Changes between Version 203 and Version 204 of UvmatHelp
 Timestamp:
 May 3, 2018, 1:42:54 PM (4 years ago)
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UvmatHelp
v203 v204 779 779 == 11  PIV: Particle Imaging Velocimetry == #Civ 780 780 === 11.1 Overview === 781 PIV can be performed by selecting the Matlab function ''civ_series'' as '''[!ActionName]''' in the GUI '''series.fig''': then the '''GUI civ_input''' appears to enter the processing parameters. An image file series must be entered as input of '''series.fig'''. 781 Particle Image Velocimetry (PIV) measures the displacement of features in a pair of images by maximising the correlation. 782 783 * Usual PIV provides the 2D velocity components in a illuminated plane. 784 * The 3 velocity components in a plane can be obtained by combining two views of this plane by two differents cameras: this is the stereoscopic 3C PIV described in [wiki:[#sec11.6 section 11.6]. 785 * PIV can be performed in a volume by scanning the illuminated plane, see below. 786 * The same image correlation technique can be used to measure 2D displacement from a single reference image (see mode 'displacement' below). 787 * Image correlation can be used also to get the shape of a 3D surface by comparing the images of two stereoscopic views of the surface marked by some visual patterns. Then image correlation is used to identify the common points on the two images, see [wiki:[#sec11.7] section 11.7]. 788 789 Usual PIV and 2D displacement measurements can be performed by selecting the Matlab function ''civ_series'' as '''[!ActionName]''' in the GUI '''series.fig''': then the '''GUI civ_input''' appears to enter the processing parameters. An image file series must be entered as input of '''series.fig'''. 782 790 783 791 An alternative obsolete possibility (not advised) is to use the older Fortran program ''CivX'' from the GUI '''civ.fig'''. This can be called directly on the Matlab prompt, by typing ''>>civ'', or from UVMAT by the menu bar command '''[RUN/PIV (old civ)]'''. … … 878 886 * To get the 3 velocity components in phys coordinates, combine 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). 879 887 880 === 11.7 Topography measurement by image correlation from stereoscopic views === 888 === 11.7 Topography measurement by image correlation from stereoscopic views === #sec11.7 881 889 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. 882 890