khoo
Posts:
63
Registered:
8/9/08
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Re: How to calculate the angle between two images?
Posted:
Apr 4, 2009 12:40 PM
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"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?
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