OptiPNG will also help you convert external formats (GIF, PNM, BMP, and TIFF) to optimized PNG, and will perform 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.

In order to achieve this goal, OptiPNG performs the following tasks:
· OptiPNG losslessly reduces the bit depth, the color type and the color palette of the image. This step reduces the size of the uncompressed image, which, indirectly, reduces the size of the compressed image (i.e. the size of the PNG file).
· OptiPNG runs a suite of compression methods and strategies and selects the parameters that yield the smallest output file.
· OptiPNG concatenates all the IDAT data in a single chunk, minimizing the overhead incurred by chunk headers and CRCs.

In addition, OptiPNG has the following capabilities:
· OptiPNG offers a suite of optimization level presets. Given the number of trials that the user is willing to run, these presets will select the range of compression parameters that are most likely to yield the smallest output.
· OptiPNG offers a high degree of freedom to advanced users who can choose among many kinds of compression parameters. The optimization level presets and the custom compression parameters may be used in any combination.
· OptiPNG allows advanced users to disable certain image reductions.
· OptiPNG corrects integrity errors found in the input files. Bad CRCs, invalid data in ancillary chunks, incomplete PNG files, or extraneous data in IDAT, are examples of such integrity errors.
· OptiPNG recognizes several external file formats:
· BMP
The reading capabilities are limited to uncompressed BMP files. This is not a big problem, since few programs produce compresses BMPs. On the other hand, it is easy to uncompress a BMP file, by opening it in a BMP processing program (such as MS Paint), and re-saving it uncompressed.
· GIF
All static (i.e. non-animated) GIF images are supported. OptiPNG preserves the transparency information, but it does not preserve the textual comments embedded in GIFs. Animated GIFs cannot be converted to PNG. Instead, only the first frame is converted, and a warning is issued.
· PNM (PBM, PGM, PPM)
All these Unix-based formats are fully supported by OptiPNG. However, losslessness is only guaranteed when the maximum sample value is one of the following: 1, 3, 7, 15, 255, 65535. (This limitation applies to any PNM-to-PNG format converter, not only to OptiPNG.)
· TIFF
The read support is currently limited to uncompressed TIFF images that are encoded in a PNG-compatible color space (grayscale, RGB or RGBA). Enhanced TIFF support is under development.

What's New in This Release: [ read full changelog ]

· Upgraded libpng to version 1.2.15-optipng [private]
· Used a previously-missed RGB-to-palette reduction opportunity for images containing 256 distinct colors. (Thanks to Joachim Kluge for the report.)
· Fixed conversion of bKGD info during rgb-to-palette reductions. (Thanks to Matthew Fearnley for the report.)
· Fixed pre-computation of iterations. (Thanks to Matthew Fearnley for the report.)
· Eliminated a false alarm when processing RGB images with tRNS in Unix-secure mode. (Thanks to Till Mass for the report, and thanks to Nelson A. de Oliveira for the fix.)
· Fixed behavior of -out and -dir when changing interlacing. (Thanks to Martin Packman for the report.)
· Fixed behavior of "-dir [DRIVE]:" on Windows.