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.

Pngrewrite will:

 * Remove any unused palette entries, and write a palette that is only as large
   as needed.
 * Remove (collapse) any duplicate palette entries.
 * Convert non-palette image to palette images, provided they contain no more
   than 256 different colors.
 * Move any colors with transparency to the beginning of the palette, and write
   a tRNS chunk that is a small as possible.
 * Reduce the bit-depth (bits per pixel) as much as possible.
 * 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.

--WARNING--

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.

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This is a very inefficient program. It is (relatively) slow, and may use a lot
of memory. To be specific, it uses about 5 bytes per pixel, no matter what the
bit depth of the image is.

This program is (hopefully) reasonably portable, and should compile without too
much effort on most C compilers. It requires the libpng and zlib libraries.

The pngrewrite code is structured as a library that could be used in other
applications, but I have not documented the interface.

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How to use:

From a command-line, run

    pngrewrite.exe <input-file.png> <output-file.png>

To read from standard-input, or write to standard-output, use "-" for the
filename.