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13:20
Anyone taking Algorithms I starting on August 12-th?
Yeah, but I'm leaving on a vacation 2 weeks later
I'm also still in Udacity's Algo-class, but I'm not particularly motivated
@IvoFlipse Nice. I'll be on vacation the first week of that class. Hopefully, I'll be able to meet the deadlines on the first assignment.
@Phonon you familiar with Gaussian mixture models?
@IvoFlipse A little bit. I know that Expectation Maximization is usually used to solve them.
Don't remember why
They use them in computer vision, but I can't really visualize what effect it would have
and/or what input I would have to put in it, the entire plate, a slice/region of interest
13:49
Thank you Youtube:
@IvoFlipse Haha nice
@IvoFlipse Expectation Maximization for image proc is used as a way to segment an image
@Mohammad But how? :P
@IvoFlipse The data will determine how to segment, using EM. EM at the end, will label each pixel with a probability of belonging to a gaussian for foreground, or gaussian for background. The label is a probability. "This pixel is 90% belonging to foregorund". "This pixel is 87% belonging to bacjground", etc
@IvoFlipse Basically the pixels are assume to have come from 2 gaussians. A foreground one, and a background one.
@Mohammad So it only helps separate foreground/background not really between objects (which have their own gaussian distribution)
Because my data has no background
13:58
@IvoFlipse So now task is, I give you a point, so which gaussian did it probably come from? That is what EM solves. It says "Oh ok, based on this point you gave me, and 2 fuzzy clusters of points I see, it probably came from the top one"
@IvoFlipse Thats basically what it is doing
@IvoFlipse Foreground/background can mean anything. I your case, can mean objects, sure
That would be very useful to distinguish between two paws if they are really close
To decide in which cluster/paw a pixel should go
@IvoFlipse Yeah, it can be used for that AFAIK
Obviously the question is: how :P
But don't bother explaining trying to figure that out, because I have plenty of other things to try before getting to that problem
@IvoFlipse How it runs exactly, I am not sure, but I know what it does. :P (How it runs is on my to do list hehe)
14:27
@IvoFlipse That link on GMM has EM after it no? Ill probably watch it - did you see it?
I'm watching it right now, or actually the third one (there's one that explains GMM before it)
Only downside is that I don't know what this really means when applied to images
14:54
@IvoFlipse Ill watch and get back to you
No rush, I first need to make a tool to create a ground truth for my tracking
 
1 hour later…
16:18
@IvoFlipse How was the algorithms final exam last time?
 
1 hour later…
17:24
@Phonon Lots of formula's
Recall the Partition subroutine that we used in both QuickSort and RSelect. Suppose that the following array has just been partitioned around some pivot element: 3, 1, 2, 4, 5, 8, 7, 6, 9.
Which of these elements could have been the pivot element? (Hint: There could be more than one possibility!)
What is the asymptotic worst-case running time of MergeSort, as a function of the input array length n?
θ(nlogn)
θ(n^2)
θ(n)
θ(logn)
When does a directed graph have a unique topological ordering?
Whenever it is directed acyclic
None of the other options
Whenever it is a complete directed graph
Whenever is has a unique cycle
Stuff like that, I scored 25/30 I believe
@IvoFlipse Nice, thanks.
You should be fine, especially given your math skills
17:49
@IvoFlipse lol, ok. Sounds good.
I won't spoil the fun by telling you all questions :P
@IvoFlipse No, no, I would have to drop the course = )
 
3 hours later…
20:27
so anyone know any good encryption algorithms?
@jellyksong RSA? AES? WPA? What's your application?
for encrypting some text with javascript
just for a project
@jellyksong I know that @Kortuk was doing a class on cryptography.
@jellyksong Yeah, let's see what he says.
hm okay
@Phonon ha!
20:30
@Kortuk Sorry for pointing fingers = )
@Kortuk I just thought you could help an occasional visitor with a school project
@Phonon :)
@jellyksong You want to use an algorithm someone else has made. Ideally it uses AES for the file and a password is hashed for the key.
so this would be good?
@Kortuk also, the length of the key determines how "secure" the encryption is right?
 
1 hour later…
21:37
@jellyksong Sorta, yes
sorry, got distracted by work messages
@jellyksong Yes, it does, there are a lot of other factors, but in general if you have at least a 256 bit AES you are 100% secure.
@kort
@Kortuk ahh kk
The reason I said to take someone else's implementation is that there can be many nuances to encryption.
In cryptography, a side channel attack is any attack based on information gained from the physical implementation of a cryptosystem, rather than brute force or theoretical weaknesses in the algorithms (compare cryptanalysis). For example, timing information, power consumption, electromagnetic leaks or even sound can provide an extra source of information which can be exploited to break the system. Some side-channel attacks require technical knowledge of the internal operation of the system on which the cryptography is implemented, although others such as differential power analysis are eff...
Those are the primary way that encryption is broken now.
21:53
"Electromagnetic attacks — attacks based on leaked electromagnetic radiation which can directly provide plaintexts and other information."
that's pretty awesome
22:27
@jellyksong You think that until you have to make a system resilient to it. I dont do encryption as a job but I know how easy it is to leak EM because that is a major part of my background.

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