As I mentioned in title default vlfeat sift returns less features than sift of D.Lowe. How could I let vlfeat sift return as much as features as D.Lowe's does. Or can we only offer position info of key-points to extract sift features with vlfeat sift? Thank you.
under deault settings, vlfeat sift return less features implementation of D.Lowe. The document of vlfeat point out:
first octave index: set to -1 to extract very small features
number of scale: can affect the number of extracted keypoints
edge threshold: decrease to eliminate more keypoints
peak threshold: increase to eliminate more keypoints
So by changing the first octave index and number of scale can increase the number of returned feature.
matlab code example:
[f, d] = vl_sift(img, 'levels', 4, 'firstoctave', -1);
Related
in a text I have found the following:
"The LASSO regerssion method offers a sparse solution and as such the interpretability of the model can be improved".
Can someone help me to understand what is meant by this? As far as I know, a sparse decomposition of a solution to a system of equation is that vector of dimension l with minimum pseudo-l norm such that the system is still satisfied. How would a sparse solution, which is setting some regression coefficients to zero, be of help in the interpretation?
Sparse matrix/array or whatever is by definition when your matrix contains mostly zeros and few non-zero entries. In the other hand, a dense matrix/array is when you have few zeros.
When you apply LASSO regression, the sparsity of your learned coefficients depends on the amount of the penalty (lambda). The higher the penalty, the more sparse coefficients you get. That is, the non-zero coefficients (selected variables). For example, if you have 100 independent variables in your regression, the LASSO may return only 10 non-zero variables. That means 10 non-zero variables and 90 zero variables. This is exactly what is the meaning of sparsity.
Having few selected variables (non-zero) means interpretable model as you can explain it with few variables (in the above example 10 variables) instead of using the 100 variables.
Lasso Regression's penalty method is different than Ridge, which is using L1 regularization. And there is "alpha" parameter which you can set it on scikit-learn. For high values of "alpha", many coefficients are exactly zeroed.
This method is also using absolute sum of the coeeficients ( |w| ). For instance, if there are high correlated features on your dataset, Lasso is doing one of the correlated predictor to largest coefficient, while the rest of them are set to 0
If there are two or more highly collinear variables then LASSO regression select one of them randomly (which is not good for the interpretation of data)
You can find more details here => https://www.geeksforgeeks.org/lasso-vs-ridge-vs-elastic-net-ml/
Let's say I want to determine the probability that I will upvote a question on SO, based only on which tags are present or absent.
Let's also imagine that I have plenty of data about past questions that I did or did not upvote.
Is there a machine learning algorithm that could take this historical data, train on it, and then be able to predict my upvote probability for future questions? Note that it must be the probability, not just some arbitrary score.
Let's assume that there will be up-to 7 tags associated with any given question, these being drawn from a superset of tens of thousands.
My hope is that it is able to make quite sophisticated connections between tags, rather than each tag simply contributing to the end result in a "linear" way (much as words do in a Bayesian spam filter).
So for example, it might be that the word "java" increases my upvote probability, except when it is present with "database", however "database" might increase my upvote probability when present with "ruby".
Oh, and it should be computationally reasonable (training within an hour or two on millions of questions).
What approaches should I research here?
Given that there probably aren't many tags per message, you could just create "n-gram" tags and apply naive Bayes. Regression trees would also produce an empirical probability at the leaf nodes, using +1 for upvote and 0 for no upvote. See http://www.stat.cmu.edu/~cshalizi/350-2006/lecture-10.pdf for some readable lecture notes and http://sites.google.com/site/rtranking/ for an open source implementation.
You can try several methods (linear regression, SMV, neural networks). The input vector should consist of all possible tags, where each tag represents one dimension.
Then each record in a training set has to be transformed to the input vector according to the tags. For example let's say you have different combinations of 4 tags in your training set (php, ruby, ms, sql) and you define an unweighted input vector [php, ruby, ms, sql]. Let's say you have the following 3 records whic are transformed to weighted input vectors:
php, sql -> [1, 0, 0, 1]
ruby -> [0, 1, 0, 0]
ms, sql -> [0, 0, 1, 1]
In case you use linear regression you use the following formula
y = k * X
where y represents an answer (upvote/downvote) in your case and by inserting known values (X - weighted input vectors).
