In 1977 and 1978, two Israeli researchers, Jacob Ziv and Abraham Lempel worked on a set of compression algorithms, which were later named LZ77 and LZ78. Later, in 1983, Terry Welch from Sperry (which later merged with Burroughs to form Unisys) developed a very fast variant of LZ78 called LZW. Welch, together with two IBM researchers, Victor Miller and Mark Wegman, filled for a patent for LZW.
At the same time, Compuserve, specifically Bob Berry, was designing a new algorithm for a compressed image format, where they used the LZW
This material is intended to inform the simple users about the features of the PNG format. So, it will pass over some details like for example the freedom from patents, as this has no connection to the end-user, being more of a concern for the developers.
The PNG format was intended to be used like a substitute for the GIF and TIFF formats. So, it was created with two main ways of usage in view:
World Wide Web
Image editing
PNG can be widely used for the web, having several advantages over the GIF format: the possibility of
The PNG format is a format for lossless raster images, which is able to replace such classic formats as TIFF and GIF. It was designed to work mainly with WWW applications; it is robust and provides integrity checks for the file.
It was designed for the purpose of replacing the GIF format, but it also has additional design properties that were added with small effort from the designers. The features that are implemented in PNG and are absent in GIF format include:
Support for images with up to 48 bits per pixel, in True Color.
APNG comes as a modification of the well-known PNG format, but with additional support for animation. It supports 24-bit images and 8-bit transparency, having a much better quality then the classic GIF format.
An APNG file contains ordinary PNG frames, with two additional data chunks: aDAT (animation data chunks) and fCTL(the frame control chunk). Any PNG decoder should be able to decode the first frame of the APNG file as an ordinary PNG image. Any APNG data file must be subjected to certain rules:
The size of the entire APNG
APNG is an extension of the PNG format, adding support for animated images. It is intended to be a replacement for simple animated images that have traditionally used the GIF format, while adding support for 24-bit images and 8-bit transparency. APNG is a simpler alternative to MNG, providing a spec suitable for the most common usage of animated images on the Internet.APNG is backwards-compatible with PNG; any PNG decoder should be able to decode the first frame of an APNG and treat it as a normal single-frame PNG.
PNG Structure
An APNG stream is a
One of the biggest complaints with the Internet is that web pages load too slowly. People don't want to wait while graphics with large file sizes load. To overcome this problem graphics need to be compressed.
Graphics for the web are usually in 72dpi resolution this is a much lower resolution than used in the print industry.
The most common graphic formats supported by web browsers for compressing graphics are JPEG, GIF and PNG.
JPEG - This format is usually used for photos where there are lots of subtle colour changes and detail. The JPEG
Portable Network Graphic (PNG, pronounced "ping") is an image format developed by a consortium of graphic software developers as a nonproprietary alternative to the GIF image format. As mentioned earlier, CompuServe developed the GIF format, and GIF uses the proprietary LZW compression scheme owned by Unisys Corporation. Any graphics tool developer who makes software that saves in GIF format must pay a royalty to Unisys and CompuServe.
PNG graphics were designed specifically for use on Web pages, and they offer a range of attractive features that
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