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1500 questions
49
votes
3 answers
What algorithm does Google use for its "Search By Image" site?
What is your best guess how Google Image Search works? I can upload a photo and can search for similar images. What algorithm does it use to identify similar images?
Cory
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48
votes
5 answers
Point Pattern Recognition
Having two different size of sets of points (2D for simplicity) dispersed within two different size squares the question are that:
1- how to find any occurrence of the the small one through the large one?
2- Any idea on how to rank the occurrences…
Developer
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48
votes
6 answers
Simplest way of detecting where audio envelopes start and stop
Below is a signal which represents a recording of someone talking. I would like to create a series of smaller audio signals based on this. The idea being to detect when 'important' sound starts and ends and use those for markers to make new snippet…
Eric Brotto
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45
votes
1 answer
How to flatten the image of a label on a food jar?
I'd like to take pictures of labels on a jar of food, and be able to transform them so the label is flat, with the right and left side resized to be even with the center of the image.
Ideally, I'd like to use the contrast between the label and the…
mahboudz
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43
votes
6 answers
What properties make certain wavelets "better" than others in image compression?
I am trying to teach myself more about image compression using the wavelet transform method. What is it about certain wavelets that make them preferable when compressing images? Are they easier to compute? Do they produce smoother images?
Example:…
user807566
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43
votes
5 answers
Difference between Discrete Time Fourier Transform and Discrete Fourier Transform
I have read many articles about DTFT and DFT but am not able to discern the difference between the two except for a few visible things like DTFT goes till infinity while DFT is only till $N-1$. Can anyone please explain the difference and when to…
BaluRaman
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41
votes
5 answers
Determining the mean and standard deviation in real time
What would be the ideal way to find the mean and standard deviation of a signal for a real time application. I'd like to be able to trigger a controller when a signal was more than 3 standard deviation off of the mean for a certain amount of time. …
jonsca
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40
votes
2 answers
What's the difference between the Hough and Radon transforms?
I'm familiar with the Radon transform from learning about CT scans, but not the Hough transform. Wikipedia says
The (r,θ) plane is sometimes referred to as Hough space for the set of straight lines in two dimensions. This representation makes the…
endolith
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39
votes
7 answers
What is the most lucid, intuitive explanation for the various FTs - CFT, DFT, DTFT and the Fourier Series?
Even after having studied these for quite sometime, I tend to forget (if I'm out of touch for a while) how they are related to each other and what each stands for (since they have such similar sounding names). I'm hoping you'd come up with an…
Vighnesh
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39
votes
1 answer
Does the autocorrelation function completely describe a stochastic process?
Is a stochastic process completely described by its autocorrelation function?
If not, which additional properties would be needed?
Andreas
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39
votes
4 answers
Hilbert transform to compute signal envelope?
I've heard that the Hilbert transform can be used to calculate the envelope of a signal. How does this work? And how is this "Hilbert envelope" different from the envelope one gets by simply rectifying a signal?
I'm interested specifically in…
dB'
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39
votes
3 answers
What are the most common algorithms for adaptive thresholding?
Adaptive thresholding has been discussed in a few questions earlier:
Adaptive Thresholding for liver segmentation using Matlab
What are the best algorithms for document image thresholding in this example?
Of course, there are many algorithms for…
Dipan Mehta
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38
votes
3 answers
What is the advantage of MATLAB's filtfilt
MATLAB's filtfilt does a forward-backward filtering, i.e., filter, reverse the signal, filter again and then reverse again. Apparently this done to reduce phase lags? What are the advantages/disadvantages of using such a filtering (I guess it would…
user4744
38
votes
7 answers
Understanding the Matched Filter
I have a question about matched filtering. Does the matched filter maximise the SNR at the moment of decision only? As far as I understand, if you put, say, NRZ through a matched filter, the SNR will be maximised at the decision point only and…
user968243
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38
votes
4 answers
What are good ways to detect signal clipping in a recording?
Given a recording I need to detect whether any clipping has occurred.
Can I safely conclude there was clipping if any (one) sample reaches the maximum sample value, or should I look for a series of subsequent samples at the maximum level?
The…
Han
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