Changes between Version 205 and Version 206 of UvmatHelp


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Timestamp:
May 14, 2018, 7:15:33 PM (3 years ago)
Author:
sommeria
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  • UvmatHelp

    v205 v206  
    112112
    113113The PIV software provided in UVMAT can deal with any image or movie format recognised by Matlab, while the older fortran version CIVx requires B&W images in the format png (portable network graphics). It is a binary format for images with lossless (reversible) compression, recommended by w3c (http://www.w3.org/Graphics/PNG). It is an open source patent-free replacement of GIF and can also replace many common uses of TIFF. It can be read directly by all standard programs of image visualisation and processing.  Compressing a raw binary image to its png form typically saves disk storage by a factor of 3.
     114
     115In addition to standard formats, uvmat can also read the proprietary formats from the companies Lavision (.im7), cameras Phantom (.cine), and the company Rdvision.
    114116
    115117For 3D PIV**, 'volume' images, with file extension .vol are used. These are images in the  png format, where the npz slices are concatenated along the y direction, forming a composite image of dimension (npy x npz, npx) from the images (npy x npx).
     
    787789 * 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 [#sec11.7 section 11.7].
    788790
    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'''.
     791Usual 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'''. Any standard image format can be used, as well as the proprietary format from the companies Lavision (.im7), cameras Phantom (.cine), and the company Rdvision.
    790792
    791793An 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)]'''.
     794
     795'''- Timing and calibration''': PIV requires information of time intervals between images. uvmat can read the time in the input file for usual movie formats, but for other image types, the timing needs to be described in an ancillary file with the format xml, see [#ImaDoc section 3.5]. Examples of such XML files are provided in /XML_SCHEMAS/ImaDoc_templates. In the absence of  information, the time is taken by default as the index i in seconds and index j in ms. Actual velocity measurements also require geometric calibration, see [#GeometryCalib section 8]. This is also documented in the same xml file !ImaDoc, but it can be performed after the PIV process as the PIV results are stored in image (pixel) coordinates.
     796
     797'''- Image pre-processing''': it may be useful to prepare the images series by a pre-processing operation before performing PIV. Removing a fixed background is often needed. This can been done by the function sub_backgroud.m, see [#a10.8OtherfunctionsAction section 10.8]. This function can also rescale the image intensity to reduce high luminosity spots.
    792798
    793799'''- Modes of frame pair indexing''': PIV is performed by comparing two images in a pair. The images are generally labelled by two indices i and j, see [#a3.3Filenamingandindexing: section 3.3]. A first menu '''[!ListCompareMode]''' on the top left selects one among four modes of operation:
     
    816822This series of operations is chosen by selecting the corresponding check boxes on the left of the GUI '''civ_series''', which give access to the corresponding parameter input panels. Note that the result of each of these operations is stored in the output NetCDF file, so the process can be split in several runs. When an existing NetCDF velocity file has been initially opened, the GUI '''civ.fig''' is automaticaly configured to perform the next operation (fix1, patch1, civ2...) needed in the process.
    817823
     824At the end of the process, it is advised to interpolate the velocity data on a regular grid in physical coordinates, using the function ''merge_proj.m'', see [#a10.7Actionfunctionsofgeneraluse section 10.7]. This function can be used to merge together the view fields from several cameras on a common grid in physical coordinates.
     825
    818826=== 11.2 CIV1: ===
    819827The time interval of the image pair (selected by '''[!ListPairCiv1]''', see above) must be chosen sufficiently small to provide a good image correlation, and sufficiently large to provide good measurement precision: a displacement of 5-10 pixels is generally optimum.
    820 
    821   Examples of XML files are provides in /XML_SCHEMAS/ImaDoc_templates.
    822828
    823829('''[num_!CorrBoxSize_1,_2,_3])''' set the size (in pixels) of the 'correlation box', the sliding window used to get image correlations. '''[num_!SearchBoxSize_1,_2,_3]''' set the size of the 'search box' in which image correlation is calculated, see figure.  This search box can be shifted with respect to the correlation box by parameters  ('''[num_!SearchBoxShift_1,_2,_3]'''). This is useful in the presence of a known mean flow. The default value '''[!SearchBoxSize]'''=(25,25) is generally good, use a larger size for images with few particles, use an elongated box , e.g. (11,41), to optimise the resolution in one direction (for instance in a boundary layer).