velocity components in a plane with stereoscopic PIV, use the following procedure: 863 863 864 Install 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. 865 866 Make 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 section 6]. This 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. 867 868 Perform usual PIV for each image series. 869 870 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 upper menu bar option '''[!Tools/Make Grid]''' in the GUI '''uvmat''' (see section [#a9.2Grids Grids]). 871 872 Combine PIV fields: TO UPDATE**. 873 874 === 11.7 Description of the velocity files: === #civdata 864 * Install 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. 865 866 * Make 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 section 6]. This 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. 867 868 * Perform usual PIV for each image series. For PIV near a straight wall, it may be useful 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 Grids]). 869 870 * To get the 3 velocity components in phys coordinates, ombine the two PIV data series with the function 'civ2vel_3C' activated by the GUI ''series''. The data are assumed by default in a plane ''x,y'' (defined by a laser sheet). A transverse ''z'' displacement can be taken into account, introduced then in the third input line of the GUI ''series''. This ''z'' displacement can be obtained by stereo comparison of the two images (at the same time). 871 872 === 11.7 Topography measurement by image correlation from stereoscopic views === 873 A displacement ''z(x,y)'' can be obtained by comparing two stereoscopic views (at the same time), using the function ''stereo_civ'' activated by the GUI '''series'''. This identifies corresponding points using image correlation, in the same way as ''civ''. A transform of the image to physical coordinates is performed as a first step to minimize the image difference between the two views. The final result is expressed as a set of ''(x,y,z)'' positions in physical coordinates (unlike PIV data which are expressed in image coordinates). See [attachment:3D_view.pdf] for full algorithm description. 874 875 876 === 11.8 Description of the velocity files: === #civdata 875 877 The velocity fields obtained by PIV, as well as their spatial derivatives, are stored in the machine independent binary format [#netcdf NetCDF]. The file contains constants ('global attributes') and fields ('variables') whose values can be directly accessed by their name. 876 878 … … 1101 1103 * 'avi2png': copy an avi movie to a series of B/W .png images (take the average of green and blue color components) 1102 1104 * 'check_data_files': check the existence, type and status of the files selected by series.fig. 1105 * 'civ2vel_3C': combine velocity fields from two cameras to get three velocity components 1103 1106 * 'ima_levels': rescale the image intensity to reduce strong luminosity peaks (their blinking 1104 1107 * 'civ_input': function associated with the GUI 'civ_input.fig' to set the input parameters for civ_series. 1105 1108 * 'civ_series': PIV function activated by the general GUI series (replaces civ). 1106 1109 * 'merge_proj': concatene several fields from series, can project them on a regular grid in phys coordinates. 1110 * 'stereo_civ': determination of topography by image correlation of two stereo views 1111 * 'stereo_input': function associated with the GUI 'stereo_input.fig' to set the input parameters for stereo_civ 1107 1112 * 'sub_background': subtract a sliding background to an image series. 1108 1113 * 'time_series': extract a time series after projection on an object (points , line..). 1109 1114 1110 1115 === Transform functions === 1116 * 'diff_vel': calculate the difference of two input velocity fields. 1117 * 'ima_color2BW': transform a color image to grey scale image 1111 1118 * 'ima_filter': low-pass filter of an image (builtin filtering parameter). 1119 * 'ima_ratio': take the ratio of two input images with same size 1112 1120 * 'ima_remove_background': removes backgound from an image (using the local minimum). 1113 1121 * 'ima_remove_particles': removes particles from an image (keeping the local minimum). … … 1115 1123 * 'phys': transforms image (Unit='pixel') to real world (phys) coordinates using geometric calibration parameters. It acts if the input field contains the tag '!CoordUnit' with value 'pixel'. 1116 1124 * 'phys_polar': this transforms the fields to polar coordinates, radius in abscissa (same unit as x, y) and azimuth in ordinate (unit =degree). 1125 * 'signal_spectrum': calculate and display power spectrum of the current field 1126 * 'sub_field': combines two input fields, taking the difference if of the same nature.
