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3
votes
1answer
185 views

Why does a color video compress better than a black and white video?

It was asked in an exam why a color video compress better than a black and white (grayscale) video using MPEG but can't find anything explaining it. In other words, we would apparently get a better ...
6
votes
2answers
453 views

Compression type that can be searched

Is there ANY compression type that can compress a file, and then that compressed file can be searched without uncompressing the file?
6
votes
4answers
60 views

Minimal size of a context-free grammar which defines $L_n=\{a^k\mid 1\le k\le n\}$

I am looking for the minimal size of a context-free grammar which defines the finite language $$L_n=\{a^k\mid 1\le k\le n\}.$$ The size of a grammar is defined as the total length of all right-hand ...
10
votes
5answers
2k views

Data compression using prime numbers

I have recently stumbled upon the following interesting article which claims to efficiently compress random data sets by always more than 50%, regardless of the type and format of the data. Basically ...
1
vote
1answer
35 views

Do compression tables exist and where are they used?

Let's take the zip compression for example, from what scraps I can gather on the Internet, the 2 major simplified ways it works is by looking for arrangements of bits that repeat throughout the file, ...
4
votes
0answers
60 views

How to calculate Huffman code length without generating the actual huffman code?

Given a vector of elements, how can I calculate the length of the Huffman codewords without generating the Huffman code itself? Using Matlab, I was able to compute the Huffman code and and get the ...
3
votes
2answers
343 views

Are there any compression algorithms based on PI?

What we know is that π is infinite and quite likely it contains every possible finite string of digits (disjunctive sequence). I've seen recently some prototype of πfs which assume that every file ...
0
votes
1answer
46 views

How to compress a bit string? [closed]

I'm aware of some techniques to compress a string of characters such as Huffman coding. I need to know whether there is any technique to compress a long string of arbitrary bits? (ideally with high ...
1
vote
1answer
472 views

Why don't we use simple binary values to compress data?

In Huffman's algorithm, we form a tree and than replace each character with the tree value of 1 and 0. Why don't we simply use the binary digits like $a=0$, $b=1$, $c=10$, $d=01$, $e=11$ and so on ...
0
votes
0answers
69 views

The algorithm yields optimal ternary codes

Steps to build Huffman Tree Input is array of unique characters along with their frequency of occurrences and output is Huffman Tree. Create a leaf node for each unique character and build a min ...
5
votes
3answers
124 views

Compression functions are only practical because “The bit strings which occur in practice are far from random”?

I would have made a comment, as this pertains to Andrej Bauer's answer in this thread; however, I believe it is worth a question. Andrej explains that given the set of all bit strings of length 3 or ...
2
votes
0answers
25 views

Looking for some lossless compression theory [duplicate]

I'll apologize in advance if anything in here is ineloquent. Suppose we have a pair of lossless compression (C) and decompression (...
5
votes
3answers
192 views

Compression of Random Data is Impossible?

A few days ago this appeared on HN http://www.patrickcraig.co.uk/other/compression.htm. This refers to a challenge from 2001 - where someone was offering a prize of \$5000 for any kind of reduction to ...
0
votes
1answer
34 views

Using Huffman Encoding to improve Lempel-Ziv Encoding

Can anyone think of a way to use Huffman Encoding to improve the compression rate of Lempel-Ziv Encoding? My initial idea was to encode with higher probabilities given to shorter strings (i.e. the ...
3
votes
0answers
101 views

LZ-type compression vs. entropy-encoding of BWT data

Why does there seem to be a preference for using Huffman (or Arithmetic) coding instead of a Lempel-Ziv type compression algorithm for Burrows-Wheeler Transformed data? I noticed that data ...
1
vote
1answer
28 views

Temporal compression protocol

Recently it occurred to me that one could potentially design an interactive "compression" scheme which seems to allow for arbitrarily high compression ratios, at the sole expense of time. This already ...
1
vote
1answer
126 views

How can GIF compression be lossless if the maximum # of colors is 256?

