Question

im using the SLsharpziplip to try to compress a byte[] before sending it on the network to a server. the byte[] contains jpeg data which is already compressed by the jpeg encoder.

you may ask , if jpeg already compress the image, why do i need to compress it more, well because i tried it and it worked.

here is what happened:

I wrote the bytes in the byte[] to a txt file , the size of the txt file is ~5k , i compressed it with winzip and the result file was ~2k , so thats about 50% reduction in the file size. however , when i try to do it with the byte[] and use the slsharziplip to compress the byte[] , the reduction in size is minimal.

here is the code i used:

 MemoryStream msCompressed = new MemoryStream();

 GZipOutputStream gzCompressed = new GZipOutputStream(msCompressed);

 gzCompressed.SetLevel(9);

  // allframes is a byte array. 

  gzCompressed.Write(allframes, 0, allframes.Length);

  gzCompressed.Finish();

  gzCompressed.IsStreamOwner = false;

  gzCompressed.Close();

  // i used byte[] compresseddata = msCompressed.ToArray() but i           thought i'll try this too.

  msCompressed.Seek(0, SeekOrigin.Begin);

  byte[] compresseddata = new byte[msCompressed.Length];

  msCompressed.Read(compresseddata, 0, compresseddata.Length);

==================================================================================

from debugging the code, i can see that the difference of size between allframes.Length and compresseddata.lenght is minimal. but if that same data is written to a text file and zipped with winzip its size is reduced by 50%.

this is how i write the same data to a txt file:

       TextWriter tw = new StreamWriter(MainPage.fs); // fs is a filestream. 

                            foreach (byte b in  allframes )

                            {

                                tw.Write(b);

                            } 

===============================================================================

am i doing something wrong?! am i misunderstanding something!!

thanks up front 🙂

Answer 1

Probably not, I would imagine WinZip has a superior zip algorithm to SLSharpZipLib. You can try varying the compression ratio but other than that, I would try different Silverlight compatible zip libraries.

JPEG as you’ve correctly pointed out is already a highly compressed file type, so finding a compression algorithm that can find further redundancy is going to be difficult.

Best regards,