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VP8L (WebP lossless) encoder

The program loads an image (in any of the widely supported formats such as JPEG, PNG or BMP) and encodes it in the WebP lossless format (VP8L codec). The program uses most of the features of the VP8L format to achieve better compression than PNG and is not much worse than the official libwebp implementation. However, the program is quite slow compared to the heavily optimized image compression libraries currently in use, but this is mostly due to the focus on clarity rather than on performance.

The implemented VP8L features include:

  • entropy coding of image data using canonical Huffman codes
  • LZ77 backward references
  • local color cache references (currently disabled, as it does not improve compression)
  • palettization, possibly using sub-byte packing
  • subtract-green color transform to crudely decorrelate color channels
  • spatially-varying prediction

The notable features that are not implemented are:

  • spatially-varying Huffman codes
  • advanced cross-color transform

The encoded images are usually 65%--90% size of PNG and 110%--160% the size of lossless WebP images produced by libwebp.

Usage

Compile all the C# files in the VP8L/ directory and add a reference to System.Drawing to get an executable VP8L.exe. A Makefile is attached that invokes mcs that will do the trick. The executable can be invoked with two command-line arguments, as

VP8L.exe [input] [output]

where [input] is the path to the input image and [output] is the name of the produced .webp file. The default paths are input.png and output.webp.

Test images

The distribution includes a small corpus of images in test/input, used to test the program. The included script test.sh invokes the program and stores the generated images in test/output. It also converts the images into WebP using cwebp, the tool shipped with libwebp, and saves them to test/ref for comparison. To check that the output images are valid, the script converts them back to PNG using dwebp, which also serves as a check that the produced WebP images are valid.

Note that the script expects to find the dwebp and cwebp binaries in the test/ directory and executes the program using mono VP8L.exe.

Source guide

The Main is located in VP8L/VP8L.cs, other source files are mostly named after the main class they contain (Argb.cs, Image.cs, ...) or the static class that must be used to wrap functions (Transform.cs, ImageData.cs, ...). Read the comments to gain basic understanding of the inner workings.