LZW Compression Via Gif Transformation

Hi everyone, i can compress an array which be formed binary data but i can't decompress it. I use "gif" transformation to compress the array.

%Generates bits with equal probabilities
n=100000;
prob = [.5 .5]; 
Input=[0 1];
fp =randsrc(1,n,[Input;prob]);

data1 = uint8(fp);
imwrite(data1,'1KB.gif');
fp2 = fopen('1KB.gif','rb');
data = uint8(fread(fp2));

it works but i need to de decompress it.

i need the first binary array after the decompression. How can i do that ?

Thanks for your Help.

[Jan]

Note that the GIF does not contain only the compressed data, but some header data also. What is "the first binary"?

fread(fp2, Inf, '*uint8') is more efficient than uint8(fread(fp2)).

[samet ercetin]

thanks, how can i ignore these header data?(of course if it is possible)

[Jan]

It is not trivial to interpret the specifications of the GIF 89a standard, see https://www.w3.org/Graphics/GIF/spec-gif89a.txt. It will be much easier to use one of the functions of the file exchange. It would help to post a suggestion, if you explain, what you want to do.

Answer 1

[Jan]

Writing GIF compresses the data with LZW, but adds a header also. If you really want this, imread can import the data again.

I'd prefer a direct approach, e.g. FEX: lzw compression or FEX: lzw image codec.

[samet ercetin]

Thanks "imread" is worked but when the source(n) is bigger than "60000"(60KB), it doesn't give me the correct out_put of decompression. How can i fix that and the codes which are you give me in a URL above are too complicated for me, i can't understand.

[Jan]

 bigger than "60000"(60KB), it doesn't give me the correct out_put
 of decompression

Please show us the code which let you think so.

[samet ercetin]

Code is that;

clc;
clear;
clear all;
%Generates bits with equal probabilities
n=60000;
prob = [.5 .5]
Input=[0 1]
fp =randsrc(1,n,[Input;prob])

input = uint8(fp);
imwrite(input,'1KB.gif');
fp2 = fopen('1KB.gif','wb');
output = uint8(fread(fp2));

let me explain clearly what i want to do.

First i want to compress data which is formed binary data by using LZW.(I choose the gif transformation for that because it looks easier)

i can compress data by using first step but i can't access the binary files after the compression. I need the compressed binary data after the compression. I couldn't access these data by using "imread".

At least, i want to decopress these data. I want to convert them first binary format.

Is it possible by using "gif" transformation?

[Jan]

"GIF" is not a "transformation" or a LZW compression method, but a format of image files. Although the data are compressed by LZW, it contains color tables and further header information. Therefore it is a bad idea to use a GIF files as a compression method.

Although you can get the binary data of the GIF image file by fread, you will get the additional header information also.

Therefore I strongly recommend, to search in the FileExchange and the internet for "MATLAB lzw" and use one of the these tools. All you have to do is to download and run it. This will be less complicated than struggling with the creation of image files and trying to access the compressed binary data.