How ta calculate weights in case you use linear regression you can read here but the point is to create binary input vectors which size is equal (or larger in case you take into account some other variables) to the number of all tags and then for each record you set weights for each tag (0 if it is not included or 1 otherwise).
I have two images of real world. (IMPORTANT)I approximately know transformation of one real world to another. Due to texture problem I don't get enough matches between two images. How can I bring transformation information into account to get more and correct matches by using SIFt.
Any idea will be helpful.
Have you tried other alternatives? Are you sure SIFT is the answer? First, OpenCV provides SIFT, among other tools. (At the moment, I can't speak highly enough of OpenCV).
If I were solving this problem, I would first try:
Downsample your two images to reduce the influence of "texture", i.e. cvPyrDown.
Perform some feature detection: edge detection, etc. OpenCV provides a Harris corner detector, among others. Google "cvGoodFeaturesToTrack" for some detail.
If you have good confidence in your transformations, take advantage of your a priori information and look for features in neighborhoods corresponding to the transformed locations.
If you still want to look at SIFT or SURF, OpenCV provides those capabilities, as well.
If you know the transform, then apply the transform and then apply SURF/SIFT to the transformed image. That's one standard way to extend the robustness of feature descriptors/matchers across large perspective changes.
There is another alternative:
In sift parameters, Contrast Threshold is set to 0.04. If you reduce it and set it to a lower value ( 0.02,0.01) SIFT would find more enough matches:
SIFT(int nfeatures=0, int nOctaveLayers=3, double contrastThreshold=0.04, double edgeThreshold=10, double sigma=1.6)
The first step I think is to try with the settings of the SIFT algorithm to find the best efficiency with respect to your problem.
One another way to use SIFT more effectively is adding the COLOR information to SIFT. So you can add the color information (RGB) of the points which are being used in the descriptor to it. For instance if your descriptor size is 10x128 then it shows that you are using 10 points in each descriptor. Now you can extract and add three column and make the size 10x(128+3) [R-G-B for each point]. In this way the SIFT algorithm will work more efficient. But remember, you need to apply weight to your descriptor and make the last three columns be stronger than the other 128 columns. Actually I do not know in your case how the images are. but this method helped me a lot. and you can see that this modification makes SIFT a stronger method than before.
A similar implementation can be find here.
Given two recorded voices in digital format, is there an algorithm to compare the two and return a coefficient of similarity?
I recommend to take a look into the HTK toolkit for speech recognition http://htk.eng.cam.ac.uk/, especially the part on feature extraction.
Features that I would assume to be good indicators:
Mel-Cepstrum coefficients (general timbre)
LPC (for the harmonics)
Given your clarification I think what you are looking for falls under speech recognition algorithms.
Even though you are only looking for the measure of similarity and not trying to turn speech into text, still the concepts are the same and I would not be surprised if a large part of the algorithms would be quite useful.
However, you will have to define this coefficient of similarity more formally and precisely to get anywhere.
EDIT:
I believe speech recognition algorithms would be useful because they do abstraction of the sound and comparison to some known forms. Conceptually this might not be that different from taking two recordings, abstracting them and comparing them.
From wikipedia article on HMM
"In speech recognition, the hidden
Markov model would output a sequence
of n-dimensional real-valued vectors
(with n being a small integer, such as
10), outputting one of these every 10
milliseconds. The vectors would
consist of cepstral coefficients,
which are obtained by taking a Fourier
transform of a short time window of
speech and decorrelating the spectrum
using a cosine transform, then taking
the first (most significant)
coefficients."
So if you run such an algorithm on both recordings you would end up with coefficients that represent the recordings and it might be far easier to measure and establish similarities between the two.
But again now you come to the question of defining the 'similarity coefficient' and introducing dogs and horses did not really help.
(Well it does a bit, but in terms of evaluating algorithms and choosing one over another, you will have to do better).
There are many different algorithms - the general name for this task is Speaker Identification - start with this Wikipedia page and work from there: http://en.wikipedia.org/wiki/Speaker_recognition
I'm not sure this will work for soundfiles, but it gives you an idea how to proceed i hope. That is a basic way how to find a pattern (image) in another image.
You first have to calculate the fft of both the soundfiles and then do a correlation. In formular it would look like (pseudocode):
fftSoundFile1 = fft(soundFile1);
fftConjSoundFile2 = conj(fft(soundFile2));
result_corr = real(ifft(soundFile1.*soundFile2));
Where fft= fast Fourier transform, ifft = inverse, conj = conjugate complex.
