PNG

(

Portable Network Graphics

), sometimes pronounced as "ping", is a relatively new
"/wiki/Bitmap" title="Bitmap">bitmap image format that

is becoming popular on the World Wide Web and elsewhere. PNG was

largely developed to deal with some of the shortcomings of the GIF format and allows storage

of images with greater color depth and other important information.

PNG

is supported by the

libpng

reference library, a platform

independent library that contains C functions for

handling PNG images.

The motivation for creating the PNG format came in early 1995, after
"/wiki/Unisys_Corporation" title="Unisys Corporation">Unisys announced that it would be enforcing
"/wiki/Software_patents" title="Software patents">software patents on the LZW
"/wiki/Data_compression" title="Data compression">data compression algorithm used for GIF. The algorithm is described in U.S.

Patent 4,558,302 and other patents around the world. There were also other problems

with the GIF format which made a replacement desirable, notably its limitation to 256
"Color">colours at a time when computers capable of displaying far more than 256 colours were becoming common. Although GIF

allows for animation, it was decided that PNG should be a

single-image format. A companion format called MNG has been defined for animation. PNG gained

some additional popularity in August 1999, after Unisys terminated its royalty-free patent

licenses to developers of free software and non-commercial software.

PNG uses a non-patented lossless data

compression method known as deflation. This method is combined with prediction,

where for each image line, a filter method is chosen that predicts the colour of each pixel based on the colours of

previous pixels and subtracts the predicted colour of the pixel from the actual color. An image line filtered in this way is

often more compressible than the raw image line would be. On most images, PNG can achieve greater compression than GIF, but some

implementations make poor choices of filter methods and therefore produce unnecessarily large PNG files.