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Subject: Re: data compression
Date: Sat, 17 May 2008 14:56:02 +0000 (UTC)
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"Ahmed " <mogwari2000@yahoo.com> wrote in message
<g0kt96$hog$1@fred.mathworks.com>...
> 1. i compute radon transform of the signature image for 
> angles 0 (horizontal projection) and 90 (vertical 
> projection).
> it is deffinitly that the signature images can't be 
> reconstructed from these two projections (it seems you 
> like to tell me that these two projections not enough to 
> classify signatures).
> 
> 2. projections here means horizontal and vertical 
> projections.

I am surprised that you go to the extent of doing a Radon
Transform, I would have thought a simple use of the Matlab
function 'sum(A,Dim)' (see doc sum) would do what you
require - I don't know, it may just be possible that the
Radon transform is smart enough to check for this ,so you
may not get any speed advantage, but my guess is that you
will find it a lot faster.

As you cannot reconstruct your signatures from the
projections, it is clear you don't really need an absolute
compression. I would also think that as you are integrating
your signal across the rows/columns they would tend to be
relatively slow functions, e.g. it would be rare to get a
full scale transition occurring in 1 pixel (but I may be
wrong here ;-) So I would think that some scale space
compression techniques based upon Wavelets would perform
very effectively (after all it is the basis of JPEG 2000
compression). So to repeat my advice on a previous response,
either have a look at the demo provided in the Wavelet
toolbox, else have a Google for wavelet compression.

When comparing the Signature 'signature's' check out dynamic
programming techniques, which will allow you to compensate
for variations in length and evenness of the original writing.

Hope that helps

Dave Robinson