The fft is performed on the sample values of the soundfiles.
The peaks in the result_corr vector will then give you the positions of high correlation.
Note that both soundfiles must in this case be of the same size-otherwise you have to place the shorter one into a file of max(soundFileLength) vector.
Regards
Edit: .* means (in matlab style) a component wise mult, you must not do a vector mult!
Next Edit: Note that you have to operate with complex numbers - but there are several Complex classes out there so I think you don't have to bother about this.
I have a number of tracks recorded by a GPS, which more formally can be described as a number of line strings.
Now, some of the recorded tracks might be recordings of the same route, but because of inaccurasies in the GPS system, the fact that the recordings were made on separate occasions and that they might have been recorded travelling at different speeds, they won't match up perfectly, but still look close enough when viewed on a map by a human to determine that it's actually the same route that has been recorded.
I want to find an algorithm that calculates the similarity between two line strings. I have come up with some home grown methods to do this, but would like to know if this is a problem that's already has good algorithms to solve it.
How would you calculate the similarity, given that similar means represents the same path on a map?
Edit: For those unsure of what I'm talking about, please look at this link for a definition of what a line string is: http://msdn.microsoft.com/en-us/library/bb895372.aspx - I'm not asking about character strings.
Compute the Fréchet distance on each pair of tracks. The distance can be used to gauge the similarity of your tracks.
Math alert: Fréchet was a pioneer in the field of metric space which is relevant to your problem.
I would add a buffer around the first line based on the estimated probable error, and then determine if the second line fits entirely within the buffer.
To determine "same route," create the minimal set of normalized path vectors, calculate the total power differences and compare the total to a quality measure.
Normalize the GPS waypoints on total path length,
walk the vectors of the paths together, creating a new set of path vectors for each path based upon the shortest vector at each waypoint,
calculate the total power differences between endpoints of each vector in the normalized paths weighting for vector length, and
compare against a quality measure.
Tune the power of the differences (start with, say, squared differences) and the quality measure (say as a percent of the total power differences) visually. This algorithm produces a continuous quality measure of the path match as well as a binary result (Are the paths the same?)
Paul Tomblin said: I would add a buffer
around the first line based on the
estimated probable error, and then
determine if the second line fits
entirely within the buffer.
You could modify the algorithm as the normalized vector endpoints are compared. You could determine if any endpoint difference was above a certain size (implementing Paul's buffer idea) or perhaps, if the endpoints were outside the "buffer," use that fact to ignore that endpoint difference, allowing a comparison ignoring side trips.
You could walk along each point (Pa) of LineString A and measure the distance from Pa to the nearest line-segment of LineString B, averaging each of these distances.
This is not a quick or perfect method, but should be able to give use a useful number and is pretty quick to implement.
Do the line strings start and finish at similar points, or are they of very different extents?
If you consider a single line string to be a sequence of [x,y] points (or [x,y,z] points), then you could compute the similarity between each pair of line strings using the Needleman-Wunsch algorithm. As described in the referenced Wikipedia article, the Needleman-Wunsch algorithm requires a "similarity matrix" which defines the distance between a pair of points. However, it would be easy to use a function instead of a matrix. In your case you could simply use the 2D Euclidean distance function (or a 3D Euclidean function if your points have elevation) to provide the distance between each pair of points.
I actually side with the person (Aaron F) who said that you might be interested in the Levenshtein distance problem (and cited this). His answer seems to me to be the best so far.
More specifically, Levenshtein distance (also called edit distance), does not measure strictly the character-by-character distance, but also allows you to perform insertions and deletions. The best algorithm for this distance measure can be computed in quadratic time (pretty slow if your strings are long), but the computational biologists have pretty good heuristics for this, that might be of interest to you on their own. Check out BLAST and FASTA.
In your problem, it seems that you are dealing with differences between strings of numbers, and you care about the numbers. If you give more information, I might be able to direct you to the right variant of BLAST/FASTA/etc for your purposes. In any case, you might consider adapting BLAST and FASTA for your needs. They're quite simple.
1: http://en.wikipedia.org/wiki/Levenshtein_distance, http://www.nist.gov/dads/HTML/Levenshtein.html