SliTaz Packages

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.

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.

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.