I'm reading "Computer Science Made Simple" by V. Anton Spraul, and in the chapter about Graphics, he describes GIF files as follows: A GIF file uses lossless compression to store an image with a ...
4
votes
0answers
49 views

Polynomial time algorithms on strings [closed]

I am looking for familiar problems on strings of finite length over an finite alphabet, where a polynomial time algorithm is known. To be more precise, let $\Sigma$ be a finite alphabet. I am looking ...
1
vote
1answer
75 views

Shannon-Fano puzzle

I was playing around with Shannon-Fano (SF) entropy encoding when I ran into this issue. I am aware that the compression that can be achieved with SF is sometimes inferior to that of Huffman ...
1
vote
1answer
83 views

Designing a prefix code for integers?

trying to figure out how to approach this problem. Suppose a sender in a network wanted to communicate the length n of the block it will be using in a subsequent transmission. If there were not ...
2
votes
2answers
1k views

difference between lzw and huffman coding technique

What is the difference between the LZW and Huffman's coding of text compression ? I've read this and this , but I'm not able to distinguish ?
5
votes
0answers
96 views

LZ-type compression with a limited recursion depth

I would like to begin by apologizing. I am not a computer scientist, so my use of terminology will be off. I came across an interesting problem, which I believe to be pretty hard and would love to get ...
6
votes
5answers
2k views

Using base 80 for compressing files

I want to compress file size through making my own numbering system which is 80-based number, I do really want to know whether this even possible ? I learnt that Hexadecimal uses symbols like A, B, C, ...
12
votes
2answers
269 views

Enumerate all non-isomorphic graphs of a certain size

I'd like to enumerate all undirected graphs of size $n$, but I only need one instance of each isomorphism class. In other words, I want to enumerate all non-isomorphic (undirected) graphs on $n$ ...
0
votes
0answers
16 views

How to decode multiple-digit gamma codes and get the gap sequence? [duplicate]

How to decode gamma code ($\gamma$ code): 1110001110101011111101101111011 and get the gap sequence? Detailed information about Gamma codes ($\gamma$ codes) ...
2
votes
2answers
454 views

How to decode multiple-digit gamma codes and get the gap sequence?

How to decode gamma code ($\gamma$ code): 1110001110101011111101101111011 and get the gap sequence? Detailed information about Gamma codes ($\gamma$ codes) ...
0
votes
1answer
48 views

Number of phrases of LZ compression

It is known that for the number $c(n)$ of phrases / tupel of the LZ compression for binary words of length $n$ the following relation holds: $$c(n)\leq\frac{n}{(1-\epsilon_n)\log_2 n}$$ With ...
3
votes
2answers
111 views

How does one choose an optimal alphabet for finding a Huffman encoding?

Huffman encoding will perform best when the distribution of symbols of an alphabet that the string to be encoded uses is dyadic. Given an arbitrary bit string S, ...
0
votes
1answer
46 views

Compression technique exploiting common similarities

I asked this on StackOverflow, but it was determined more appropriate to post here. So it goes like this.. For people concerned about users having slow connections and timeout issues, it is known ...
3
votes
3answers
101 views

Compression of non-adjacent structure using grammar

I'm working with compression algorithms that use context-free grammars (e.g. RE-PAIR and SEQUITUR). These grammars look for frequently occurring digrams (pairs of adjacent symbols) in an input string ...
3
votes
1answer
116 views

What determines the entropy of a program's source code?

a few days ago I asked a question about the limits of compression: Can PRNGs be used to magically compress-stuff? The idea common to all the answers was that if you consider all programs of length ...
5
votes
2answers
178 views

What are all the differences between the Gibbs phenomenon artefects and JPEG artefacts?

I thought the Gibbs phenomenom is the result of Fourier analysis estimation (but was it Fourier Series estimation or can it also be Fourier Transform estimation?) JPEG uses the Discrete cosines ...
26
votes
7answers
3k views

Can PRNGs be used to magically compress stuff?

This idea occurred to me as a kid learning to program and on first encountering PRNG's. I still don't know how realistic it is, but now there's stack exchange. Here's a 14 year-old's scheme for an ...
1
vote
1answer
52 views

Do the two huffman trees have the same corpus?

