Reduce large palettes on PNG files with this tool. Pngrewrite is command-line utility that reduces the unnecessarily large palettes that some programs write into PNG files. It also optimizes transparency data, and reduces the bits-per-pixel if possible. Handy for post-processing PNG files before putting them on a web site.

It can probably run on most any platform on which libpng can run. A Windows version is included. For other platforms, you'll have to compile it yourself.

Pngrewrite will:
1. Remove any unused palette entries, and write a palette that is only as large as needed.
2. Remove (collapse) any duplicate palette entries.
3. Convert non-palette image to palette images, provided they contain no more than 256 different colors.
4. Move any colors with transparency to the beginning of the palette, and write a tRNS chunk that is a small as possible.
5. Reduce the bit-depth (bits per pixel) as much as possible.
6. Write images as grayscale when possible, if that is compatible with the goal of using the minimum possible bit depth.

Under no circumstances does pngrewrite change the actual pixel colors, or background color, or transparency of the image. If it ever does, that's a bug.

Please Note:
pngrewrite removes most extra (ancillary) information from the PNG file, such as text comments. Although this does make the file size smaller, the removed information may sometimes be important.

The only ancillary chunks that are NOT removed are:
* gAMA - Image gamma setting
* sRGB - srgb color space indicator
* tIME - creation time
* pHYs - physical pixel size
* bKGD and tRNS - Background color and transparency are maintained. The actual chunk may be modified according to the new color structure.

If the original image was interlaced, the new one will also be interlaced.

Pngrewrite will not work at all on images that have more than 256 colors. Colors with the same RGB values but a different level of transparency count as different colors. The background color counts as an extra color if it does not occur in the image.

It will also not work at all on images that have a color depth of 16 bits, since they cannot have a palette.

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