OptiPNG is a PNG optimizer that recompresses image files to a smaller size, without losing any information. This program also converts external formats (BMP, GIF, PNM and TIFF) to optimized PNG, and performs PNG integrity checks and corrections.

The main purpose of OptiPNG is to optimize PNG files, i.e. to reduce their size to a minimum, without losing any information.

Like any other compression scheme, PNG compression depends on the statistics of the input data. In addition, it depends on the following PNG-specific parameters:
1.The PNG image type
2.The PNG delta filters
3.The strategy of searching LZ77 matches
4.The size of the Huffman buffers inside the Deflate encoder
Depending on how these parameters are chosen by the implementation, PNG compression may vary within wide limits. The process of selecting the best configuration is computationally infeasible, but heuristics to select a satisfactory configuration are available. The problem of improving these heuristics constitutes an interesting subject for research.

The OptiPNG program shall attempt to optimize PNG files, i.e. reduce their size to a minimum, without losing any information. In addition, this program shall perform a suite of auxiliary functions like integrity checks, metadata recovery and pixmap-to-PNG conversion.

The optimization attempts are not guaranteed to succeed. Valid PNG files that cannot be optimized by this program are normally left intact; their size will not grow. The user may request to override this default behavior.

The input files are raster image files encoded either in PNG format (the native format), or in an external format. The currently supported external formats are GIF, BMP, PNM and TIFF.

OptiPNG processes each image file given in the command line as follows:
1. If it is in PNG format:
Attempt to optimize the given file in-place. If optimization is successful, or if the option -force is in effect, replace the original file with its optimized version. The original file is backed up if the option -keep is in effect.

2. If it is in an external format:
Create an optimized PNG version of the given file. The output file name is composed from the original file name and the .png extension.

The PNG optimization algorithm consists of the following steps:
1. Reduce the bit depth, the color type and the color palette of the image. This step may reduce the size of the uncompressed image, which, indirectly, may reduce the size of the compressed image (i.e. the size of the output PNG file).
2. Run a suite of compression methods and strategies and select the compression parameters that yield the smallest output file.
3. Store all IDAT contents into a single chunk, eliminating the overhead incurred by repeated IDAT headers and CRCs.
4. Set the zlib window size inside IDAT to a mininum that does not affect the compression ratio, reducing the memory requirements of PNG decoders.

The license of this software is Free, you can free download and free use this image optimizer software.