zopfli-sharp

A C# wrapper for the Zopfli Compression Algorithm

This library is available on Nuget as libzopfli-sharp.

What is Zopfli?

Zopfli Compression Algorithm is a new zlib (gzip, deflate) compatible compressor. This compressor takes more time (~100x slower), but compresses around 5% better than zlib and better than any other zlib-compatible compressor we have found 1.

PNG Compression Usage

If you are working with .Net Image objects simply call SaveAsPNG() Image extension method.

Image testImage = Image.FromFile("files/ev.png");
testImage.SaveAsPNG(path_to_save_compressed_PNG);

You can compress *.PNG files directly using the ZopfliPNG.compress() method.

string path = "files/ev.png";
ZopfliPNG.compress(path);

We also implemented a derived class of Stream called ZopfliPNGStream

byte[] uncompressed = File.ReadAllBytes("files/ev.png");
int before = uncompressed.Length;
byte[] compressed;
int after = 0;

using (MemoryStream compressStream = new MemoryStream())
using (ZopfliPNGStream compressor = new ZopfliPNGStream(compressStream))
{
    compressor.Write(uncompressed, 0, before);
    compressor.Close();
    compressed = compressStream.ToArray();
    after = compressed.Length;
}

In addition to using the default compression options Zopfli exposes some additional options to fine tune compression. We extended this in the ZopfliPNGOptions object.

public class ZopfliPNGOptions
{
    // Allow altering hidden colors of fully transparent pixels
    public Boolean lossy_transparent;

    // Convert 16-bit per channel images to 8-bit per channel
    public Boolean lossy_8bit;

    // Filter strategies to try
    public ZopfliPNGFilterStrategy[] filter_strategies;

    // Automatically choose filter strategy using less good compression
    public Boolean auto_filter_strategy;

    // PNG chunks to keep
    // chunks to literally copy over from the original PNG to the resulting one
    public String[] keepchunks;

    // Use Zopfli deflate compression
    public Boolean use_zopfli;

    // Zopfli number of iterations
    public Int32 num_iterations;

    // Zopfli number of iterations on large images
    public Int32 num_iterations_large;

    // 0=none, 1=first, 2=last, 3=both
    public Int32 block_split_strategy;
}

Gzip, Deflate and Zlib Compression Usage

For all 3 compression types we implemented a derived class of Stream ZopfliStream

byte[] uncompressed = File.ReadAllBytes("files/fp.log");
int before = uncompressed.Length;
byte[] compressed;
int after = 0;

using (MemoryStream compressStream = new MemoryStream())
using (ZopfliStream compressor = new ZopfliStream(compressStream, ZopfliFormat.ZOPFLI_FORMAT_DEFLATE))
{
    compressor.Write(uncompressed, 0, before);
    compressor.Close();
    compressed = compressStream.ToArray();
    after = compressed.Length;
}

The second parameter for our derived Stream class is the type of compression to use.

public enum ZopfliFormat
{
    ZOPFLI_FORMAT_GZIP,
    ZOPFLI_FORMAT_ZLIB,
    ZOPFLI_FORMAT_DEFLATE
};

In addition to using the default options Zopfli exposes some additional options used to fine tune compression. We extended this in the ZopfliOptions object which can also be passed into the Stream.

public class ZopfliOptions
{
    // Whether to print output
    public Int32 verbose;
        
    // Whether to print more detailed output
    public Int32 verbose_more;

    // Maximum amount of times to rerun forward and backward pass to optimize LZ77
    // compression cost. Good values: 10, 15 for small files, 5 for files over
    // several MB in size or it will be too slow.
    public Int32 numiterations;

    // If true, splits the data in multiple deflate blocks with optimal choice
    // for the block boundaries. Block splitting gives better compression. Default:
    // true (1).
    public Int32 blocksplitting;

    // If true, chooses the optimal block split points only after doing the iterative
    // LZ77 compression. If false, chooses the block split points first, then does
    // iterative LZ77 on each individual block. Depending on the file, either first
    // or last gives the best compression. Default: false (0).
    public Int32 blocksplittinglast;

    // Maximum amount of blocks to split into (0 for unlimited, but this can give
    // extreme results that hurt compression on some files). Default value: 15.
    public Int32 blocksplittingmax;
}

Credit

Zopfli was written by Jyrki Alakuijala, Ph.D. and Lode Vandevenne, M.Sc. from Google Inc. You will find the projects homepage here: http://code.google.com/p/zopfli/ and the whitepaper here: http://code.google.com/p/zopfli/downloads/detail?name=Data_compression_using_Zopfli.pdf.