khoo
Posts:
63
Registered:
8/9/08


Re: How to calculate the angle between two images?
Posted:
Apr 4, 2009 12:40 PM


"khoo" <jim_khoo@hotmail.com> wrote in message <gr4v19$60a$1@fred.mathworks.com>... > > One method that I found for easily determining the relative > > angle of 'twist' between a known reference image and a rotated image was accomplished very reliably even for arbitrary images. > > > > 1) Compute the vertical and horizontal gradient of every > > pixel > > 2) Use the atan2 function to convert the image to an angle > > image. Note that pixels on a featureless region returns > > angles that are purely dictated by noise. > > 3) Calculate an edge image  using edge, Canny is a good > > choice, but Sobel will probably work  depending on the > > complexity of the image. > > 4) Fatten up the edge lines using morphology operators > > dilate/erode. Binarize into a mask image > > 5) Mask the angle image with above mask, thus removing > > meaningless angle pixels. We are only looking at gradients > > that are real. > > 6) Apply this to the reference image, and the target image > > 7) Histogram the results. Remember as the pixels represent > > angles, the histogram essentially wraps round 0 mapping to > > 2Pi radians. > > 8) Compare the histograms, I found correlation a good way > > to do this, the peak on the correlogram provides you with > > the relative rotation between the two images. > > > > One danger point is that if your image suffers badly from > > digital stepping (aliasing) you can get peaks at 0 and 90 > > degrees on both images. So you need to do something > > intelligent at removing this from your histograms prior to > > doing your comparison. > > > > One of the fun things that I tried, was to use fast > > correlation to do the matching. I FFT'd both histograms, > > then extended the spectra by zero packing, done the > > multiplication in the frequency domain, then followed up by > > the IFFT. This generated a very nicely interpolated measure > > of the relative rotation between the two images. > > > > Hope that this is helpful, and makes sense > > > > Regards > > > > Dave Robinson >
I FFT'd both histograms, > > then extended the spectra by zero packing, done the > > multiplication in the frequency domain, then followed up by > > the IFFT. This generated a very nicely interpolated measure > > of the relative rotation between the two images.
bro, i no understand wat this mean?? and how to extend the spctra by zero packing??can explain and teach me?

