Pivlab – towards user-friendly, affordable and accurate digital particle image velocimetry in matlab image representation in digital image processing

The most sensitive part of a DPIV analysis is the cross correlation algorithm: Small sub images (interrogation areas) of an image pair are cross correlated to derive the most probable particle displacement in the interrogation areas. Digital image processing nptel In essence, the cross-correlation is a statistical pattern matching technique that tries to find the particle pattern from interrogation area A back in interrogation area B. Digital image processing rafael c gonzalez This statistical technique is implemented with the discrete cross correlation function [ 12]:

Correlation matrices of the DCC (top) and the DFT approach (bottom), interrogation area A is 64·64 pixels for both DCC and DFT. Representation of digital image Area B is 128·128 pixels in DCC and 64·64 pixels in DFT. 3 image digital album In DCC, the background noise does not increase up to a displacement of 32 pixels. Digital image stabilization In DFT, background noise immediately increases if the displacement is larger than 0 pixels. Digital image recovery A displacement of more than 32 pixels will flip the correlation peak to the opposite side of the correlation matrix, and thus makes correct measurements impossible.


A: Calculation speed of DCC in comparison with DFT (both calculations performed in MATLAB). Applications of digital image processing in military For the FFT calculations, FFTW is used [ 17] which accepts inputs of arbitrary size, but is slow for sizes that are prime or which have large prime factors (note the peaks in the graph). Digital image recovery freeware Generally, the DFT approach is much faster. Microsoft digital image 2006 suite B: Principle of the window deformation technique. Digital image recovery indir Left: After the first interrogation pass, displacement information is present at nine positions inside the interrogation area. Definition of digital image This information is interpolated to derive the displacement of every pixel of the interrogation area. Digital image processing gonzalez 3rd edition pdf download Subsequently, interrogation area B is deformed, followed by several additional interrogation passes.

This disadvantage can be offset by running several passes of the DFT on the same dataset [ 18]: The integer result of the first analysis pass is used to offset the interrogation area in the following passes. Digital image library The loss of information due to particle displacement is hence minimized. Digital image processing youtube The interrogation grid can be refined with every pass [ 19], yielding a high spatial resolution in the final vector map, together with a high dynamic velocity range and an optimal signal to noise ratio.

In real flows, the particle patterns will additionally be sheared and rotated; the non uniform particle motion will broaden the intensity peak in the correlation matrix and deteriorate the result. Digital image processing projects Several methods that account for the transformation of the interrogation areas have been proposed [ 20 , 21 , 22]. Digital image recovery windows 8 In PIVlab, the following procedure is implemented: The analysis is started with a regular DFT analysis. Digital image processing introduction The first pass yields displacement information at the centre of each interrogation area. Digital image representation pdf When the areas overlap one another by e. Digital image processing images g. Digital image processing book 50%, there is additional displacement information at the borders and corners of each interrogation area (nine positions in total, see Figure 6B, left). Digital image processing notes vtu This information is used to calculate displacement information at every pixel of the interrogation areas via bilinear interpolation. Image of digital computer Next, interrogation area B is deformed according to this displacement information (see Figure 6B, right) using either bilinear interpolation (faster) or spline interpolation (higher precision, but slower). Digital image processing objective questions with answer pdf The next interrogation pass correlates the original interrogation area A with the deformed area B. What is a digital image The remaining displacement information of each pass is accumulated. Digital image processing third edition pdf After a few passes, the displacement has been determined with high accuracy. S jayaraman digital image processing Between the passes, but not after the final pass, the velocity information is smoothed and validated and missing information is interpolated. Digital image processing tools Data validation can be relatively strict, as any deteriorating effect of interpolation and smoothing will be corrected in the correlation of the following pass.

The choice of the peak finding technique is – similar to the choice of the cross correlation technique – another important factor for the accuracy of DPIV. Digital image pro The integer displacement of two interrogation areas can be determined straightforward from the location of the intensity peak of the correlation matrix. Digital image manipulation The location can be refined with sub-pixel precision using a range of methods [ 8 , 23 , 24]. Digital image processing multiple choice questions with answers The standard procedure is to fit a Gaussian function to the integer intensity distribution (see Figure 7). Digital image processing pratt It is sufficient to use only the directly adjacent vertical and horizontal pixels (two times a 3-point fit = 2·3-point fit) and to evaluate the x and y axis separately. Image of digital camera The peak of the fitted function is used to determine the particle displacement with sub-pixel precision.

After the removal of outliers, missing vectors should be replaced by interpolated data [ 26]. Digital image editing One common technique is the 3·3 neighbourhood (3·3 mean) interpolation. Digital image representation ppt Two-dimensional linear or spline interpolation [ 5] are other alternatives. Digital image processing gonzalez ebook PIVlab uses a boundary value solver for interpolation 1. Digital image processing ebook The approach provides an interpolation that is generally fairly smooth, and over larger regions with missing data, it will tend towards the average of the boundary velocities, which prevents overshooting.

Funtional testing has been performed on Windows XP and Windows 7 with MATLAB releases R2010a, R2011a and R2013b. Digital image suite Although the authors do not have access to additional operation systems and MATLAB versions, the reports from many users claim that PIVlab works flawlessly on Mac OS X and UNIX/Linux too. Digital image technology Bugs that have been discovered in some of the last ten PIVlab releases have been corrected before the next release. Digital image processing gonzalez pdf The user acceptance of PIVlab has been monitored and optimized while the software was used by more than 150 supervised students with several different operating systems and MATLAB releases.

Furthermore, extensive tests on the quality of the results obtained with PIVlab were performed using more than 6·10 4 synthetic particle images. Digital image processing previous year question papers The image properties were modified in a way that allowed to calculate the accuracy of all important PIV image parameters according to [ 8]. Digital image processing gonzalez and woods 3rd edition pdf Detailed results of these tests are reported in [ 29] 2. Digital image processing 3rd pdf Furthermore, the PIVlab toolbox contains a script that performs a fully automatic analysis of the accuracy of the DPIV analysis: The script ‘Accuracy.m’ will generate random PIV images, evaluate these images, and perform a comparison between the calculated and the real displacement of the interrogation areas. Define digital image This script helps users to make sure that PIVlab works accurately on their combination of MATLAB and operating system.

The quality of the DPIV measurements in PIVlab was extensively evaluated using synthetic particle images with known properties. Digital image processing by gonzalez The effect of particle image diameter, particle density, sensor noise, particle pair loss, motion blur and shear was determined and is reported in detail elsewhere [ 29]². Digital image processing gonzalez 3rd edition These quality tests revealed that DFT using window deformation outperforms the basic DCC and DFT correlation, especially under challenging conditions. Digital image processing pdf free download The additional computational load is compensated by the increased robustness and accuracy of the algorithm. Digital image york Under optimal conditions (particle image diameter ≈ 3 pixels, particle density ≈ 5 – 15 particles/window, no noise, no particle pair loss, no motion blur, no shear) the bias error of the window deformation DPIV algorithms is smaller than 0.005 pixels and the random error is below 0.02 pixels.