"The type of a PNG image is defined in the IHDR image header. The image has a certain bit depth, up to 16 bits per sample, and a certain color type, from Grayscale to RGB+Alpha. If two PNG files of different types represent exactly the same image, each file can be regarded as a lossless transformation of the other. A lossless transformation can reduce the uncompressed stream, and such a transformation is named image reduction. In most cases, image reductions are capable of reducing the compressed stream (which is, in fact, our interest), as an indirect
"For the Web, PNG really has three main advantages over GIF: alpha channels (variable transparency), gamma correction (cross-platform control of image brightness), and two-dimensional interlacing (a method of progressive display). PNG also compresses better than GIF in almost every case, but the difference is generally only around 5% to 25%, not a large enough factor to encourage folks to switch on that basis alone. One GIF feature that PNG does not try to reproduce is multiple-image support, especially animations; PNG was and is intended to be a
For those of you who simply wish to buy a commercial application or download a zipfile full of executables and start looking at or making pictures, the pages below have pointers to a large number of applications that are known to support PNG in one form or other, or that are going to support PNG in the near future. Some of the programs (particularly those for Unix) are provided as source code and may require libpng and/or zlib (see PNG Source Code and Libraries), but often even these have pre-built binaries for popular systems somewhere nearby.
Also
Compression
PNG's compression is among the best that can be had without losing image information and without paying patent fees, but not all implementations take full advantage of the available power. Even those that do can be thwarted by unwise choices on the part of the user.
PNG supports three main image types: truecolor, grayscale and palette-based ("8-bit"). JPEG only supports the first two; GIF only the third (although it can fake grayscale by using a gray palette). The impact on compression comes from the ability to mix up image types in
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