Consider the following Huffman trees: I was asked if those trees can have the same corpus. My answer was no, based on these calculations: For the right tree: $a_1 \le a_2$ $a_1 + a_2 \le a_5$ ...
4
votes
3answers
123 views

Data Compression: Which one is Better “Compress before Encrypt” or “Encrypt before Compression”

Which one is better approach do data compression before encrypting it or Encrypt data before compression ? I think it is biased towards the application requirement. Please share your views.
3
votes
1answer
390 views

Does there exist a data compression algorithm that uses a large dataset distributed with the encoder/decoder?

If my goal were to compress say 10,000 images and I could include a dictionary or some sort of common database that the compressed data for each image would reference, could I use a large dictionary ...
3
votes
2answers
303 views

Compressing normally distributed data

Given normally distributed integers with a mean of 0 and a standard deviation $\sigma$ around 1000, how do I compress those numbers (almost) perfectly? Given the entropy of the Gaussian distribution, ...
0
votes
2answers
49 views

Arithmetic code: from interval to code value

I've not clear how to pass from final interval to code value, for example: Suppose we have the set of symbols={0,1,2,3} with probability={0.2, 0.5, 0.2 , 0.1} and that we have to encode a source ...
6
votes
2answers
101 views

How to find the minimal description for an array?

The following array occupies 10000 slots in memory: a = [0,1,2,3,4,5,6,7,8,9,10,...,10000] But one could easily represent the same array as: ...
3
votes
0answers
40 views

Approximate estimation of stream compressibility?

I wish to estimate the compression ratio of a stream of bytes. Specifically, I'm interested in DEFLATE compression. I'm looking for some algorithm/heuristic that can estimate roughly the ...
4
votes
4answers
257 views

compressed information = randomness?

Suppose I have a compressed file and it is not possible to compress it more without loss of information. We say that this file is random or pseudorandom. So, if the randomness means not ...
12
votes
1answer
247 views
2
votes
5answers
264 views

Is there a correlation of zip compression ratio and density of information provided by a text?

I'll phrase my question using an intuitive and rather extreme example: Is the expected compression ratio (using zip compression) of a children's book higher than that of a novel written for adults? ...
5
votes
1answer
110 views

To what factor can I compress information from binary to Quinary

I am trying to think if we can have an injective mapping $\mathcal{f}$, say something like $\Sigma_{bool}= \{0,1\}^* $ to $\Sigma_{5} = \{0,1,2,3,4\}^* $ such that $|x| \leq 2.|\mathcal{f}(x)|$ The ...
4
votes
2answers
134 views

Lossless data compression must make some messages longer? [duplicate]

I read on Wikipedia and in lecture notes that if a lossless data compression algorithm makes a message shorter, it must make another message longer. E.g. In this set of notes, it says: Consider, ...
1
vote
2answers
205 views

How optimal is Lempel-Ziv at reaching the Shannon limit?

I find this a bit difficult to describe, but I am interested in the following idea : The LZ algorithm factors (verb) an input stream into adjacent factors, these are by definition the maximal ...
5
votes
2answers
129 views

Behavior of iterative application of LZ77

I have been experimenting with LZ77 (naively $O(n^2)$ runtime, infinite window). Applying it to the 7th Fibonacci word $abaababaabaab$ yields the correct LZ factorization: $\qquad ...
3
votes
0answers
51 views

Bayesian Coding

Suppose you have a sequence generated by an i.i.d. process (such as repeatedly rolling a die and recording the values in order) parameterized by some K-dimensional vector $\vec{\gamma}$ (the ...
4
votes
1answer
354 views

Algorithm for determining minimal set of covering prefixes

I have a set of strings. My goal is to find a minimal set of longest prefixes which will match most of that set. For instance, if my set is: ...
5
votes
4answers
487 views

No compression algorithm can compress all input messages?

I just started reading a book called Introduction to Data Compression, by Guy E. Blelloch. On page one, he states: The truth is that if any one message is shortened by an algorithm, then some ...